lec7b.chn+eng.ass

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Dialogue: 0,0:00:17.21,0:00:17.96,EN,,0,0,0,,PROFESSOR: Well, let's see.
Dialogue: 0,0:00:19.52,0:00:21.29,EN,,0,0,0,,What we did so far was a lot of fun,
Dialogue: 0,0:00:21.52,0:00:23.05,EN,,0,0,0,,was it useful for anything?
Dialogue: 0,0:00:26.33,0:00:27.96,EN,,0,0,0,,I suppose the answer is going to be yes.
Dialogue: 0,0:00:29.38,0:00:31.92,EN,,0,0,0,,If these metacircular interpreters
Dialogue: 0,0:00:32.96,0:00:34.60,EN,,0,0,0,,are a valuable thing to play with.
Dialogue: 0,0:00:34.62,0:00:36.17,EN,,0,0,0,,I spend, say
Dialogue: 0,0:00:38.05,0:00:41.85,EN,,0,0,0,,there have been times I spend 50% of my time, over a year,
Dialogue: 0,0:00:42.86,0:00:45.26,EN,,0,0,0,,trying various design alternatives
Dialogue: 0,0:00:45.76,0:00:48.19,EN,,0,0,0,,by experimenting with them with metacircular interpreters--
Dialogue: 0,0:00:49.47,0:00:52.01,EN,,0,0,0,,metacircular interpreters like the sort you just saw.
Dialogue: 0,0:00:52.57,0:00:54.11,EN,,0,0,0,,Metacircular is because
Dialogue: 0,0:00:54.72,0:00:56.94,EN,,0,0,0,,they are defined in terms of themselves in such a way
Dialogue: 0,0:00:56.97,0:00:59.71,EN,,0,0,0,,that the language they interpret contains itself.
Dialogue: 0,0:01:01.27,0:01:03.87,EN,,0,0,0,,Such interpreters are a convenient medium
Dialogue: 0,0:01:03.88,0:01:05.58,EN,,0,0,0,,for exploring language issues.
Dialogue: 0,0:01:06.80,0:01:09.44,EN,,0,0,0,,If you want to try adding a new feature,
Dialogue: 0,0:01:10.51,0:01:12.38,EN,,0,0,0,,it's sort of a snap, it's easy,
Dialogue: 0,0:01:12.73,0:01:15.10,EN,,0,0,0,,you just do it and see what happens.
Dialogue: 0,0:01:15.49,0:01:17.20,EN,,0,0,0,,You play with that language for a while you say,
Dialogue: 0,0:01:17.24,0:01:18.24,EN,,0,0,0,,gee, I'm didn't like that,
Dialogue: 0,0:01:18.52,0:01:19.47,EN,,0,0,0,,you throw it away.
Dialogue: 0,0:01:20.96,0:01:23.55,EN,,0,0,0,,Or you might want to see what
Dialogue: 0,0:01:23.64,0:01:27.37,EN,,0,0,0,,the difference is if you'd make a slight difference in the binding strategy,
Dialogue: 0,0:01:28.81,0:01:31.90,EN,,0,0,0,,or some more complicated things that might occur.
Dialogue: 0,0:01:33.72,0:01:35.48,EN,,0,0,0,,In fact, these metacircular interpreters
Dialogue: 0,0:01:36.17,0:01:37.88,EN,,0,0,0,,are an excellent medium for people
Dialogue: 0,0:01:38.20,0:01:42.56,EN,,0,0,0,,exchanging ideas about language design,
Dialogue: 0,0:01:43.98,0:01:45.74,EN,,0,0,0,,because they're pretty easy to understand,
Dialogue: 0,0:01:46.28,0:01:48.46,EN,,0,0,0,,and they're short, and compact, and simple.
Dialogue: 0,0:01:49.32,0:01:50.80,EN,,0,0,0,,If I have some idea
Dialogue: 0,0:01:51.53,0:01:53.77,EN,,0,0,0,,that I want somebody to criticize
Dialogue: 0,0:01:54.25,0:01:58.32,EN,,0,0,0,,like say, Dan Friedman at Indiana,
Dialogue: 0,0:01:59.05,0:02:02.00,EN,,0,0,0,,I'd write a little metacircular interpreter
Dialogue: 0,0:02:02.56,0:02:03.79,EN,,0,0,0,,send him some network mail
Dialogue: 0,0:02:04.65,0:02:05.45,EN,,0,0,0,,with this interpreter in it.
Dialogue: 0,0:02:05.45,0:02:07.90,EN,,0,0,0,,He could whip it up on his machine and play with it
Dialogue: 0,0:02:07.92,0:02:09.82,EN,,0,0,0,,and say, that's no good.
Dialogue: 0,0:02:11.94,0:02:13.10,EN,,0,0,0,,And then send it back to me and say,
Dialogue: 0,0:02:13.13,0:02:14.83,EN,,0,0,0,,well, why don't you try this one, it's a little better.
Dialogue: 0,0:02:16.88,0:02:19.36,EN,,0,0,0,,So I want to show you some of that technology.
Dialogue: 0,0:02:20.16,0:02:24.20,EN,,0,0,0,,See, because, really, it's the essential, simple technology
Dialogue: 0,0:02:24.72,0:02:28.68,EN,,0,0,0,,for getting started in designing your own languages for particular purposes.
Dialogue: 0,0:02:30.79,0:02:32.08,EN,,0,0,0,,Let's start by adding
Dialogue: 0,0:02:32.51,0:02:34.21,EN,,0,0,0,,a very simple feature to a Lisp.
Dialogue: 0,0:02:40.64,0:02:44.37,EN,,0,0,0,,Now, one thing I want to tell you about is features, before I start.
Dialogue: 0,0:02:49.56,0:02:52.17,EN,,0,0,0,,There are many languages that have made a mess of themselves
Dialogue: 0,0:02:53.05,0:02:54.91,EN,,0,0,0,,by adding huge numbers of features.
Dialogue: 0,0:02:56.86,0:02:58.38,EN,,0,0,0,,Computer scientists have a joke
Dialogue: 0,0:02:59.28,0:03:02.52,EN,,0,0,0,,about bugs that transform it to features all the time.
Dialogue: 0,0:03:05.03,0:03:06.46,EN,,0,0,0,,But I like to think of it is that
Dialogue: 0,0:03:08.91,0:03:11.44,EN,,0,0,0,,many systems suffer from what's called creeping featurism.
Dialogue: 0,0:03:12.82,0:03:13.44,EN,,0,0,0,,Which is that
Dialogue: 0,0:03:14.94,0:03:18.16,EN,,0,0,0,,George has a pet feature he'd like in the system,
Dialogue: 0,0:03:18.72,0:03:19.36,EN,,0,0,0,,so he adds it.
Dialogue: 0,0:03:20.17,0:03:22.14,EN,,0,0,0,,And then Harry says, go says
Dialogue: 0,0:03:22.17,0:03:24.20,EN,,0,0,0,,gee, this system is no longer what exactly I like,
Dialogue: 0,0:03:24.24,0:03:25.92,EN,,0,0,0,,so I'm going to add my favorite feature.
Dialogue: 0,0:03:26.64,0:03:30.24,EN,,0,0,0,,And then Jim adds his favorite feature.
Dialogue: 0,0:03:30.83,0:03:31.79,EN,,0,0,0,,And, after a while,
Dialogue: 0,0:03:31.80,0:03:34.81,EN,,0,0,0,,the thing has a manual 500 pages long
Dialogue: 0,0:03:35.15,0:03:36.51,EN,,0,0,0,,that no one can understand.
Dialogue: 0,0:03:37.79,0:03:39.32,EN,,0,0,0,,And sometimes it's the same person
Dialogue: 0,0:03:39.90,0:03:41.37,EN,,0,0,0,,who writes all of these features
Dialogue: 0,0:03:41.39,0:03:43.23,EN,,0,0,0,,and produces this terribly complicated thing.
Dialogue: 0,0:03:44.14,0:03:46.09,EN,,0,0,0,,In some cases, like editors,
Dialogue: 0,0:03:47.37,0:03:49.12,EN,,0,0,0,,it's sort of reasonable to have lots of features,
Dialogue: 0,0:03:50.92,0:03:52.65,EN,,0,0,0,,because there are a lot of things you want to be able to do
Dialogue: 0,0:03:52.68,0:03:53.76,EN,,0,0,0,,and many of them arbitrary.
Dialogue: 0,0:03:56.11,0:03:57.29,EN,,0,0,0,,But in computer languages,
Dialogue: 0,0:03:57.85,0:03:58.91,EN,,0,0,0,,I think it's a disaster
Dialogue: 0,0:04:00.01,0:04:01.29,EN,,0,0,0,,to have too much stuff in them.
Dialogue: 0,0:04:04.03,0:04:08.00,EN,,0,0,0,,The other alternative you get into is something called feeping creaturism,
Dialogue: 0,0:04:09.52,0:04:11.39,EN,,0,0,0,,which is where you have a box
Dialogue: 0,0:04:11.80,0:04:15.29,EN,,0,0,0,,which has a display, a fancy display, and a mouse,
Dialogue: 0,0:04:15.95,0:04:20.04,EN,,0,0,0,,and there is all sorts of complexity associated with all this fancy IO.
Dialogue: 0,0:04:21.01,0:04:22.80,EN,,0,0,0,,And your computer language becomes
Dialogue: 0,0:04:23.34,0:04:25.37,EN,,0,0,0,,a dismal, little, tiny thing that barely works
Dialogue: 0,0:04:25.40,0:04:27.90,EN,,0,0,0,,because of all the swapping, and disk twitching, and so on,
Dialogue: 0,0:04:28.09,0:04:29.36,EN,,0,0,0,,caused by your Window system.
Dialogue: 0,0:04:30.08,0:04:31.82,EN,,0,0,0,,And every time you go near the computer,
Dialogue: 0,0:04:31.93,0:04:33.45,EN,,0,0,0,,the mouse process wakes up and says,
Dialogue: 0,0:04:33.85,0:04:35.95,EN,,0,0,0,,gee do you have something for me to do,
Dialogue: 0,0:04:36.14,0:04:37.23,EN,,0,0,0,,and then it goes back to sleep.
Dialogue: 0,0:04:37.44,0:04:39.44,EN,,0,0,0,,And if you accidentally push mouse with you elbow,
Dialogue: 0,0:04:39.61,0:04:42.32,EN,,0,0,0,,a big puff of smoke comes out of your computer and things like that.
Dialogue: 0,0:04:42.94,0:04:45.29,EN,,0,0,0,,So two ways to disastrously
Dialogue: 0,0:04:45.55,0:04:47.21,EN,,0,0,0,,destroy a system by adding features.
Dialogue: 0,0:04:47.50,0:04:49.73,EN,,0,0,0,,But try right now to add a little, simple feature.
Dialogue: 0,0:04:52.60,0:04:53.77,EN,,0,0,0,,This actually is a good one,
Dialogue: 0,0:04:53.85,0:04:56.17,EN,,0,0,0,,and in fact, real Lisps have it.
Dialogue: 0,0:04:57.25,0:04:58.17,EN,,0,0,0,,As you've seen,
Dialogue: 0,0:04:59.29,0:05:03.13,EN,,0,0,0,,there are procedures like plus and times
Dialogue: 0,0:05:03.37,0:05:04.89,EN,,0,0,0,,that take any number of arguments.
Dialogue: 0,0:05:05.43,0:05:06.44,EN,,0,0,0,,So we can write things
Dialogue: 0,0:05:06.57,0:05:10.94,EN,,0,0,0,,like the sum of the product of a and x and x,
Dialogue: 0,0:05:12.09,0:05:16.99,EN,,0,0,0,,and the product of b and x and c.
Dialogue: 0,0:05:17.54,0:05:18.68,EN,,0,0,0,,As you can see here,
Dialogue: 0,0:05:18.92,0:05:21.76,EN,,0,0,0,,addition takes three arguments or two arguments,
Dialogue: 0,0:05:22.30,0:05:24.81,EN,,0,0,0,,multiplication takes two arguments or three arguments,
Dialogue: 0,0:05:25.08,0:05:26.76,EN,,0,0,0,,taking numbers of arguments
Dialogue: 0,0:05:26.78,0:05:28.49,EN,,0,0,0,,all of which are to be treated in the same way.
Dialogue: 0,0:05:30.00,0:05:32.17,EN,,0,0,0,,This is a valuable thing,
Dialogue: 0,0:05:32.28,0:05:34.01,EN,,0,0,0,,indefinite numbers of arguments.
Dialogue: 0,0:05:34.96,0:05:38.41,EN,,0,0,0,,Yet the particular Lisp system that I show you
Dialogue: 0,0:05:39.23,0:05:41.85,EN,,0,0,0,,is one where the numbers of arguments is fixed,
Dialogue: 0,0:05:42.62,0:05:45.28,EN,,0,0,0,,because I had to match the arguments against the formal parameters
Dialogue: 0,0:05:45.63,0:05:47.92,EN,,0,0,0,,in the binder, where there's a pairup.
Dialogue: 0,0:05:50.81,0:05:53.80,EN,,0,0,0,,Well, I'd like to be able to define new procedures like this
Dialogue: 0,0:05:54.89,0:05:57.32,EN,,0,0,0,,that can have any number of arguments.
Dialogue: 0,0:05:58.75,0:06:00.40,EN,,0,0,0,,Well there's several parts to this problem.
Dialogue: 0,0:06:01.34,0:06:04.81,EN,,0,0,0,,The first part is coming up with the syntactic specification,
Dialogue: 0,0:06:05.72,0:06:11.21,EN,,0,0,0,,some way of notating the additional arguments,
Dialogue: 0,0:06:12.17,0:06:13.63,EN,,0,0,0,,of which you don't know how many there are.
Dialogue: 0,0:06:15.48,0:06:16.62,EN,,0,0,0,,And then there's the other thing,
Dialogue: 0,0:06:17.10,0:06:18.70,EN,,0,0,0,,which is once we've notated it,
Dialogue: 0,0:06:19.07,0:06:20.78,EN,,0,0,0,,how are we going to interpret that notation
Dialogue: 0,0:06:21.74,0:06:23.10,EN,,0,0,0,,so as to do the right thing,
Dialogue: 0,0:06:23.85,0:06:25.37,EN,,0,0,0,,whatever the right thing is?
Dialogue: 0,0:06:26.98,0:06:28.80,EN,,0,0,0,,So let's consider an example of a sort of thing
Dialogue: 0,0:06:28.84,0:06:30.27,EN,,0,0,0,,we might want to be able to do.
Dialogue: 0,0:06:33.07,0:06:34.51,EN,,0,0,0,,So an example might be,
Dialogue: 0,0:06:35.42,0:06:37.34,EN,,0,0,0,,that I might want to be able to define a procedure
Dialogue: 0,0:06:37.95,0:06:41.36,EN,,0,0,0,,which is a procedure of one required argument x
Dialogue: 0,0:06:42.20,0:06:45.26,EN,,0,0,0,,and a non-required -- bunch of arguments,
Dialogue: 0,0:06:45.28,0:06:47.23,EN,,0,0,0,,I don't know how many there are, called y.
Dialogue: 0,0:06:49.09,0:06:50.36,EN,,0,0,0,,So x is required,
Dialogue: 0,0:06:55.88,0:06:57.44,EN,,0,0,0,,and there are many y's,
Dialogue: 0,0:06:59.53,0:07:05.99,EN,,0,0,0,,many arguments-- y will be the list of them.
Dialogue: 0,0:07:14.48,0:07:16.06,EN,,0,0,0,,Now, with such a thing,
Dialogue: 0,0:07:16.09,0:07:17.68,EN,,0,0,0,,we might be able to say something like,
Dialogue: 0,0:07:19.02,0:07:21.98,EN,,0,0,0,,map-- I'm going to do something to every one--
Dialogue: 0,0:07:22.52,0:07:25.76,EN,,0,0,0,,of that procedure of one argument u,
Dialogue: 0,0:07:27.00,0:07:34.54,EN,,0,0,0,,which multiplies x by u, and we'll apply that to y.
Dialogue: 0,0:07:36.89,0:07:38.04,EN,,0,0,0,,I've used a dot here
Dialogue: 0,0:07:38.59,0:07:41.31,EN,,0,0,0,,to indicate that the thing after this
Dialogue: 0,0:07:42.19,0:07:44.30,EN,,0,0,0,,is a list of all the rest of the arguments.
Dialogue: 0,0:07:46.30,0:07:48.12,EN,,0,0,0,,I'm making a syntactic specification.
Dialogue: 0,0:07:53.32,0:07:54.64,EN,,0,0,0,,Now, what this depends upon,
Dialogue: 0,0:07:55.71,0:07:58.06,EN,,0,0,0,,the reason why this is sort of a reasonable thing to do,
Dialogue: 0,0:07:59.77,0:08:01.96,EN,,0,0,0,,is because this happens to be a syntax
Dialogue: 0,0:08:02.00,0:08:03.60,EN,,0,0,0,,that's used in the Lisp reader
Dialogue: 0,0:08:04.41,0:08:07.15,EN,,0,0,0,,for representing conses.
Dialogue: 0,0:08:08.94,0:08:11.08,EN,,0,0,0,,We've never introduced that before.You never see.
Dialogue: 0,0:08:11.08,0:08:12.78,EN,,0,0,0,,You may have seen when playing with the system
Dialogue: 0,0:08:13.04,0:08:14.62,EN,,0,0,0,,if you cons two things together, you get the
Dialogue: 0,0:08:14.89,0:08:18.12,EN,,0,0,0,,first, space, dot, the second, space--
Dialogue: 0,0:08:19.79,0:08:22.83,EN,,0,0,0,,the first, space, dot, space, the second
Dialogue: 0,0:08:23.08,0:08:24.64,EN,,0,0,0,,with parentheses around the whole thing.
Dialogue: 0,0:08:26.98,0:08:28.16,EN,,0,0,0,,So that, for example,
Dialogue: 0,0:08:28.97,0:08:35.04,EN,,0,0,0,,this x dot y corresponds to a pair,
Dialogue: 0,0:08:36.33,0:08:39.29,EN,,0,0,0,,which has got an x in it and a y in it.
Dialogue: 0,0:08:41.48,0:08:43.98,EN,,0,0,0,,The other notations that you've seen so far
Dialogue: 0,0:08:44.94,0:08:46.67,EN,,0,0,0,,are things like, like
Dialogue: 0,0:08:46.92,0:08:55.24,EN,,0,0,0,,a procedure of arguments x and y and z which do things
Dialogue: 0,0:08:55.71,0:08:57.63,EN,,0,0,0,,and that looks like--
Dialogue: 0,0:09:02.00,0:09:03.61,EN,,0,0,0,,Just looking at the bound variable list,
Dialogue: 0,0:09:04.22,0:09:05.29,EN,,0,0,0,,it looks like this,
Dialogue: 0,0:09:09.93,0:09:17.32,EN,,0,0,0,,x, y, z, and the empty thing.
Dialogue: 0,0:09:18.28,0:09:21.08,EN,,0,0,0,,If I have a list of arguments I wish to match this against,
Dialogue: 0,0:09:22.60,0:09:25.60,EN,,0,0,0,,I have a list of arguments one, two, three,
Dialogue: 0,0:09:25.87,0:09:27.26,EN,,0,0,0,,I want to match these against.
Dialogue: 0,0:09:28.38,0:09:37.10,EN,,0,0,0,,OK? So I might have here a list of three things,
Dialogue: 0,0:09:42.44,0:09:46.94,EN,,0,0,0,,one, two, three.
Dialogue: 0,0:09:48.99,0:09:53.16,EN,,0,0,0,,And I want to match x, y, z against one, two, three.
Dialogue: 0,0:09:54.22,0:09:56.28,EN,,0,0,0,,Well, it's clear that the one matches the x,
Dialogue: 0,0:09:56.32,0:09:58.01,EN,,0,0,0,,because I can just sort of follow the structure,
Dialogue: 0,0:09:58.86,0:10:01.56,EN,,0,0,0,,and the two matches the y,
Dialogue: 0,0:10:02.46,0:10:04.04,EN,,0,0,0,,and the three matches the z.
Dialogue: 0,0:10:05.48,0:10:09.53,EN,,0,0,0,,But now, supposing I were to compare this x dot y.
Dialogue: 0,0:10:09.55,0:10:11.84,EN,,0,0,0,,this is x dot y--
Dialogue: 0,0:10:12.51,0:10:16.91,EN,,0,0,0,,supposing I compare that with a list of three arguments, one, two, three.
Dialogue: 0,0:10:19.08,0:10:20.00,EN,,0,0,0,,Let's look at that again.
Dialogue: 0,0:10:28.00,0:10:30.32,EN,,0,0,0,,One, two, three--
Dialogue: 0,0:10:30.86,0:10:32.88,EN,,0,0,0,,Well, I can walk along here
Dialogue: 0,0:10:32.99,0:10:35.50,EN,,0,0,0,,and say, oh yes, x matches the one,
Dialogue: 0,0:10:37.56,0:10:41.84,EN,,0,0,0,,Ah, the y matches the list, which is two and three.
Dialogue: 0,0:10:43.74,0:10:46.22,EN,,0,0,0,,So the notation I'm choosing here
Dialogue: 0,0:10:46.41,0:10:50.16,EN,,0,0,0,,is one that's very natural for Lisp system.
Dialogue: 0,0:10:52.66,0:10:54.14,EN,,0,0,0,,But I'm going to choose this as a notation
Dialogue: 0,0:10:54.17,0:10:55.80,EN,,0,0,0,,for representing a bunch of arguments.
Dialogue: 0,0:10:58.29,0:11:00.09,EN,,0,0,0,,Now, there's an alternative possibility.
Dialogue: 0,0:11:00.59,0:11:02.78,EN,,0,0,0,,If I don't want to take one special out,
Dialogue: 0,0:11:03.00,0:11:05.00,EN,,0,0,0,,or two special ones out or something like that,
Dialogue: 0,0:11:06.54,0:11:07.56,EN,,0,0,0,,if I don't want to do that,
Dialogue: 0,0:11:08.78,0:11:10.44,EN,,0,0,0,,if I want to talk about
Dialogue: 0,0:11:10.52,0:11:12.52,EN,,0,0,0,,just the list of all the arguments like in addition，
Dialogue: 0,0:11:13.88,0:11:17.96,EN,,0,0,0,,well then the argument list I'm going to choose to be
Dialogue: 0,0:11:18.20,0:11:23.45,EN,,0,0,0,,that procedure of all the arguments x, which does something with x
Dialogue: 0,0:11:25.14,0:11:26.30,EN,,0,0,0,,And which, for example,
Dialogue: 0,0:11:26.81,0:11:27.96,EN,,0,0,0,,if I take the procedure,
Dialogue: 0,0:11:28.06,0:11:30.44,EN,,0,0,0,,which takes all the arguments x
Dialogue: 0,0:11:31.12,0:11:32.70,EN,,0,0,0,,and returned the list of them,
Dialogue: 0,0:11:34.81,0:11:38.67,EN,,0,0,0,,OK? That's list. That's the procedure list.
Dialogue: 0,0:11:45.85,0:11:46.67,EN,,0,0,0,,How does this work?
Dialogue: 0,0:11:46.84,0:11:50.06,EN,,0,0,0,,Well, indeed what I had as the bound variable list in this case,
Dialogue: 0,0:11:50.60,0:11:51.45,EN,,0,0,0,,whatever it is,
Dialogue: 0,0:11:51.61,0:11:53.68,EN,,0,0,0,,is being matched against a list of arguments.
Dialogue: 0,0:11:55.14,0:11:57.14,EN,,0,0,0,,This symbol now is all of the arguments.
Dialogue: 0,0:12:01.49,0:12:05.13,EN,,0,0,0,,And so this is the choice I'm making for a particular syntactic specification,
Dialogue: 0,0:12:05.64,0:12:07.63,EN,,0,0,0,,for the description of procedures
Dialogue: 0,0:12:08.04,0:12:10.56,EN,,0,0,0,,which take indefinite numbers of arguments.
Dialogue: 0,0:12:13.45,0:12:14.60,EN,,0,0,0,,There are two cases of it,
Dialogue: 0,0:12:15.40,0:12:16.35,EN,,0,0,0,,this one and this one.
Dialogue: 0,0:12:17.44,0:12:18.36,EN,,0,0,0,,And none of this.
Dialogue: 0,0:12:18.42,0:12:20.11,EN,,0,0,0,,When you make syntactic specifications,
Dialogue: 0,0:12:20.44,0:12:22.54,EN,,0,0,0,,it's important that it's unambiguous,
Dialogue: 0,0:12:23.56,0:12:27.36,EN,,0,0,0,,that neither of these can be confused with
Dialogue: 0,0:12:27.66,0:12:31.20,EN,,0,0,0,,a representation we already have, this one.
Dialogue: 0,0:12:33.61,0:12:35.82,EN,,0,0,0,,I can always tell whether I have
Dialogue: 0,0:12:36.54,0:12:39.80,EN,,0,0,0,,a fixed number of explicitly named arguments
Dialogue: 0,0:12:40.28,0:12:41.76,EN,,0,0,0,,made by these formal parameters,
Dialogue: 0,0:12:42.64,0:12:43.13,EN,,0,0,0,,or
Dialogue: 0,0:12:43.28,0:12:45.36,EN,,0,0,0,,a fixed number of named formal parameters
Dialogue: 0,0:12:45.44,0:12:48.01,EN,,0,0,0,,followed by a thing which picks up all the rest of them,
Dialogue: 0,0:12:49.42,0:12:53.52,EN,,0,0,0,,or a list of all the arguments
Dialogue: 0,0:12:53.68,0:12:56.52,EN,,0,0,0,,which will be matched against this particular formal parameter called x,
Dialogue: 0,0:12:56.99,0:12:58.84,EN,,0,0,0,,because these are syntactically distinguishable.
Dialogue: 0,0:13:02.25,0:13:04.62,EN,,0,0,0,,Many languages make terrible errors in that form
Dialogue: 0,0:13:05.04,0:13:08.03,EN,,0,0,0,,where whole segments of interpretation are cut off,
Dialogue: 0,0:13:08.64,0:13:13.92,EN,,0,0,0,,because there are syntactic ambiguities in the language.
Dialogue: 0,0:13:14.56,0:13:16.67,EN,,0,0,0,,They are the traditional problems with ALGOL like languages
Dialogue: 0,0:13:16.67,0:13:23.47,EN,,0,0,0,,having to do with the nesting of ifs in the predicate part.
Dialogue: 0,0:13:25.06,0:13:25.93,EN,,0,0,0,,In any case,
Dialogue: 0,0:13:27.52,0:13:29.44,EN,,0,0,0,,now, so I've told you about the syntax,
Dialogue: 0,0:13:30.27,0:13:34.83,EN,,0,0,0,,now, what are we going to do about the semantics of this?
Dialogue: 0,0:13:35.25,0:13:36.11,EN,,0,0,0,,How do we interpret it?
Dialogue: 0,0:13:36.59,0:13:37.96,EN,,0,0,0,,Well this is just super easy.
Dialogue: 0,0:13:38.44,0:13:42.57,EN,,0,0,0,,I'm going to modify the metacircular interpreter to do it.
Dialogue: 0,0:13:43.71,0:13:44.76,EN,,0,0,0,,And that's a one liner.
Dialogue: 0,0:13:45.98,0:13:46.57,EN,,0,0,0,,There it is.
Dialogue: 0,0:13:47.53,0:13:49.56,EN,,0,0,0,,I'm changing the way you pair things up.
Dialogue: 0,0:13:50.81,0:13:54.19,EN,,0,0,0,,OK? Here we have procedure that pairs --
Dialogue: 0,0:13:56.76,0:14:02.03,EN,,0,0,0,,Here's the procedure that pairs the
Dialogue: 0,0:14:04.81,0:14:09.56,EN,,0,0,0,,the variables, the formal parameters, with the arguments that were passed
Dialogue: 0,0:14:12.16,0:14:16.68,EN,,0,0,0,,from the last description of the metacircular interpreter.
Dialogue: 0,0:14:18.96,0:14:21.93,EN,,0,0,0,,And here's some things that are the same as they were before.
Dialogue: 0,0:14:22.67,0:14:23.23,EN,,0,0,0,,In other words,
Dialogue: 0,0:14:23.31,0:14:25.07,EN,,0,0,0,,if the list of variables is empty,
Dialogue: 0,0:14:25.52,0:14:27.31,EN,,0,0,0,,then if the list of values is empty,
Dialogue: 0,0:14:27.45,0:14:29.61,EN,,0,0,0,,then I have an empty list.
Dialogue: 0,0:14:31.05,0:14:33.00,EN,,0,0,0,,Otherwise, I have too many arguments,
Dialogue: 0,0:14:33.98,0:14:40.19,EN,,0,0,0,,If I have, that is if I have empty variables but not empty values.
Dialogue: 0,0:14:41.58,0:14:44.00,EN,,0,0,0,,If I have empty values,
Dialogue: 0,0:14:44.96,0:14:47.47,EN,,0,0,0,,OK? But the variables are not empty,
Dialogue: 0,0:14:47.48,0:14:48.56,EN,,0,0,0,,that I have too few arguments.
Dialogue: 0,0:14:48.94,0:14:51.31,EN,,0,0,0,,However if I have a variable -- the variables are a symbol--
Dialogue: 0,0:14:55.53,0:14:56.49,EN,,0,0,0,,interesting case--
Dialogue: 0,0:14:58.30,0:15:04.40,EN,,0,0,0,,then, what I should do is say, oh yes, this is the special case
Dialogue: 0,0:15:04.59,0:15:06.51,EN,,0,0,0,,that I have a symbolic tail.
Dialogue: 0,0:15:08.35,0:15:14.11,EN,,0,0,0,,OK. I have here a thing just like we looked over here.
Dialogue: 0,0:15:14.90,0:15:17.87,EN,,0,0,0,,This is a tail which is a symbol, y.
Dialogue: 0,0:15:18.63,0:15:19.39,EN,,0,0,0,,It's not a nil.
Dialogue: 0,0:15:20.73,0:15:21.72,EN,,0,0,0,,It's not the empty list.
Dialogue: 0,0:15:23.26,0:15:25.60,EN,,0,0,0,,Here's a symbolic tail that is just the very beginning of the tail.
Dialogue: 0,0:15:25.98,0:15:26.81,EN,,0,0,0,,There is nothing else.
Dialogue: 0,0:15:27.79,0:15:28.72,EN,,0,0,0,,In that case,
Dialogue: 0,0:15:29.96,0:15:37.20,EN,,0,0,0,,I wish to match that variable with all the values
Dialogue: 0,0:15:38.03,0:15:42.52,EN,,0,0,0,,and add that to the pairing that I'm making.
Dialogue: 0,0:15:44.50,0:15:46.91,EN,,0,0,0,,Otherwise, I go through the normal arrangement
Dialogue: 0,0:15:47.15,0:15:48.52,EN,,0,0,0,,of making up the whole pairing.
Dialogue: 0,0:15:52.02,0:15:53.82,EN,,0,0,0,,I suppose that's very simple.
Dialogue: 0,0:15:54.51,0:15:55.84,EN,,0,0,0,,And that's all there is to it.
Dialogue: 0,0:15:57.08,0:15:58.33,EN,,0,0,0,,And now I'll answer some questions.
Dialogue: 0,0:16:02.62,0:16:05.05,EN,,0,0,0,,The first one-- Are there any questions?
Dialogue: 0,0:16:06.60,0:16:06.94,EN,,0,0,0,,Yes?
Dialogue: 0,0:16:07.37,0:16:09.92,EN,,0,0,0,,AUDIENCE: Could you explain that third form?
Dialogue: 0,0:16:09.98,0:16:12.12,EN,,0,0,0,,PROFESSOR: Third form. This one? OK.
Dialogue: 0,0:16:12.59,0:16:14.27,EN,,0,0,0,,Well, maybe we should look at the thing
Dialogue: 0,0:16:14.30,0:16:16.24,EN,,0,0,0,,as a piece of list structure.
Dialogue: 0,0:16:18.57,0:16:22.73,EN,,0,0,0,,This is a procedure which contains a lambda.
Dialogue: 0,0:16:25.85,0:16:29.61,EN,,0,0,0,,I'm just looking at the list structure which represents this.
Dialogue: 0,0:16:31.26,0:16:32.44,EN,,0,0,0,,Here's x.
Dialogue: 0,0:16:32.73,0:16:33.98,EN,,0,0,0,,These are our symbols.
Dialogue: 0,0:16:37.41,0:16:39.58,EN,,0,0,0,,And then the body is nothing but x.
Dialogue: 0,0:16:44.84,0:16:48.75,EN,,0,0,0,,If I were looking for the bound variable list part of this procedure,
Dialogue: 0,0:16:50.09,0:16:51.58,EN,,0,0,0,,I would go looking at the CADR,
Dialogue: 0,0:16:52.14,0:16:53.16,EN,,0,0,0,,and I'd find a symbol.
Dialogue: 0,0:16:54.01,0:16:57.16,EN,,0,0,0,,So the, matcher, which is this pairup thing I just showed you,
Dialogue: 0,0:16:58.24,0:17:00.44,EN,,0,0,0,,is going to be matching a symbolic object
Dialogue: 0,0:17:01.56,0:17:04.40,EN,,0,0,0,,against a list of arguments that were passed.
Dialogue: 0,0:17:05.76,0:17:09.55,EN,,0,0,0,,And it will bind that symbol to the list of arguments.
Dialogue: 0,0:17:11.37,0:17:16.48,EN,,0,0,0,,The -- In this case, if I'm looking for it,
Dialogue: 0,0:17:16.92,0:17:20.97,EN,,0,0,0,,the match will be against this in the bound variable list position.
Dialogue: 0,0:17:24.14,0:17:26.14,EN,,0,0,0,,Now, if what this does is
Dialogue: 0,0:17:26.17,0:17:29.13,EN,,0,0,0,,it gets a list of arguments and returns it, that's list
Dialogue: 0,0:17:30.40,0:17:31.39,EN,,0,0,0,,That's the procedure is.
Dialogue: 0,0:17:34.51,0:17:35.48,EN,,0,0,0,,Oh well, thank you.
Dialogue: 0,0:17:36.14,0:17:37.28,EN,,0,0,0,,Let's take a break.
Dialogue: 0,0:17:37.83,0:17:55.36,EN,,0,0,0,,[JESU, JOY OF MAN'S DESIRING]
Dialogue: 0,0:17:55.36,0:17:59.02,EN,,0,0,0,,
Dialogue: 0,0:18:03.53,0:18:07.56,EN,,0,0,0,,
Dialogue: 0,0:18:07.56,0:18:11.69,EN,,0,0,0,,
Dialogue: 0,0:18:12.25,0:18:16.11,EN,,0,0,0,,
Dialogue: 0,0:18:20.86,0:18:21.61,EN,,0,0,0,,PROFESSOR: Well let's see.
Dialogue: 0,0:18:23.26,0:18:26.32,EN,,0,0,0,,Now, I'm going to tell you about a rather more substantial variation
Dialogue: 0,0:18:27.45,0:18:31.04,EN,,0,0,0,,one that's a famous variation
Dialogue: 0,0:18:31.60,0:18:36.80,EN,,0,0,0,,hat many early Lisps had.
Dialogue: 0,0:18:38.25,0:18:40.06,EN,,0,0,0,,It's called dynamic binding of variables.
Dialogue: 0,0:18:41.77,0:18:44.68,EN,,0,0,0,,And we'll investigate a little bit about that right now.
Dialogue: 0,0:18:47.62,0:18:50.16,EN,,0,0,0,,I'm going to first introduce this by showing you the sort of thing
Dialogue: 0,0:18:50.35,0:18:52.36,EN,,0,0,0,,that would make someone want this idea.
Dialogue: 0,0:18:53.74,0:18:55.23,EN,,0,0,0,,I'm not going to tell what it is yet,
Dialogue: 0,0:18:55.40,0:18:57.60,EN,,0,0,0,,I'm going to show you why you might want it.
Dialogue: 0,0:18:58.64,0:18:59.93,EN,,0,0,0,,Suppose, for example,
Dialogue: 0,0:19:00.75,0:19:02.59,EN,,0,0,0,,we looked at the sum procedure again
Dialogue: 0,0:19:05.02,0:19:06.43,EN,,0,0,0,,for summing up a bunch of things.
Dialogue: 0,0:19:08.14,0:19:09.47,EN,,0,0,0,,To be that procedure,
Dialogue: 0,0:19:09.60,0:19:10.78,EN,,0,0,0,,of a term,
Dialogue: 0,0:19:13.04,0:19:14.41,EN,,0,0,0,,lower bound,
Dialogue: 0,0:19:15.24,0:19:17.04,EN,,0,0,0,,method of computing the next index,
Dialogue: 0,0:19:17.24,0:19:18.56,EN,,0,0,0,,and upper bound,
Dialogue: 0,0:19:19.36,0:19:20.16,EN,,0,0,0,,such that,
Dialogue: 0,0:19:23.16,0:19:26.94,EN,,0,0,0,,if a is greater than b
Dialogue: 0,0:19:27.15,0:19:28.64,EN,,0,0,0,,then the result is 0,
Dialogue: 0,0:19:30.24,0:19:31.08,EN,,0,0,0,,otherwise,
Dialogue: 0,0:19:33.68,0:19:39.82,EN,,0,0,0,,it's the sum, of the term, procedure, applied to a
Dialogue: 0,0:19:40.60,0:19:44.24,EN,,0,0,0,,and the result of adding up, terms,
Dialogue: 0,0:19:47.68,0:19:52.64,EN,,0,0,0,,with the next a being the a,
Dialogue: 0,0:20:00.30,0:20:03.56,EN,,0,0,0,,the next procedure passed along,
Dialogue: 0,0:20:06.40,0:20:08.25,EN,,0,0,0,,and the upper bound being passed along.
Dialogue: 0,0:20:14.51,0:20:15.76,EN,,0,0,0,,Blink, blink, blink--
Dialogue: 0,0:20:17.82,0:20:21.45,EN,,0,0,0,,OK? Now, when I use this sum procedure,
Dialogue: 0,0:20:21.96,0:20:24.35,EN,,0,0,0,,I can use it, for example, like this.
Dialogue: 0,0:20:25.45,0:20:38.04,EN,,0,0,0,,We can define the sum of the powers to be,
Dialogue: 0,0:20:38.08,0:20:40.33,EN,,0,0,0,,for example, sum of a bunch of powers x to the n,
Dialogue: 0,0:20:41.10,0:20:45.93,EN,,0,0,0,,to be that procedure of a, b, and n--
Dialogue: 0,0:20:45.95,0:20:47.69,EN,,0,0,0,,lower bound, the upper bound, and n--
Dialogue: 0,0:20:48.06,0:20:53.34,EN,,0,0,0,,which is sum, of lambda of x,
Dialogue: 0,0:20:53.60,0:20:59.31,EN,,0,0,0,,the procedure of one argument x, which exponentiates x to the n,
Dialogue: 0,0:21:02.19,0:21:09.29,EN,,0,0,0,,with the a, the incrementer, and b, being passed along.
Dialogue: 0,0:21:11.82,0:21:15.76,EN,,0,0,0,,So we're adding up x to n, given an x.
Dialogue: 0,0:21:16.14,0:21:19.74,EN,,0,0,0,,x takes on values from a to b, incrementing by one.
Dialogue: 0,0:21:22.94,0:21:24.38,EN,,0,0,0,,I can also write the--
Dialogue: 0,0:21:27.68,0:21:28.20,EN,,0,0,0,,That's right.
Dialogue: 0,0:21:29.78,0:21:31.02,EN,,0,0,0,,Product, excuse me.
Dialogue: 0,0:21:31.91,0:21:33.36,EN,,0,0,0,,The product of a bunch of powers.
Dialogue: 0,0:21:38.08,0:21:39.12,EN,,0,0,0,,It's a strange name.
Dialogue: 0,0:21:40.02,0:21:40.80,EN,,0,0,0,,I'm going to leave it there.
Dialogue: 0,0:21:41.96,0:21:46.32,EN,,0,0,0,,Weird-- OK? I write up what I have.
Dialogue: 0,0:21:49.34,0:21:50.19,EN,,0,0,0,,I'm sure that's right.
Dialogue: 0,0:21:51.37,0:21:53.82,EN,,0,0,0,,And if I want the product of a bunch of powers--
Dialogue: 0,0:21:58.41,0:22:02.36,EN,,0,0,0,,That was 12 brain cells, that double-take.
Dialogue: 0,0:22:03.00,0:22:06.81,EN,,0,0,0,,I can for example use the procedure which is like sum,
Dialogue: 0,0:22:06.81,0:22:08.22,EN,,0,0,0,,which is for making products,
Dialogue: 0,0:22:08.56,0:22:11.05,EN,,0,0,0,,but it's similar to that, that you've seen before.
Dialogue: 0,0:22:11.45,0:22:16.38,EN,,0,0,0,,There's a procedure of three arguments again.
Dialogue: 0,0:22:17.00,0:22:25.42,EN,,0,0,0,,Which is the product of terms that are constructed, or factors in this case,
Dialogue: 0,0:22:25.66,0:22:31.60,EN,,0,0,0,,constructed from exponentiating x to the n,
Dialogue: 0,0:22:34.43,0:22:37.85,EN,,0,0,0,,where I start with a, I increment, and I go to b.
Dialogue: 0,0:22:41.53,0:22:41.88,EN,,0,0,0,,Now,
Dialogue: 0,0:22:46.83,0:22:49.50,EN,,0,0,0,,there's some sort of thing here that should disturb you immediately.
Dialogue: 0,0:22:50.75,0:22:52.01,EN,,0,0,0,,These look the same.
Dialogue: 0,0:22:53.18,0:22:55.20,EN,,0,0,0,,Why am I writing this code so many times?
Dialogue: 0,0:22:56.59,0:22:59.72,EN,,0,0,0,,Here I am, in the same boat I've been in before.
Dialogue: 0,0:23:01.00,0:23:03.15,EN,,0,0,0,,Right? Wouldn't it be nice to make an abstraction here?
Dialogue: 0,0:23:03.81,0:23:05.76,EN,,0,0,0,,What's an example of a good abstraction to make?
Dialogue: 0,0:23:05.85,0:23:07.55,EN,,0,0,0,,Well, I see some codes that's identical.
Dialogue: 0,0:23:08.47,0:23:09.32,EN,,0,0,0,,Here's one,
Dialogue: 0,0:23:09.98,0:23:11.08,EN,,0,0,0,,and here's another.
Dialogue: 0,0:23:14.45,0:23:16.22,EN,,0,0,0,,And so maybe I should be able to pull that out.
Dialogue: 0,0:23:17.09,0:23:19.23,EN,,0,0,0,,I should be able to say, oh yes,
Dialogue: 0,0:23:20.51,0:23:22.67,EN,,0,0,0,,the sum of the powers could be written in terms of
Dialogue: 0,0:23:22.88,0:23:24.52,EN,,0,0,0,,something called the nth power procedure.
Dialogue: 0,0:23:25.71,0:23:27.40,EN,,0,0,0,,Imagine somebody wanted to write
Dialogue: 0,0:23:27.74,0:23:30.03,EN,,0,0,0,,a slightly different procedure that looks like this.
Dialogue: 0,0:23:37.63,0:23:45.18,EN,,0,0,0,,The sum powers to be a procedure
Dialogue: 0,0:23:46.44,0:23:48.46,EN,,0,0,0,,of a, b, and n,
Dialogue: 0,0:23:48.75,0:23:52.27,EN,,0,0,0,,which the result of summing up the nth power.
Dialogue: 0,0:23:53.88,0:23:55.42,EN,,0,0,0,,We're going to give a name to that idea,
Dialogue: 0,0:23:58.35,0:24:02.27,EN,,0,0,0,,for starting at a, going by one, and ending at b.
Dialogue: 0,0:24:05.74,0:24:06.91,EN,,0,0,0,,And similarly,
Dialogue: 0,0:24:10.65,0:24:12.76,EN,,0,0,0,,I might want to write the product powers this way,
Dialogue: 0,0:24:12.89,0:24:15.24,EN,,0,0,0,,abstracting out this idea.
Dialogue: 0,0:24:16.27,0:24:17.37,EN,,0,0,0,,I might want this.
Dialogue: 0,0:24:22.10,0:24:23.02,EN,,0,0,0,,Product powers,
Dialogue: 0,0:24:29.48,0:24:34.94,EN,,0,0,0,,to be a procedure of a, b, and n,
Dialogue: 0,0:24:35.31,0:24:42.33,EN,,0,0,0,,which is the product of the nth power operation
Dialogue: 0,0:24:46.44,0:24:50.30,EN,,0,0,0,,on a with the incrementation and b being
Dialogue: 0,0:24:53.50,0:24:57.56,EN,,0,0,0,,being my arguments for the analogous-thing product.
Dialogue: 0,0:24:58.38,0:25:00.24,EN,,0,0,0,,And I'd like to be able to define,
Dialogue: 0,0:25:02.04,0:25:03.88,EN,,0,0,0,,I'd like to be able to define nth power--
Dialogue: 0,0:25:04.89,0:25:05.93,EN,,0,0,0,,I'll put it over here.
Dialogue: 0,0:25:12.22,0:25:12.99,EN,,0,0,0,,I'll put it at the top.
Dialogue: 0,0:25:25.41,0:25:29.04,EN,,0,0,0,,--to be, in fact, my procedure of one argument x
Dialogue: 0,0:25:29.60,0:25:34.56,EN,,0,0,0,,which is the result of exponentiating x to the n.
Dialogue: 0,0:25:35.93,0:25:36.96,EN,,0,0,0,,But I have a problem.
Dialogue: 0,0:25:38.64,0:25:39.93,EN,,0,0,0,,My environment model,
Dialogue: 0,0:25:40.57,0:25:43.23,EN,,0,0,0,,that is my means of interpretation
Dialogue: 0,0:25:44.00,0:25:45.95,EN,,0,0,0,,of interpretation for the language that we've defined so far,
Dialogue: 0,0:25:46.27,0:25:48.81,EN,,0,0,0,,does not give me a meaning for this n.
Dialogue: 0,0:25:52.76,0:25:59.26,EN,,0,0,0,,Because, as you know, the, as you know
Dialogue: 0,0:26:00.76,0:26:04.25,EN,,0,0,0,,this n is free in this procedure.
Dialogue: 0,0:26:06.41,0:26:07.98,EN,,0,0,0,,The environment model tells us
Dialogue: 0,0:26:08.60,0:26:10.20,EN,,0,0,0,,that the meaning of a free variable
Dialogue: 0,0:26:11.21,0:26:14.99,EN,,0,0,0,,is determined in the environment in which this procedure is defined.
Dialogue: 0,0:26:16.64,0:26:17.47,EN,,0,0,0,,In a way I have written it,
Dialogue: 0,0:26:17.48,0:26:19.84,EN,,0,0,0,,assuming these things are defined on the blackboard as is,
Dialogue: 0,0:26:21.64,0:26:23.63,EN,,0,0,0,,this is defined in the global environment,
Dialogue: 0,0:26:24.06,0:26:25.15,EN,,0,0,0,,where there is no n.
Dialogue: 0,0:26:25.93,0:26:27.63,EN,,0,0,0,,Therefore, n is unbound variable.
Dialogue: 0,0:26:28.72,0:26:31.66,EN,,0,0,0,,But it's perfectly clear, to most of us,
Dialogue: 0,0:26:32.60,0:26:36.32,EN,,0,0,0,,that we would like it to be this n and this n.
Dialogue: 0,0:26:38.99,0:26:42.67,EN,,0,0,0,,On the other hand, OK, it would be nice.
Dialogue: 0,0:26:42.84,0:26:44.28,EN,,0,0,0,,Certainly we've got to be careful here
Dialogue: 0,0:26:44.56,0:26:46.06,EN,,0,0,0,,of keeping this to be this,
Dialogue: 0,0:26:48.96,0:26:52.83,EN,,0,0,0,,and this one over here, wherever it is to be this one.
Dialogue: 0,0:26:57.39,0:26:59.74,EN,,0,0,0,,Well, the desire to make this work
Dialogue: 0,0:27:00.67,0:27:02.72,EN,,0,0,0,,has led to a very famous bug.
Dialogue: 0,0:27:04.65,0:27:06.04,EN,,0,0,0,,I'll tell you about the famous bug.
Dialogue: 0,0:27:07.15,0:27:09.40,EN,,0,0,0,,Look at this slide.
Dialogue: 0,0:27:10.66,0:27:12.70,EN,,0,0,0,,This is an idea called dynamic binding.
Dialogue: 0,0:27:13.99,0:27:16.91,EN,,0,0,0,,Where, instead of the free variable being interpreted
Dialogue: 0,0:27:17.76,0:27:21.23,EN,,0,0,0,,in the environment of definition of a procedure,
Dialogue: 0,0:27:22.40,0:27:25.16,EN,,0,0,0,,the free variable is interpreted as having its value
Dialogue: 0,0:27:25.44,0:27:29.31,EN,,0,0,0,,in the environment of the caller of the procedure.
Dialogue: 0,0:27:31.85,0:27:34.84,EN,,0,0,0,,So what you have is a system
Dialogue: 0,0:27:34.86,0:27:39.68,EN,,0,0,0,,where you search up the chain of callers of a particular procedure,
Dialogue: 0,0:27:40.43,0:27:42.65,EN,,0,0,0,,and, of course, in this case,
Dialogue: 0,0:27:42.84,0:27:44.30,EN,,0,0,0,,since nth power is called
Dialogue: 0,0:27:44.33,0:27:45.98,EN,,0,0,0,,from inside product whatever it is--
Dialogue: 0,0:27:46.41,0:27:48.68,EN,,0,0,0,,I had to write our own sum which is the analogous procedure--
Dialogue: 0,0:27:50.51,0:27:54.92,EN,,0,0,0,,and product is presumably called from product powers,
Dialogue: 0,0:27:55.13,0:27:56.14,EN,,0,0,0,,as you see over here,
Dialogue: 0,0:27:56.83,0:27:59.37,EN,,0,0,0,,then since product powers bind with variable n ,
Dialogue: 0,0:28:00.09,0:28:04.09,EN,,0,0,0,,then nth powers n would be derived through that chain.
Dialogue: 0,0:28:08.14,0:28:09.64,EN,,0,0,0,,Similarly, this n,
Dialogue: 0,0:28:10.11,0:28:12.01,EN,,0,0,0,,the nth power in n in this case,
Dialogue: 0,0:28:12.32,0:28:15.80,EN,,0,0,0,,would come through nth power here being called from inside sum.
Dialogue: 0,0:28:15.80,0:28:18.27,EN,,0,0,0,,You can see it being called from inside sum here.
Dialogue: 0,0:28:20.73,0:28:21.69,EN,,0,0,0,,It's called term here.
Dialogue: 0,0:28:22.90,0:28:25.72,EN,,0,0,0,,But sum was called from inside of sum powers,
Dialogue: 0,0:28:26.94,0:28:27.96,EN,,0,0,0,,which bound n.
Dialogue: 0,0:28:28.93,0:28:30.65,EN,,0,0,0,,Therefore, there would be an n available
Dialogue: 0,0:28:32.75,0:28:36.11,EN,,0,0,0,,that n to get it's value from.
Dialogue: 0,0:28:37.95,0:28:39.24,EN,,0,0,0,,This is called dynamic --
Dialogue: 0,0:28:39.28,0:28:43.10,EN,,0,0,0,,What we have below this white line plus over here
Dialogue: 0,0:28:43.31,0:28:46.04,EN,,0,0,0,,is what's called a dynamic binding view of the world.
Dialogue: 0,0:28:46.59,0:28:49.00,EN,,0,0,0,,If that works, that's a dynamic binding view.
Dialogue: 0,0:28:50.85,0:28:52.65,EN,,0,0,0,,Now, let's take a look, for example,
Dialogue: 0,0:28:54.54,0:28:55.99,EN,,0,0,0,,at just what it takes to implement that.
Dialogue: 0,0:28:55.99,0:28:56.96,EN,,0,0,0,,That's real easy.
Dialogue: 0,0:28:57.48,0:28:59.34,EN,,0,0,0,,In fact, the very first Lisps
Dialogue: 0,0:29:00.01,0:29:02.52,EN,,0,0,0,,had any form of interpretations of the free variables at all,
Dialogue: 0,0:29:03.31,0:29:05.98,EN,,0,0,0,,had dynamic binding interpretations for the free variables.
Dialogue: 0,0:29:06.40,0:29:10.14,EN,,0,0,0,,APL has dynamic binding interpretation for the free variables,
Dialogue: 0,0:29:11.68,0:29:14.32,EN,,0,0,0,,not lexical or static binding.
Dialogue: 0,0:29:15.22,0:29:17.00,EN,,0,0,0,,So, of course, the change is in eval.
Dialogue: 0,0:29:19.31,0:29:20.59,EN,,0,0,0,,And it's really in two places.
Dialogue: 0,0:29:22.78,0:29:25.61,EN,,0,0,0,,First of all, one thing we see,
Dialogue: 0,0:29:26.52,0:29:28.49,EN,,0,0,0,,is that things become a little simpler.
Dialogue: 0,0:29:29.39,0:29:33.63,EN,,0,0,0,,If I don't have to have the -- If I don't have to have the
Dialogue: 0,0:29:33.64,0:29:36.20,EN,,0,0,0,,environment be the environment of definition for procedure,
Dialogue: 0,0:29:36.44,0:29:38.04,EN,,0,0,0,,the procedure need not capture
Dialogue: 0,0:29:38.43,0:29:40.17,EN,,0,0,0,,the environment at the time it's defined.
Dialogue: 0,0:29:42.03,0:29:44.96,EN,,0,0,0,,And so if we look here at this slide,
Dialogue: 0,0:29:45.84,0:29:50.08,EN,,0,0,0,,we see that the clause for a lambda expression,
Dialogue: 0,0:29:50.73,0:29:52.43,EN,,0,0,0,,which is the way a procedure is defined,
Dialogue: 0,0:29:53.92,0:29:56.73,EN,,0,0,0,,does not make up a thing which has a type closure
Dialogue: 0,0:29:56.75,0:30:01.05,EN,,0,0,0,,and a attached environment structure.
Dialogue: 0,0:30:01.29,0:30:02.54,EN,,0,0,0,,It's just the expression itself.
Dialogue: 0,0:30:02.54,0:30:04.76,EN,,0,0,0,,And we'll decompose that some other way somewhere else.
Dialogue: 0,0:30:06.44,0:30:09.40,EN,,0,0,0,,The other thing we see is the applicator
Dialogue: 0,0:30:10.36,0:30:13.69,EN,,0,0,0,,applicator must be able to get the environment of the caller.
Dialogue: 0,0:30:14.29,0:30:17.24,EN,,0,0,0,,The caller of a procedure is right here.
Dialogue: 0,0:30:17.26,0:30:19.45,EN,,0,0,0,,If the procedure is a application--
Dialogue: 0,0:30:19.56,0:30:21.63,EN,,0,0,0,,If the expression we're evaluating is a combination,
Dialogue: 0,0:30:21.79,0:30:23.71,EN,,0,0,0,,then we're going to call a combination
Dialogue: 0,0:30:23.93,0:30:25.50,EN,,0,0,0,,then we're going to call a procedure
Dialogue: 0,0:30:25.66,0:30:27.37,EN,,0,0,0,,which is the value of the operator.
Dialogue: 0,0:30:29.84,0:30:31.44,EN,,0,0,0,,The environment of the caller
Dialogue: 0,0:30:31.98,0:30:34.51,EN,,0,0,0,,is the environment we have right here, available now.
Dialogue: 0,0:30:35.89,0:30:40.72,EN,,0,0,0,,So all I have to do is pass that environment to the applicator, to apply.
Dialogue: 0,0:30:41.49,0:30:42.75,EN,,0,0,0,,And if we look at that here,
Dialogue: 0,0:30:43.58,0:30:44.97,EN,,0,0,0,,the only change we have to make
Dialogue: 0,0:30:45.71,0:30:48.41,EN,,0,0,0,,is that fellow takes that environment
Dialogue: 0,0:30:48.78,0:30:55.68,EN,,0,0,0,,uses that environment for the purpose of extending that environment
Dialogue: 0,0:30:56.67,0:30:59.02,EN,,0,0,0,,when abiding the formal parameters
Dialogue: 0,0:30:59.02,0:31:01.37,EN,,0,0,0,,of the procedure to the arguments that were passed,
Dialogue: 0,0:31:03.08,0:31:05.98,EN,,0,0,0,,not an environment that was captured in the procedure.
Dialogue: 0,0:31:06.81,0:31:09.45,EN,,0,0,0,,The reason why the first Lisps were implemented this way,
Dialogue: 0,0:31:09.66,0:31:11.96,EN,,0,0,0,,is the sort of the obvious, accidental implementation.
Dialogue: 0,0:31:14.13,0:31:16.68,EN,,0,0,0,,And, of course, as usual, people got used to it and liked it.
Dialogue: 0,0:31:17.25,0:31:18.27,EN,,0,0,0,,And there were some people said,
Dialogue: 0,0:31:18.40,0:31:19.50,EN,,0,0,0,,this is the way to do it.
Dialogue: 0,0:31:21.59,0:31:24.09,EN,,0,0,0,,Unfortunately that causes some serious problems.
Dialogue: 0,0:31:25.40,0:31:27.24,EN,,0,0,0,,The most important, serious problem
Dialogue: 0,0:31:27.53,0:31:29.84,EN,,0,0,0,,in using dynamic binding
Dialogue: 0,0:31:31.00,0:31:33.56,EN,,0,0,0,,is there's a modularity crisis that's involved it.
Dialogue: 0,0:31:35.46,0:31:37.66,EN,,0,0,0,,If two people are working together on some big system,
Dialogue: 0,0:31:38.57,0:31:40.01,EN,,0,0,0,,then an important thing to one
Dialogue: 0,0:31:40.35,0:31:42.19,EN,,0,0,0,,is that the names used by each one
Dialogue: 0,0:31:42.99,0:31:44.58,EN,,0,0,0,,don't interfere with the names of the other.
Dialogue: 0,0:31:47.93,0:31:50.78,EN,,0,0,0,,It's important that when I invent some segment of code
Dialogue: 0,0:31:51.07,0:31:53.13,EN,,0,0,0,,that no one can make my code stop working
Dialogue: 0,0:31:53.88,0:31:56.57,EN,,0,0,0,,by using my names that I use internal to my code,
Dialogue: 0,0:31:56.75,0:31:57.71,EN,,0,0,0,,internal to his code.
Dialogue: 0,0:31:59.85,0:32:00.46,EN,,0,0,0,,However,
Dialogue: 0,0:32:01.04,0:32:05.18,EN,,0,0,0,,dynamic binding violates that particular modularity constraint in a clear way.
Dialogue: 0,0:32:06.67,0:32:08.08,EN,,0,0,0,,Consider, for example,
Dialogue: 0,0:32:09.18,0:32:10.35,EN,,0,0,0,,what happens over here.
Dialogue: 0,0:32:12.54,0:32:13.79,EN,,0,0,0,,Suppose it was the case
Dialogue: 0,0:32:15.47,0:32:19.81,EN,,0,0,0,,that I decided to change the word next.
Dialogue: 0,0:32:19.81,0:32:24.41,EN,,0,0,0,,Supposing somebody is writing, somebody is writing sum,
Dialogue: 0,0:32:25.10,0:32:26.68,EN,,0,0,0,,and somebody else is going to use sum.
Dialogue: 0,0:32:28.97,0:32:30.32,EN,,0,0,0,,The writer of sum
Dialogue: 0,0:32:30.49,0:32:32.30,EN,,0,0,0,,has a choice of what names he may use.
Dialogue: 0,0:32:33.66,0:32:34.84,EN,,0,0,0,,Let's say, I'm that writer.
Dialogue: 0,0:32:36.83,0:32:39.30,EN,,0,0,0,,Well, by gosh, just happens I didn't want to call this next.
Dialogue: 0,0:32:39.30,0:32:40.09,EN,,0,0,0,,I called it n.
Dialogue: 0,0:32:41.74,0:32:43.10,EN,,0,0,0,,So all places where you see next,
Dialogue: 0,0:32:44.28,0:32:45.26,EN,,0,0,0,,I called it n.
Dialogue: 0,0:32:48.14,0:32:48.48,EN,,0,0,0,,Whoops.
Dialogue: 0,0:32:49.94,0:32:52.22,EN,,0,0,0,,I changed nothing about the specifications of this program,
Dialogue: 0,0:32:53.32,0:32:54.86,EN,,0,0,0,,but this program stops working.
Dialogue: 0,0:32:56.11,0:32:57.96,EN,,0,0,0,,Not only that, unfortunately, this one does too.
Dialogue: 0,0:32:59.50,0:33:01.40,EN,,0,0,0,,Why do these programs stop working?
Dialogue: 0,0:33:02.26,0:33:03.24,EN,,0,0,0,,Well, it's sort of clear.
Dialogue: 0,0:33:04.48,0:33:09.29,EN,,0,0,0,,Instead of chasing out the value of the n
Dialogue: 0,0:33:09.31,0:33:13.72,EN,,0,0,0,,that occurs in nth power over here or over here,
Dialogue: 0,0:33:14.97,0:33:17.16,EN,,0,0,0,,through the environment of definition,
Dialogue: 0,0:33:17.20,0:33:19.58,EN,,0,0,0,,where this one is always linked to this one,
Dialogue: 0,0:33:19.87,0:33:21.48,EN,,0,0,0,,if it was through the environment of definition,
Dialogue: 0,0:33:21.55,0:33:23.63,EN,,0,0,0,,because here is the definition.
Dialogue: 0,0:33:24.37,0:33:26.25,EN,,0,0,0,,This lambda expression was executed
Dialogue: 0,0:33:26.59,0:33:28.59,EN,,0,0,0,,in the environment where that n was defined.
Dialogue: 0,0:33:30.70,0:33:31.84,EN,,0,0,0,,If instead of doing that,
Dialogue: 0,0:33:32.01,0:33:33.68,EN,,0,0,0,,I have to chase through the call chain,
Dialogue: 0,0:33:34.78,0:33:36.27,EN,,0,0,0,,then look what horrible thing happens.
Dialogue: 0,0:33:37.32,0:33:41.18,EN,,0,0,0,,Well, this was called from inside sum as term, term a.
Dialogue: 0,0:33:41.76,0:33:42.38,EN,,0,0,0,,term a.
Dialogue: 0,0:33:44.78,0:33:46.19,EN,,0,0,0,,I'm looking for a value of n.
Dialogue: 0,0:33:47.35,0:33:48.40,EN,,0,0,0,,Instead of getting this one,
Dialogue: 0,0:33:48.84,0:33:49.76,EN,,0,0,0,,I get that one.
Dialogue: 0,0:33:50.70,0:33:52.54,EN,,0,0,0,,So by changing the insides of this program,
Dialogue: 0,0:33:52.86,0:33:54.09,EN,,0,0,0,,this program stops working.
Dialogue: 0,0:33:56.77,0:34:00.08,EN,,0,0,0,,So I no longer have a quantifier, as I described before.
Dialogue: 0,0:34:01.12,0:34:05.13,EN,,0,0,0,,Which is a symbol -- The lambda symbol is supposed to be a quantifier.
Dialogue: 0,0:34:05.43,0:34:06.70,EN,,0,0,0,,A thing which has the property
Dialogue: 0,0:34:06.89,0:34:11.42,EN,,0,0,0,,that the names that are bound by it are unimportant,
Dialogue: 0,0:34:12.65,0:34:15.71,EN,,0,0,0,,that I can uniformly substitute any names for these
Dialogue: 0,0:34:16.92,0:34:19.98,EN,,0,0,0,,throughout this thing, so long as they don't occur in here, the new names,
Dialogue: 0,0:34:20.94,0:34:23.16,EN,,0,0,0,,and the meaning of this expression should remain unchanged.
Dialogue: 0,0:34:24.04,0:34:25.50,EN,,0,0,0,,I've just changed the meaning of the expression
Dialogue: 0,0:34:25.53,0:34:27.20,EN,,0,0,0,,by changing the one of the names.
Dialogue: 0,0:34:28.69,0:34:30.89,EN,,0,0,0,,So lambda is no longer a well defined idea.
Dialogue: 0,0:34:32.17,0:34:33.37,EN,,0,0,0,,It's a very serious problem.
Dialogue: 0,0:34:34.55,0:34:35.55,EN,,0,0,0,,So for that reason,
Dialogue: 0,0:34:36.64,0:34:42.51,EN,,0,0,0,,I and my buddies have given up this particular kind of abstraction,
Dialogue: 0,0:34:43.13,0:34:44.36,EN,,0,0,0,,which I would like to have,
Dialogue: 0,0:34:45.61,0:34:47.50,EN,,0,0,0,,in favor of a modularity principle.
Dialogue: 0,0:34:48.09,0:34:50.20,EN,,0,0,0,,But this is the kind of experiment you can do
Dialogue: 0,0:34:51.96,0:34:53.68,EN,,0,0,0,,if you want to play with these interpreters.
Dialogue: 0,0:34:54.83,0:34:56.91,EN,,0,0,0,,You can try them out this way, that way, and the other way.
Dialogue: 0,0:34:58.11,0:35:00.25,EN,,0,0,0,,You see what makes a nicer language.
Dialogue: 0,0:35:02.68,0:35:04.49,EN,,0,0,0,,So that's a very important thing to be able to do.
Dialogue: 0,0:35:04.99,0:35:06.68,EN,,0,0,0,,Now, I would like to give you a feeling
Dialogue: 0,0:35:06.72,0:35:08.49,EN,,0,0,0,,for I think the right thing to do is here.
Dialogue: 0,0:35:09.32,0:35:12.91,EN,,0,0,0,,How are you going to, how are you going to I get this kind of
Dialogue: 0,0:35:13.04,0:35:15.34,EN,,0,0,0,,of power in a lexical system?
Dialogue: 0,0:35:16.28,0:35:17.39,EN,,0,0,0,,And the answer is, of course,
Dialogue: 0,0:35:17.55,0:35:20.03,EN,,0,0,0,,what I really want is a something that makes up for me
Dialogue: 0,0:35:20.68,0:35:22.60,EN,,0,0,0,,an exponentiator for a particular n.
Dialogue: 0,0:35:23.69,0:35:24.28,EN,,0,0,0,,Given an n,
Dialogue: 0,0:35:24.32,0:35:25.66,EN,,0,0,0,,it will make me an exponentiator.
Dialogue: 0,0:35:26.28,0:35:27.40,EN,,0,0,0,,Oh, but that's easy too.
Dialogue: 0,0:35:28.17,0:35:30.57,EN,,0,0,0,,In other words, I can write my program this way.
Dialogue: 0,0:35:35.84,0:35:37.84,EN,,0,0,0,,I'm going to define a thing called PGEN,
Dialogue: 0,0:35:40.25,0:35:42.54,EN,,0,0,0,,which is a procedure of n
Dialogue: 0,0:35:43.16,0:35:45.95,EN,,0,0,0,,which produces for me an exponentiator.
Dialogue: 0,0:35:50.24,0:35:51.23,EN,,0,0,0,,--x to the n.
Dialogue: 0,0:35:56.80,0:35:57.98,EN,,0,0,0,,Given that I have that,
Dialogue: 0,0:35:58.59,0:36:00.88,EN,,0,0,0,,then I can capture the abstraction I wanted
Dialogue: 0,0:36:01.42,0:36:03.93,EN,,0,0,0,,even better, because now it's encapsulated in a way
Dialogue: 0,0:36:04.09,0:36:06.60,EN,,0,0,0,,where I can't be destroyed by a change of names.
Dialogue: 0,0:36:07.89,0:36:12.35,EN,,0,0,0,,I can define some powers
Dialogue: 0,0:36:17.28,0:36:20.70,EN,,0,0,0,,I can define some powers to be a procedure again of a, b, and n
Dialogue: 0,0:36:21.61,0:36:26.83,EN,,0,0,0,,which is the sum of the term function
Dialogue: 0,0:36:26.88,0:36:32.32,EN,,0,0,0,,generated by using this generator, PGEN, n,
Dialogue: 0,0:36:34.40,0:36:38.01,EN,,0,0,0,,with a, incrementer, and b.
Dialogue: 0,0:36:42.49,0:36:47.95,EN,,0,0,0,,And I can define the product of powers
Dialogue: 0,0:36:54.11,0:36:58.84,EN,,0,0,0,,to be a procedure of a, b, and n
Dialogue: 0,0:36:59.80,0:37:09.96,EN,,0,0,0,,which is the product PGEN, n, with a, increment, and b.
Dialogue: 0,0:37:11.28,0:37:13.28,EN,,0,0,0,,Now, of course, this is a very simple example
Dialogue: 0,0:37:13.60,0:37:16.35,EN,,0,0,0,,where this object that I'm trying to abstract over is small.
Dialogue: 0,0:37:17.28,0:37:18.83,EN,,0,0,0,,But it could be a 100 lines of code.
Dialogue: 0,0:37:20.10,0:37:23.67,EN,,0,0,0,,And so, the purpose of this is, of course, to make it simple.
Dialogue: 0,0:37:23.67,0:37:24.57,EN,,0,0,0,,I'd give a name to it,
Dialogue: 0,0:37:24.73,0:37:26.94,EN,,0,0,0,,it's just that here it's a parameterized name.
Dialogue: 0,0:37:28.20,0:37:30.27,EN,,0,0,0,,It's a name that depends upon, explicitly,
Dialogue: 0,0:37:30.49,0:37:33.63,EN,,0,0,0,,the lexically apparent value of n.
Dialogue: 0,0:37:37.13,0:37:38.59,EN,,0,0,0,,So you can think of this as a long name.
Dialogue: 0,0:37:40.21,0:37:41.58,EN,,0,0,0,,And here, I've solved my problem
Dialogue: 0,0:37:41.76,0:37:45.82,EN,,0,0,0,,by naming my... by naming the term generation
Dialogue: 0,0:37:46.12,0:37:49.22,EN,,0,0,0,,procedures within an n in them.
Dialogue: 0,0:37:55.08,0:37:55.87,EN,,0,0,0,,Are there any questions?
Dialogue: 0,0:37:57.00,0:37:58.38,EN,,0,0,0,,Oh, yes, David.
Dialogue: 0,0:37:58.57,0:38:02.27,EN,,0,0,0,,AUDIENCE: Is the only solution to um...
Dialogue: 0,0:38:03.07,0:38:06.46,EN,,0,0,0,,the problem you raise to create another procedure?
Dialogue: 0,0:38:06.47,0:38:08.92,EN,,0,0,0,,In other words, can this only work in languages that are
Dialogue: 0,0:38:08.99,0:38:11.56,EN,,0,0,0,,capable of defining objects as procedures?
Dialogue: 0,0:38:12.41,0:38:13.76,EN,,0,0,0,,PROFESSOR: Oh, I see.
Dialogue: 0,0:38:15.90,0:38:19.74,EN,,0,0,0,,My solution to making this abstraction,
Dialogue: 0,0:38:20.14,0:38:22.86,EN,,0,0,0,,when I didn't want include the procedure inside the body,
Dialogue: 0,0:38:23.26,0:38:26.81,EN,,0,0,0,,depends upon my ability to return a procedure or export one.
Dialogue: 0,0:38:27.04,0:38:27.24,EN,,0,0,0,,AUDIENCE: And that's right.
Dialogue: 0,0:38:28.19,0:38:28.88,EN,,0,0,0,,PROFESSOR: And that's right.
Dialogue: 0,0:38:29.53,0:38:31.52,EN,,0,0,0,,If I don't have that,
Dialogue: 0,0:38:32.24,0:38:35.13,EN,,0,0,0,,then I just don't have this ability to make an abstraction in a way
Dialogue: 0,0:38:35.53,0:38:41.77,EN,,0,0,0,,where I don't have possibilities of symbol conflicts that were unanticipated.
Dialogue: 0,0:38:43.00,0:38:43.48,EN,,0,0,0,,That's right.
Dialogue: 0,0:38:44.14,0:38:46.51,EN,,0,0,0,,So one of the, the essential -- I consider, I consider
Dialogue: 0,0:38:46.54,0:38:48.91,EN,,0,0,0,,being able to return the procedural value and, therefore,
Dialogue: 0,0:38:49.20,0:38:58.28,EN,,0,0,0,,and therefore, to sort of have first class procedures, in general,
Dialogue: 0,0:38:59.13,0:39:02.46,EN,,0,0,0,,as being essential to doing very good modular programming.
Dialogue: 0,0:39:03.70,0:39:06.43,EN,,0,0,0,,Now, indeed there are many other ways to skin this cat.
Dialogue: 0,0:39:07.44,0:39:09.16,EN,,0,0,0,,What you can do is take for each of the
Dialogue: 0,0:39:09.18,0:39:11.84,EN,,0,0,0,,for each of the bad things that you have to worry about,
Dialogue: 0,0:39:12.27,0:39:15.20,EN,,0,0,0,,you can make a special feature that covers that thing.
Dialogue: 0,0:39:15.84,0:39:17.12,EN,,0,0,0,,You can make a package system.
Dialogue: 0,0:39:17.74,0:39:21.16,EN,,0,0,0,,You can make a module system as in Ada, et cetera. OK?
Dialogue: 0,0:39:22.24,0:39:24.88,EN,,0,0,0,,And all of those work, or they cover little regions of it.
Dialogue: 0,0:39:26.44,0:39:28.38,EN,,0,0,0,,The thing is that returning procedures as values
Dialogue: 0,0:39:28.41,0:39:29.74,EN,,0,0,0,,cover all of those problems.
Dialogue: 0,0:39:32.68,0:39:34.60,EN,,0,0,0,,And so it's the simplest mechanism
Dialogue: 0,0:39:35.58,0:39:37.79,EN,,0,0,0,,that gives you the best modularity,
Dialogue: 0,0:39:39.21,0:39:41.31,EN,,0,0,0,,gives you all of the known modularity mechanisms.
Dialogue: 0,0:39:45.59,0:39:48.24,EN,,0,0,0,,Well, I suppose it's time for the next break, thank you.
Dialogue: 0,0:39:48.24,0:40:01.08,EN,,0,0,0,,[JESU, JOY OF MAN'S DESIRING]
Dialogue: 0,0:40:01.28,0:40:04.75,EN,,0,0,0,,
Dialogue: 0,0:40:25.69,0:40:29.42,EN,,0,0,0,,
Dialogue: 0,0:40:30.01,0:40:33.28,EN,,0,0,0,,
Dialogue: 0,0:40:34.17,0:40:37.61,EN,,0,0,0,,
Dialogue: 0,0:40:42.32,0:40:44.28,EN,,0,0,0,,PROFESSOR: Well, yesterday when you learned about streams,
Dialogue: 0,0:40:46.01,0:40:51.16,EN,,0,0,0,,Hal worried to you about the order of evaluation
Dialogue: 0,0:40:51.95,0:40:53.87,EN,,0,0,0,,and delayed arguments to procedures.
Dialogue: 0,0:40:55.61,0:40:58.30,EN,,0,0,0,,The way we played with streams yesterday,
Dialogue: 0,0:41:00.25,0:41:04.22,EN,,0,0,0,,it was the responsibility of the caller and the callee
Dialogue: 0,0:41:05.77,0:41:08.84,EN,,0,0,0,,both agree that an argument was delayed,
Dialogue: 0,0:41:09.42,0:41:13.44,EN,,0,0,0,,and the callee must force the argument if it needs the answer.
Dialogue: 0,0:41:15.13,0:41:17.87,EN,,0,0,0,,So there had to be a lot of hand shaking between
Dialogue: 0,0:41:18.17,0:41:24.32,EN,,0,0,0,,the designer of a procedure and user of it over delayedness.
Dialogue: 0,0:41:26.36,0:41:28.72,EN,,0,0,0,,That turns out, of course, to be a fairly bad thing,
Dialogue: 0,0:41:29.48,0:41:30.96,EN,,0,0,0,,it works all right with streams.
Dialogue: 0,0:41:31.74,0:41:32.86,EN,,0,0,0,,But as a general thing,
Dialogue: 0,0:41:32.92,0:41:36.32,EN,,0,0,0,,what you want is an idea to have a locus,
Dialogue: 0,0:41:36.46,0:41:38.49,EN,,0,0,0,,a decision, a design decision in general,
Dialogue: 0,0:41:38.89,0:41:41.28,EN,,0,0,0,,to have a place where it's made, explicitly,
Dialogue: 0,0:41:41.63,0:41:43.93,EN,,0,0,0,,and notated in a clear way.
Dialogue: 0,0:41:45.88,0:41:49.28,EN,,0,0,0,,And so it's not a very good idea to have to have an agreement,
Dialogue: 0,0:41:50.46,0:41:54.89,EN,,0,0,0,,between the person who writes a procedure and the person who calls it,
Dialogue: 0,0:41:55.08,0:41:57.98,EN,,0,0,0,,about such details as, maybe, the arguments of evaluation,
Dialogue: 0,0:41:58.43,0:41:59.50,EN,,0,0,0,,the order of evaluation.
Dialogue: 0,0:41:59.50,0:42:00.75,EN,,0,0,0,,Although, that's not so bad.
Dialogue: 0,0:42:01.02,0:42:03.95,EN,,0,0,0,,I mean, we have other such agreements like, the input's a number.
Dialogue: 0,0:42:05.20,0:42:06.08,EN,,0,0,0,,But it would be nice if
Dialogue: 0,0:42:06.35,0:42:09.20,EN,,0,0,0,,one of these guys could take responsibility, completely.
Dialogue: 0,0:42:11.02,0:42:13.31,EN,,0,0,0,,Now this is not a new idea.
Dialogue: 0,0:42:15.51,0:42:21.16,EN,,0,0,0,,ALGOL 60 had two different ways of calling a procedure.
Dialogue: 0,0:42:22.02,0:42:24.28,EN,,0,0,0,,The arguments could be passed by name or by value.
Dialogue: 0,0:42:25.59,0:42:27.48,EN,,0,0,0,,And what that meant was that
Dialogue: 0,0:42:27.63,0:42:29.72,EN,,0,0,0,,a name argument was delayed.
Dialogue: 0,0:42:31.11,0:42:32.84,EN,,0,0,0,,That when you passed an argument by name,
Dialogue: 0,0:42:33.64,0:42:36.52,EN,,0,0,0,,that its value would only be obtained
Dialogue: 0,0:42:36.96,0:42:39.55,EN,,0,0,0,,if you accessed that argument.
Dialogue: 0,0:42:42.29,0:42:44.20,EN,,0,0,0,,So what I'd like to do now is show you,
Dialogue: 0,0:42:44.43,0:42:46.96,EN,,0,0,0,,first of all, a little bit about, again,
Dialogue: 0,0:42:46.99,0:42:48.65,EN,,0,0,0,,we're going to make a modification to a language.
Dialogue: 0,0:42:50.32,0:42:51.79,EN,,0,0,0,,In this case, we're going to add a feature.
Dialogue: 0,0:42:53.37,0:42:55.05,EN,,0,0,0,,We're going to add the feature of,
Dialogue: 0,0:42:55.36,0:42:58.73,EN,,0,0,0,,by name parameters, if you will, or delayed parameters.
Dialogue: 0,0:43:00.43,0:43:04.41,EN,,0,0,0,,Because, in fact, the default in our Lisp system
Dialogue: 0,0:43:04.76,0:43:06.60,EN,,0,0,0,,is by the value of a pointer.
Dialogue: 0,0:43:08.22,0:43:09.15,EN,,0,0,0,,A pointer is copied,
Dialogue: 0,0:43:09.15,0:43:10.91,EN,,0,0,0,,but the data structure it points at is not.
Dialogue: 0,0:43:13.41,0:43:14.84,EN,,0,0,0,,But I'd like to, in fact, show you
Dialogue: 0,0:43:15.04,0:43:18.38,EN,,0,0,0,,is how you add name arguments as well.
Dialogue: 0,0:43:19.99,0:43:22.12,EN,,0,0,0,,Now again, why would we need such a thing?
Dialogue: 0,0:43:23.10,0:43:24.72,EN,,0,0,0,,Well supposing we wanted to invent
Dialogue: 0,0:43:25.24,0:43:28.44,EN,,0,0,0,,certain kinds of what otherwise would be special forms,
Dialogue: 0,0:43:28.73,0:43:29.72,EN,,0,0,0,,reserve words?
Dialogue: 0,0:43:29.72,0:43:31.48,EN,,0,0,0,,But I'd rather not take up reserve words.
Dialogue: 0,0:43:32.18,0:43:34.76,EN,,0,0,0,,I want procedures that can do things like if.
Dialogue: 0,0:43:36.36,0:43:39.42,EN,,0,0,0,,If is special, or cond, or whatever it is.
Dialogue: 0,0:43:39.42,0:43:40.43,EN,,0,0,0,,It's the same thing.
Dialogue: 0,0:43:40.59,0:43:42.86,EN,,0,0,0,,It's special in that it determines whether or not
Dialogue: 0,0:43:42.92,0:43:45.02,EN,,0,0,0,,to evaluate the consequent or the alternative
Dialogue: 0,0:43:46.22,0:43:49.76,EN,,0,0,0,,based on the value of the predicate part of an expression.
Dialogue: 0,0:43:50.84,0:43:53.12,EN,,0,0,0,,So taking the value of one thing
Dialogue: 0,0:43:53.44,0:43:55.36,EN,,0,0,0,,determines whether or not to do something else.
Dialogue: 0,0:43:57.27,0:43:58.88,EN,,0,0,0,,Whereas all the procedures like plus,
Dialogue: 0,0:43:59.15,0:44:01.20,EN,,0,0,0,,evaluate... the ones that we can define right now,
Dialogue: 0,0:44:01.42,0:44:06.56,EN,,0,0,0,,evaluate all of their arguments before application.
Dialogue: 0,0:44:08.67,0:44:09.64,EN,,0,0,0,,So, for example,
Dialogue: 0,0:44:10.46,0:44:12.41,EN,,0,0,0,,supposing I wish to be able to define something like
Dialogue: 0,0:44:15.39,0:44:18.75,EN,,0,0,0,,the reverse of if in terms of if.
Dialogue: 0,0:44:19.85,0:44:20.70,EN,,0,0,0,,Call it unless.
Dialogue: 0,0:44:24.89,0:44:27.47,EN,,0,0,0,,We've a predicate, a consequent, and an alternative.
Dialogue: 0,0:44:28.67,0:44:30.44,EN,,0,0,0,,Now what I would like to sort of be able to do is
Dialogue: 0,0:44:30.46,0:44:32.08,EN,,0,0,0,,say-- oh, I'll do it in terms of cond.
Dialogue: 0,0:44:32.64,0:44:36.72,EN,,0,0,0,,Cond, if not the predicate,
Dialogue: 0,0:44:38.96,0:44:40.32,EN,,0,0,0,,then take the consequent,
Dialogue: 0,0:44:41.58,0:44:45.63,EN,,0,0,0,,otherwise, take the alternative.
Dialogue: 0,0:44:51.29,0:44:52.76,EN,,0,0,0,,Now, what I'd like this to mean,
Dialogue: 0,0:44:53.32,0:44:55.40,EN,,0,0,0,,is supposing I do something like this.
Dialogue: 0,0:44:56.92,0:45:04.12,EN,,0,0,0,,I'd like this unless say if equals one, 0,
Dialogue: 0,0:45:05.08,0:45:06.64,EN,,0,0,0,,then the answer is two,
Dialogue: 0,0:45:07.90,0:45:11.35,EN,,0,0,0,,otherwise, the quotient of one and 0.
Dialogue: 0,0:45:15.92,0:45:18.91,EN,,0,0,0,,What I'd like that to mean is the result of substituting
Dialogue: 0,0:45:20.00,0:45:23.26,EN,,0,0,0,,equal one, 0, and the quotient of one, 0
Dialogue: 0,0:45:23.66,0:45:24.76,EN,,0,0,0,,for p, c, and a.
Dialogue: 0,0:45:25.58,0:45:27.58,EN,,0,0,0,,I'd like that to mean, and this is funny,
Dialogue: 0,0:45:28.11,0:45:30.33,EN,,0,0,0,,I'd like it to transform into or mean
Dialogue: 0,0:45:30.75,0:45:38.44,EN,,0,0,0,,cond not equal one, 0,
Dialogue: 0,0:45:40.62,0:45:42.54,EN,,0,0,0,,then the result is two,
Dialogue: 0,0:45:44.28,0:45:45.10,EN,,0,0,0,,otherwise
Dialogue: 0,0:45:48.22,0:45:51.16,EN,,0,0,0,,I want it to be the quotient one and 0.
Dialogue: 0,0:45:54.48,0:45:56.48,EN,,0,0,0,,Now, you know that if I were to type this into Lisp,
Dialogue: 0,0:45:57.74,0:45:58.59,EN,,0,0,0,,I'd get a two.
Dialogue: 0,0:45:59.97,0:46:01.32,EN,,0,0,0,,There's no problem with that.
Dialogue: 0,0:46:02.91,0:46:04.64,EN,,0,0,0,,However, if I were to type this into Lisp,
Dialogue: 0,0:46:05.28,0:46:07.79,EN,,0,0,0,,because all the arguments are evaluated before I start,
Dialogue: 0,0:46:09.12,0:46:10.73,EN,,0,0,0,,then I'm going to get an error out of this.
Dialogue: 0,0:46:13.38,0:46:15.61,EN,,0,0,0,,So that if the substitutions work at all, of course,
Dialogue: 0,0:46:16.03,0:46:16.88,EN,,0,0,0,,I would get the right answer.
Dialogue: 0,0:46:16.88,0:46:20.16,EN,,0,0,0,,But here's a case where the substitutions don't work.
Dialogue: 0,0:46:22.17,0:46:23.86,EN,,0,0,0,,I don't get the wrong answer.
Dialogue: 0,0:46:23.86,0:46:24.67,EN,,0,0,0,,I get no answer.
Dialogue: 0,0:46:24.80,0:46:25.60,EN,,0,0,0,,I get an error.
Dialogue: 0,0:46:28.42,0:46:31.21,EN,,0,0,0,,Now, however, I'd like to be able to make my definition
Dialogue: 0,0:46:31.61,0:46:32.99,EN,,0,0,0,,so that this kind of thing works.
Dialogue: 0,0:46:34.48,0:46:36.51,EN,,0,0,0,,What I want to do is say something special
Dialogue: 0,0:46:36.70,0:46:38.76,EN,,0,0,0,,about c and a.
Dialogue: 0,0:46:39.93,0:46:43.15,EN,,0,0,0,,I want them to be delayed automatically.
Dialogue: 0,0:46:44.27,0:46:48.08,EN,,0,0,0,,I don't want them to be, I don't want them to be evaluated
Dialogue: 0,0:46:48.52,0:46:49.74,EN,,0,0,0,,at the time I call.
Dialogue: 0,0:46:51.52,0:46:52.72,EN,,0,0,0,,So I'm going to make a declaration,
Dialogue: 0,0:46:52.75,0:46:55.32,EN,,0,0,0,,and then I'm going to see how to implement such a declaration.
Dialogue: 0,0:46:55.60,0:46:57.63,EN,,0,0,0,,But again, I want you to say to yourself,
Dialogue: 0,0:46:57.79,0:47:00.25,EN,,0,0,0,,oh, this is an interesting kluge he's adding in here.
Dialogue: 0,0:47:00.76,0:47:02.16,EN,,0,0,0,,A kluge, you know.
Dialogue: 0,0:47:02.25,0:47:04.72,EN,,0,0,0,,The piles of kluges make a big complicated mess.
Dialogue: 0,0:47:05.75,0:47:09.79,EN,,0,0,0,,And is this going to foul up something else that might occur.
Dialogue: 0,0:47:10.12,0:47:12.70,EN,,0,0,0,,First of all, is it syntactically unambiguous?
Dialogue: 0,0:47:13.86,0:47:15.50,EN,,0,0,0,,Well, it will be syntactically unambiguous
Dialogue: 0,0:47:15.71,0:47:16.91,EN,,0,0,0,,with what we've seen so far.
Dialogue: 0,0:47:17.84,0:47:20.76,EN,,0,0,0,,But what I'm going to do may, in fact, cause trouble.
Dialogue: 0,0:47:21.67,0:47:24.67,EN,,0,0,0,,It may be that the thing I had will conflict with
Dialogue: 0,0:47:25.15,0:47:27.10,EN,,0,0,0,,type declarations I might want to add in the future
Dialogue: 0,0:47:28.19,0:47:31.08,EN,,0,0,0,,for giving some system, some compiler or something,
Dialogue: 0,0:47:31.21,0:47:33.66,EN,,0,0,0,,the ability to optimize given the types are known.
Dialogue: 0,0:47:34.75,0:47:36.97,EN,,0,0,0,,Or it might conflict with other types of declarations
Dialogue: 0,0:47:37.00,0:47:39.71,EN,,0,0,0,,that I might want to make about the formal parameters.
Dialogue: 0,0:47:40.57,0:47:42.56,EN,,0,0,0,,So I'm not making a general mechanism here
Dialogue: 0,0:47:43.77,0:47:45.24,EN,,0,0,0,,where I can add declarations.
Dialogue: 0,0:47:45.28,0:47:46.54,EN,,0,0,0,,And I would like to be able to do that.
Dialogue: 0,0:47:46.89,0:47:48.81,EN,,0,0,0,,But I don't want to talk about that right now.
Dialogue: 0,0:47:51.01,0:47:53.88,EN,,0,0,0,,So here I'm going to do, I'm going to build a kluge.
Dialogue: 0,0:47:57.56,0:48:08.38,EN,,0,0,0,,So we're going to define unless of a predicate--
Dialogue: 0,0:48:08.81,0:48:10.27,EN,,0,0,0,,and I'm going to call these by name--
Dialogue: 0,0:48:12.78,0:48:15.28,EN,,0,0,0,,the consequent, and name the alternative.
Dialogue: 0,0:48:19.85,0:48:25.28,EN,,0,0,0,,Huh, huh-- I got caught in the corner.
Dialogue: 0,0:48:31.76,0:48:35.61,EN,,0,0,0,,If not p then the result is c,
Dialogue: 0,0:48:36.80,0:48:41.16,EN,,0,0,0,,else-- that's what I'd like.
Dialogue: 0,0:48:44.67,0:48:46.88,EN,,0,0,0,,Where I can explicitly declare
Dialogue: 0,0:48:47.55,0:48:51.65,EN,,0,0,0,,certain of the parameters to be delayed, to be computed later.
Dialogue: 0,0:48:55.60,0:48:58.48,EN,,0,0,0,,Now, this is actually a very complicated modification to an interpreter
Dialogue: 0,0:48:58.70,0:48:59.77,EN,,0,0,0,,rather than a simple one.
Dialogue: 0,0:49:00.45,0:49:03.10,EN,,0,0,0,,The ones you saw before, dynamic binding
Dialogue: 0,0:49:03.40,0:49:06.89,EN,,0,0,0,,or adding indefinite argument procedures,
Dialogue: 0,0:49:07.50,0:49:08.52,EN,,0,0,0,,is relatively simple.
Dialogue: 0,0:49:09.28,0:49:11.28,EN,,0,0,0,,But this one changes a basic strategy.
Dialogue: 0,0:49:12.32,0:49:13.39,EN,,0,0,0,,The problem here
Dialogue: 0,0:49:13.96,0:49:17.63,EN,,0,0,0,,is that our interpreter, as written
Dialogue: 0,0:49:17.96,0:49:23.40,EN,,0,0,0,,evaluates a combination by evaluating the procedure,
Dialogue: 0,0:49:24.24,0:49:25.92,EN,,0,0,0,,the operator producing the procedure,
Dialogue: 0,0:49:26.20,0:49:30.35,EN,,0,0,0,,and evaluating the operands producing the arguments,
Dialogue: 0,0:49:30.76,0:49:35.26,EN,,0,0,0,,and then doing apply of the procedure to the arguments.
Dialogue: 0,0:49:36.38,0:49:37.07,EN,,0,0,0,,However, here,
Dialogue: 0,0:49:37.36,0:49:41.48,EN,,0,0,0,,I don't want to evaluate the operands to produce the arguments
Dialogue: 0,0:49:41.74,0:49:43.66,EN,,0,0,0,,until after I examined the procedure
Dialogue: 0,0:49:44.62,0:49:46.86,EN,,0,0,0,,to see what the procedure's declarations look like.
Dialogue: 0,0:49:49.59,0:49:50.59,EN,,0,0,0,,So let's look at that.
Dialogue: 0,0:49:52.68,0:49:56.54,EN,,0,0,0,,Here we have a changed evaluator.
Dialogue: 0,0:49:57.48,0:50:01.15,EN,,0,0,0,,I'm starting with the simple lexical evaluator,
Dialogue: 0,0:50:01.72,0:50:02.65,EN,,0,0,0,,not dynamic
Dialogue: 0,0:50:04.14,0:50:08.20,EN,,0,0,0,,but we're going to have to do something sort of similar in some ways.
Dialogue: 0,0:50:09.75,0:50:11.45,EN,,0,0,0,,Because of the fact that,
Dialogue: 0,0:50:11.90,0:50:13.34,EN,,0,0,0,,if I delay a procedure--
Dialogue: 0,0:50:13.66,0:50:15.15,EN,,0,0,0,,I'm sorry-- delay an argument to a procedure,
Dialogue: 0,0:50:15.40,0:50:17.52,EN,,0,0,0,,I'm going to have to attach and environment to it.
Dialogue: 0,0:50:19.36,0:50:21.55,EN,,0,0,0,,Remember how Hal implemented delay.
Dialogue: 0,0:50:23.38,0:50:25.44,EN,,0,0,0,,Hal implemented delay as being
Dialogue: 0,0:50:25.50,0:50:27.47,EN,,0,0,0,,a procedure of no arguments
Dialogue: 0,0:50:28.56,0:50:30.52,EN,,0,0,0,,which does some expression.
Dialogue: 0,0:50:31.18,0:50:36.94,EN,,0,0,0,,That's what delay of the expression is. --of that expression.
Dialogue: 0,0:50:39.29,0:50:40.99,EN,,0,0,0,,This turned into something like this.
Dialogue: 0,0:50:44.52,0:50:46.92,EN,,0,0,0,,Now, however, if I evaluate a lambda expression,
Dialogue: 0,0:50:47.42,0:50:49.20,EN,,0,0,0,,I have to capture the environment.
Dialogue: 0,0:50:51.41,0:50:53.45,EN,,0,0,0,,The reason why is because there are
Dialogue: 0,0:50:54.60,0:50:56.32,EN,,0,0,0,,there are variables in there
Dialogue: 0,0:50:57.02,0:51:00.83,EN,,0,0,0,,who's meaning I wish to derive from the context where this was written.
Dialogue: 0,0:51:04.01,0:51:05.76,EN,,0,0,0,,So that's why a lambda does the job.
Dialogue: 0,0:51:06.62,0:51:07.50,EN,,0,0,0,,It's the right thing.
Dialogue: 0,0:51:08.07,0:51:15.12,EN,,0,0,0,,And such that the forcing of a delayed expression
Dialogue: 0,0:51:16.52,0:51:20.08,EN,,0,0,0,,was same thing as calling that with no arguments.
Dialogue: 0,0:51:21.09,0:51:22.28,EN,,0,0,0,,It's just the opposite of this.
Dialogue: 0,0:51:24.10,0:51:26.94,EN,,0,0,0,,Producing an environment of the call
Dialogue: 0,0:51:27.36,0:51:29.90,EN,,0,0,0,,which is, in fact, the environment where this was defined
Dialogue: 0,0:51:30.81,0:51:32.36,EN,,0,0,0,,with an extra frame in it that's empty.
Dialogue: 0,0:51:33.23,0:51:34.41,EN,,0,0,0,,I don't care about that.
Dialogue: 0,0:51:36.24,0:51:39.40,EN,,0,0,0,,Well, if we go back to this slide,
Dialogue: 0,0:51:40.99,0:51:43.72,EN,,0,0,0,,since it's the case, if we look at this for a second,
Dialogue: 0,0:51:44.14,0:51:46.12,EN,,0,0,0,,everything is the same as it was before
Dialogue: 0,0:51:46.35,0:51:50.65,EN,,0,0,0,,except the case of applications or combinations.
Dialogue: 0,0:51:51.98,0:51:53.71,EN,,0,0,0,,And combinations are going to do two things.
Dialogue: 0,0:51:54.68,0:51:57.79,EN,,0,0,0,,One, is I have to evaluate the procedure--
Dialogue: 0,0:51:57.92,0:51:59.88,EN,,0,0,0,,I have to get the procedure-- by evaluating the operator.
Dialogue: 0,0:52:00.70,0:52:01.69,EN,,0,0,0,,That's what you see right here.
Dialogue: 0,0:52:02.38,0:52:04.35,EN,,0,0,0,,I have to make sure that that's current,
Dialogue: 0,0:52:04.46,0:52:05.76,EN,,0,0,0,,that is not a delayed object,
Dialogue: 0,0:52:06.36,0:52:09.85,EN,,0,0,0,,and evaluate that to the point where became it's forced now.
Dialogue: 0,0:52:10.73,0:52:12.08,EN,,0,0,0,,And then I have to somehow
Dialogue: 0,0:52:12.24,0:52:17.32,EN,,0,0,0,,apply that to the, to the operands.
Dialogue: 0,0:52:18.03,0:52:19.61,EN,,0,0,0,,But I have to keep the environment,
Dialogue: 0,0:52:19.63,0:52:20.92,EN,,0,0,0,,pass that environmental along.
Dialogue: 0,0:52:21.53,0:52:23.71,EN,,0,0,0,,So some of those operands I may have to delay.
Dialogue: 0,0:52:23.71,0:52:27.53,EN,,0,0,0,,I may have to attach that environment to those operands.
Dialogue: 0,0:52:29.66,0:52:31.52,EN,,0,0,0,,This is a rather complicated thing happening here.
Dialogue: 0,0:52:32.99,0:52:34.24,EN,,0,0,0,,Looking at that in apply.
Dialogue: 0,0:52:36.40,0:52:38.72,EN,,0,0,0,,Apply, well it has a primitive procedure
Dialogue: 0,0:52:39.36,0:52:40.60,EN,,0,0,0,,thing just like before.
Dialogue: 0,0:52:42.61,0:52:44.68,EN,,0,0,0,,But the compound one is a little more interesting.
Dialogue: 0,0:52:47.25,0:52:49.52,EN,,0,0,0,,I have to evaluate the body, just as before,
Dialogue: 0,0:52:50.48,0:52:51.98,EN,,0,0,0,,in an environment which is
Dialogue: 0,0:52:52.28,0:52:54.97,EN,,0,0,0,,which is the result of binding some
Dialogue: 0,0:52:55.61,0:53:00.29,EN,,0,0,0,,formal parameters to arguments in the environment.
Dialogue: 0,0:53:00.29,0:53:01.07,EN,,0,0,0,,That's true.
Dialogue: 0,0:53:01.53,0:53:03.82,EN,,0,0,0,,The environment is the one that comes from the procedure now.
Dialogue: 0,0:53:03.82,0:53:06.65,EN,,0,0,0,,It's a lexical language, statically bound.
Dialogue: 0,0:53:08.04,0:53:11.82,EN,,0,0,0,,However, one thing I have to do is strip off the declarations
Dialogue: 0,0:53:11.84,0:53:12.84,EN,,0,0,0,,to get the names of the variables.
Dialogue: 0,0:53:12.84,0:53:15.20,EN,,0,0,0,,That's what this guy does, vnames.
Dialogue: 0,0:53:15.45,0:53:16.67,EN,,0,0,0,,And the other thing I have to do
Dialogue: 0,0:53:16.97,0:53:18.86,EN,,0,0,0,,is process these declarations,
Dialogue: 0,0:53:19.13,0:53:21.52,EN,,0,0,0,,deciding which of these operands--
Dialogue: 0,0:53:21.76,0:53:23.92,EN,,0,0,0,,that's the operands now, as opposed to the arguments--
Dialogue: 0,0:53:24.09,0:53:25.87,EN,,0,0,0,,which of these operands to evaluate,
Dialogue: 0,0:53:26.62,0:53:30.20,EN,,0,0,0,,and which of them are to be
Dialogue: 0,0:53:30.99,0:53:33.77,EN,,0,0,0,,encapsulated in delays of some sort.
Dialogue: 0,0:53:37.28,0:53:40.08,EN,,0,0,0,,The other thing you see here is that we got a primitive,
Dialogue: 0,0:53:40.60,0:53:42.38,EN,,0,0,0,,a primitive like plus,
Dialogue: 0,0:53:42.68,0:53:45.58,EN,,0,0,0,,had better get at the real operands.
Dialogue: 0,0:53:45.82,0:53:47.39,EN,,0,0,0,,So here is a place where we're going to have to force them.
Dialogue: 0,0:53:47.92,0:53:50.38,EN,,0,0,0,,And we're going to look at what evlist is going to have to do a bunch of forces.
Dialogue: 0,0:53:51.34,0:53:52.78,EN,,0,0,0,,So we have two different kinds of evlist now.
Dialogue: 0,0:53:52.78,0:53:54.09,EN,,0,0,0,,We have evlist and gevlist.
Dialogue: 0,0:53:54.52,0:53:57.16,EN,,0,0,0,,Gevlist is going to wrap delays around some things
Dialogue: 0,0:53:57.18,0:53:59.74,EN,,0,0,0,,and force others, evaluate others.
Dialogue: 0,0:53:59.87,0:54:05.85,EN,,0,0,0,,And this guy's going to do some forcing of things.
Dialogue: 0,0:54:07.90,0:54:09.16,EN,,0,0,0,,Just looking at this a little bit,
Dialogue: 0,0:54:09.69,0:54:11.98,EN,,0,0,0,,this is a game you must play for yourself, you know.
Dialogue: 0,0:54:12.25,0:54:14.67,EN,,0,0,0,,It's not something that you're going to see all possible
Dialogue: 0,0:54:14.72,0:54:18.20,EN,,0,0,0,,variations on an evaluator talking to me.
Dialogue: 0,0:54:19.52,0:54:21.24,EN,,0,0,0,,What you have to do is do this for yourself.
Dialogue: 0,0:54:21.37,0:54:23.84,EN,,0,0,0,,And after you feel this, you play this a bit,
Dialogue: 0,0:54:24.22,0:54:27.02,EN,,0,0,0,,you get to see all the possible design decisions and what they might mean,
Dialogue: 0,0:54:27.77,0:54:29.16,EN,,0,0,0,,and how they interact with each other.
Dialogue: 0,0:54:29.93,0:54:32.38,EN,,0,0,0,,So what languages might have in them.
Dialogue: 0,0:54:33.16,0:54:34.64,EN,,0,0,0,,And what are some of the consistent sets
Dialogue: 0,0:54:34.94,0:54:36.32,EN,,0,0,0,,that make a legitimate language.
Dialogue: 0,0:54:37.20,0:54:40.06,EN,,0,0,0,,Whereas what things are complicated kluges that are just piles of junk.
Dialogue: 0,0:54:41.85,0:54:44.68,EN,,0,0,0,,So evlist of course, over here, just as I said,
Dialogue: 0,0:54:44.81,0:54:46.03,EN,,0,0,0,,is a list of operands
Dialogue: 0,0:54:46.70,0:54:50.28,EN,,0,0,0,,which are going to be undelayed after evaluation.
Dialogue: 0,0:54:50.75,0:54:51.90,EN,,0,0,0,,So these are going to be forced,
Dialogue: 0,0:54:53.28,0:54:54.44,EN,,0,0,0,,whatever that's going to mean.
Dialogue: 0,0:54:56.05,0:54:58.51,EN,,0,0,0,,And gevlist, which is the next thing--
Dialogue: 0,0:55:01.26,0:55:01.85,EN,,0,0,0,,Thank you.
Dialogue: 0,0:55:04.04,0:55:06.35,EN,,0,0,0,,What we see here, uh
Dialogue: 0,0:55:07.80,0:55:09.61,EN,,0,0,0,,well there's a couple of possibilities.
Dialogue: 0,0:55:09.81,0:55:11.52,EN,,0,0,0,,Either it's a normal, ordinary thing,
Dialogue: 0,0:55:12.48,0:55:13.69,EN,,0,0,0,,a symbol sitting there
Dialogue: 0,0:55:13.74,0:55:16.20,EN,,0,0,0,,like the predicate in the unless,
Dialogue: 0,0:55:17.64,0:55:18.81,EN,,0,0,0,,and that's what we have here.
Dialogue: 0,0:55:19.39,0:55:22.49,EN,,0,0,0,,In which case, this is intended to be evaluated in applicative order.
Dialogue: 0,0:55:23.34,0:55:25.45,EN,,0,0,0,,And it's, essentially, just what we had before.
Dialogue: 0,0:55:25.63,0:55:28.84,EN,,0,0,0,,It's mapping eval down the list.
Dialogue: 0,0:55:29.95,0:55:32.14,EN,,0,0,0,,In other words, I evaluate the first expression
Dialogue: 0,0:55:32.65,0:55:37.36,EN,,0,0,0,,and continue gevlisting the CDR of the expression in the environment.
Dialogue: 0,0:55:37.93,0:55:43.20,EN,,0,0,0,,However, it's possible that this is a name parameter.
Dialogue: 0,0:55:44.00,0:55:45.05,EN,,0,0,0,,If it's a name parameter,
Dialogue: 0,0:55:45.20,0:55:46.59,EN,,0,0,0,,I want to put a delay in
Dialogue: 0,0:55:47.00,0:55:50.97,EN,,0,0,0,,which combines that expression, which I'm calling by name,
Dialogue: 0,0:55:52.14,0:55:57.74,EN,,0,0,0,,with the environment that's available at this time
Dialogue: 0,0:55:59.05,0:56:00.59,EN,,0,0,0,,and passing that as the parameter.
Dialogue: 0,0:56:02.79,0:56:05.04,EN,,0,0,0,,And this is part of the mapping process that you see here.
Dialogue: 0,0:56:09.07,0:56:11.31,EN,,0,0,0,,The only other interesting place in this procedure
Dialogue: 0,0:56:11.37,0:56:13.53,EN,,0,0,0,,in this interpreter is cond.
Dialogue: 0,0:56:14.70,0:56:15.92,EN,,0,0,0,,People tend to write this thing,
Dialogue: 0,0:56:15.93,0:56:17.24,EN,,0,0,0,,and then they leave this one out.
Dialogue: 0,0:56:18.55,0:56:19.98,EN,,0,0,0,,There's a place where you have to force.
Dialogue: 0,0:56:20.51,0:56:23.10,EN,,0,0,0,,Conditionals have to know
Dialogue: 0,0:56:24.20,0:56:25.90,EN,,0,0,0,,whether or not the answer is true or false.
Dialogue: 0,0:56:25.99,0:56:26.83,EN,,0,0,0,,It's like a primitive.
Dialogue: 0,0:56:28.55,0:56:30.56,EN,,0,0,0,,When you do a conditional, you have to force.
Dialogue: 0,0:56:31.72,0:56:33.95,EN,,0,0,0,,Now, I'm not going to look at any more of this in any detail.
Dialogue: 0,0:56:34.62,0:56:36.28,EN,,0,0,0,,It isn't very exciting.
Dialogue: 0,0:56:36.75,0:56:38.99,EN,,0,0,0,,And what's left is how you make delays.
Dialogue: 0,0:56:38.99,0:56:40.91,EN,,0,0,0,,Well, delays are data structures
Dialogue: 0,0:56:41.31,0:56:44.75,EN,,0,0,0,,which contain an expression, an environment, and a type on them.
Dialogue: 0,0:56:44.84,0:56:46.36,EN,,0,0,0,,And it says they're a thunk.
Dialogue: 0,0:56:46.96,0:56:48.46,EN,,0,0,0,,That comes from ALGOL language,
Dialogue: 0,0:56:49.07,0:56:50.81,EN,,0,0,0,,and it's claimed to be the sound of
Dialogue: 0,0:56:50.83,0:56:52.06,EN,,0,0,0,,of something being pushed on a stack.
Dialogue: 0,0:56:52.97,0:56:53.41,EN,,0,0,0,,I don't know.
Dialogue: 0,0:56:53.41,0:56:57.12,EN,,0,0,0,,I was not an ALGOLician, so or an ALGOLite or whatever,
Dialogue: 0,0:56:57.60,0:56:58.38,EN,,0,0,0,,so I don't know.
Dialogue: 0,0:56:58.74,0:56:59.64,EN,,0,0,0,,But that's what was claimed.
Dialogue: 0,0:57:00.27,0:57:01.56,EN,,0,0,0,,And undelay is something
Dialogue: 0,0:57:01.77,0:57:03.66,EN,,0,0,0,,which will recursively undelay thunks
Dialogue: 0,0:57:03.69,0:57:06.00,EN,,0,0,0,,until the thunk becomes something which isn't a thunk.
Dialogue: 0,0:57:07.72,0:57:10.94,EN,,0,0,0,,This is the way you implement a call by name like thing in ALGOL.
Dialogue: 0,0:57:12.05,0:57:13.76,EN,,0,0,0,,And that's about all there is.
Dialogue: 0,0:57:15.21,0:57:16.25,EN,,0,0,0,,Are there any questions?
Dialogue: 0,0:57:26.68,0:57:27.52,EN,,0,0,0,,AUDIENCE: Gerry?
Dialogue: 0,0:57:28.09,0:57:28.80,EN,,0,0,0,,PROFESSOR: Yes, Vesko?
Dialogue: 0,0:57:30.03,0:57:32.99,EN,,0,0,0,,AUDIENCE: I noticed you avoided calling by name
Dialogue: 0,0:57:33.44,0:57:34.89,EN,,0,0,0,,in the primitive procedures,
Dialogue: 0,0:57:36.41,0:57:38.38,EN,,0,0,0,,I was wondering what cause you have on that?
Dialogue: 0,0:57:38.41,0:57:39.21,EN,,0,0,0,,You never need that?
Dialogue: 0,0:57:40.07,0:57:41.61,EN,,0,0,0,,PROFESSOR: Vesko is asking
Dialogue: 0,0:57:42.06,0:57:46.00,EN,,0,0,0,,if it's ever reasonable to call a primitive procedure by name?
Dialogue: 0,0:57:47.14,0:57:48.70,EN,,0,0,0,,The answer is, yes.
Dialogue: 0,0:57:49.27,0:57:52.32,EN,,0,0,0,,There's one particular case where it's reasonable, actually two.
Dialogue: 0,0:57:55.53,0:57:58.27,EN,,0,0,0,,Construction of a data structure like cons
Dialogue: 0,0:57:59.02,0:58:02.00,EN,,0,0,0,,where making an array if you have arrays with any number of elements.
Dialogue: 0,0:58:03.26,0:58:07.44,EN,,0,0,0,,OK? It's unnecessary to evaluate those arguments.
Dialogue: 0,0:58:07.44,0:58:08.83,EN,,0,0,0,,All you need is promises
Dialogue: 0,0:58:09.10,0:58:10.81,EN,,0,0,0,,to evaluate those arguments if you look at them.
Dialogue: 0,0:58:11.50,0:58:15.08,EN,,0,0,0,,If I cons together a, two things,
Dialogue: 0,0:58:16.24,0:58:17.77,EN,,0,0,0,,then I could cons together the promises
Dialogue: 0,0:58:17.80,0:58:19.93,EN,,0,0,0,,just as easily as I can cons together the things.
Dialogue: 0,0:58:21.15,0:58:23.37,EN,,0,0,0,,And it's not even when I CAR CDR them
Dialogue: 0,0:58:23.39,0:58:24.30,EN,,0,0,0,,that I have to look at them.
Dialogue: 0,0:58:24.84,0:58:26.97,EN,,0,0,0,,That just gets out the promises and passes them to somebody.
Dialogue: 0,0:58:28.26,0:58:30.51,EN,,0,0,0,,That's why the lambda calculus definition, the
Dialogue: 0,0:58:30.57,0:58:34.03,EN,,0,0,0,,the Alonzo Church definition of CAR, CDR, and cons makes sense.
Dialogue: 0,0:58:34.42,0:58:36.32,EN,,0,0,0,,It's because no work is done in CAR, CDR, and cons,
Dialogue: 0,0:58:36.38,0:58:40.06,EN,,0,0,0,,it's just shuffling data, it's just routing, if you will.
Dialogue: 0,0:58:40.99,0:58:42.20,EN,,0,0,0,,However, the things that do have
Dialogue: 0,0:58:42.24,0:58:43.84,EN,,0,0,0,,to look at data are things like plus.
Dialogue: 0,0:58:45.28,0:58:46.91,EN,,0,0,0,,Because they have a look at the bits
Dialogue: 0,0:58:47.12,0:58:48.30,EN,,0,0,0,,that the numbers are made out of,
Dialogue: 0,0:58:48.32,0:58:50.44,EN,,0,0,0,,unless they're lambda calculus numbers
Dialogue: 0,0:58:50.44,0:58:51.88,EN,,0,0,0,,which are funny. OK?
Dialogue: 0,0:58:52.43,0:58:53.58,EN,,0,0,0,,They have to look at the bits to
Dialogue: 0,0:58:53.77,0:58:55.53,EN,,0,0,0,,be able to crunch them together to do the add.
Dialogue: 0,0:58:59.21,0:58:59.92,EN,,0,0,0,,So, in fact,
Dialogue: 0,0:59:00.19,0:59:02.78,EN,,0,0,0,,data constructors, data selectors,
Dialogue: 0,0:59:03.24,0:59:05.50,EN,,0,0,0,,in fact, things that side-effect data objects
Dialogue: 0,0:59:06.27,0:59:09.76,EN,,0,0,0,,don't need to do, don't need to do any forcing
Dialogue: 0,0:59:11.34,0:59:13.39,EN,,0,0,0,,in the laziest possible interpreters.
Dialogue: 0,0:59:16.46,0:59:16.99,EN,,0,0,0,,On the other hand
Dialogue: 0,0:59:17.02,0:59:18.70,EN,,0,0,0,,predicates on data structures have to.
Dialogue: 0,0:59:19.61,0:59:22.65,EN,,0,0,0,,If you want to say, is this a, is this a pair?
Dialogue: 0,0:59:23.56,0:59:24.40,EN,,0,0,0,,Or is it a symbol?
Dialogue: 0,0:59:24.64,0:59:26.57,EN,,0,0,0,,Well, you better find out. You got to look at it then.
Dialogue: 0,0:59:30.30,0:59:31.18,EN,,0,0,0,,Any other questions?
Dialogue: 0,0:59:40.05,0:59:41.61,EN,,0,0,0,,Oh, well, I suppose it's time for a break.
Dialogue: 0,0:00:00.01,0:00:01.63,Declare,,0,0,0,,{\an2\fad(500,500)}Learning-SICP学习小组\N倾情制作
Dialogue: 0,0:00:01.63,0:00:09.15,staff,,0,0,0,,{\fad(600,800)\pos(110.666,403.334)}翻译&&时间轴\N邓雄飞\N张大伟
Dialogue: 0,0:00:01.63,0:00:09.15,staff,,0,0,0,,{\fad(600,800)\pos(534.666,404)}压制&&特效\N邓雄飞\N（Dysprosium）
Dialogue: 0,0:00:01.63,0:00:09.15,staff,,0,0,0,,{\fad(600,800)\pos(574.667,277.333)}校对\N邓雄飞
Dialogue: 0,0:00:01.63,0:00:09.15,staff,,0,0,0,,{\fad(600,800)\pos(89.334,273.333)}特别感谢\N裘宗燕教授
Dialogue: 0,0:00:01.63,0:00:09.15,title,,0,0,0,,{\fad(600,800)\pos(324,32)}计算机程序的构造和解释
Dialogue: 0,0:00:09.52,0:00:13.66,Declare,,0,0,0,,{\an2\fad(500,500)}元循环求值器 II
Dialogue: 0,0:00:17.21,0:00:17.96,Default,,0,0,0,,教授：目前为止
Dialogue: 0,0:00:19.52,0:00:21.29,Default,,0,0,0,,我们学的东西都非常有趣
Dialogue: 0,0:00:21.52,0:00:23.05,Default,,0,0,0,,但它有什么实际的用途吗？
Dialogue: 0,0:00:26.33,0:00:27.96,Default,,0,0,0,,我想答案是“是的”
Dialogue: 0,0:00:29.38,0:00:31.92,Default,,0,0,0,,这些元循环解释器
Dialogue: 0,0:00:32.96,0:00:34.60,Default,,0,0,0,,非常值得琢磨
Dialogue: 0,0:00:34.62,0:00:36.17,Default,,0,0,0,,我花了大概
Dialogue: 0,0:00:38.05,0:00:41.85,Default,,0,0,0,,曾几何时 我花了半年的功夫
Dialogue: 0,0:00:42.86,0:00:45.26,Default,,0,0,0,,用上节课讲的那种
Dialogue: 0,0:00:45.76,0:00:48.19,Default,,0,0,0,,元循环解释器来试验
Dialogue: 0,0:00:49.47,0:00:52.01,Default,,0,0,0,,各种各样的设计变种
Dialogue: 0,0:00:52.57,0:00:54.11,Default,,0,0,0,,用元循环解释器是因为
Dialogue: 0,0:00:54.72,0:00:56.94,Default,,0,0,0,,它们通过自己定义自己
Dialogue: 0,0:00:56.97,0:00:59.71,Default,,0,0,0,,因此被解释的语言也包含了自己
Dialogue: 0,0:01:01.27,0:01:03.87,Default,,0,0,0,,这样的解释器是一种的媒介
Dialogue: 0,0:01:03.88,0:01:05.58,Default,,0,0,0,,方便我们探索语言问题
Dialogue: 0,0:01:06.80,0:01:09.44,Default,,0,0,0,,如果你想添加一个新的FEATURE
Dialogue: 0,0:01:10.51,0:01:12.38,Default,,0,0,0,,这就是小菜一碟
Dialogue: 0,0:01:12.73,0:01:15.10,Default,,0,0,0,,你只需要稍作修改 然后观察结果
Dialogue: 0,0:01:15.49,0:01:17.20,Default,,0,0,0,,在尝试了一会儿新语言后
Dialogue: 0,0:01:17.24,0:01:18.24,Default,,0,0,0,,你可能觉得它不好
Dialogue: 0,0:01:18.52,0:01:19.47,Default,,0,0,0,,就把它扔到一边去了
Dialogue: 0,0:01:20.96,0:01:23.55,Default,,0,0,0,,或者你也想研究
Dialogue: 0,0:01:23.64,0:01:27.37,Default,,0,0,0,,不同绑定策略的差异
Dialogue: 0,0:01:28.81,0:01:31.90,Default,,0,0,0,,或者是一些更复杂的东西
Dialogue: 0,0:01:33.72,0:01:35.48,Default,,0,0,0,,事实上 这些元循环解释器
Dialogue: 0,0:01:36.17,0:01:37.88,Default,,0,0,0,,非常适合作为交换媒介
Dialogue: 0,0:01:38.20,0:01:42.56,Default,,0,0,0,,用于承载人们关于语言设计想法
Dialogue: 0,0:01:43.98,0:01:45.74,Default,,0,0,0,,因为它们易于理解
Dialogue: 0,0:01:46.28,0:01:48.46,Default,,0,0,0,,它们短小、紧凑而且简洁
Dialogue: 0,0:01:49.32,0:01:50.80,Default,,0,0,0,,如果我有一些点子
Dialogue: 0,0:01:51.53,0:01:53.77,Default,,0,0,0,,想让其它人评论一下
Dialogue: 0,0:01:54.25,0:01:58.32,Default,,0,0,0,,比如Indiana大学的Dan Friedman教授
Dialogue: 0,0:01:59.05,0:02:02.00,Default,,0,0,0,,我就编写一个小型元循环解释器
Dialogue: 0,0:02:02.56,0:02:03.79,Default,,0,0,0,,然后给他发一封电子邮件
Dialogue: 0,0:02:04.65,0:02:05.45,Default,,0,0,0,,并附上解释器
Dialogue: 0,0:02:05.45,0:02:07.90,Default,,0,0,0,,他就可以在计算机上安装并运行
Dialogue: 0,0:02:07.92,0:02:09.82,Default,,0,0,0,,可能他会觉得这个设计并不好
Dialogue: 0,0:02:11.94,0:02:13.10,Default,,0,0,0,,然后他会给我回一封邮件
Dialogue: 0,0:02:13.13,0:02:14.83,Default,,0,0,0,,“为什么不试试这个 这个更好一点”
Dialogue: 0,0:02:16.88,0:02:19.36,Default,,0,0,0,,所以我将会讲一些这方面的技术
Dialogue: 0,0:02:20.16,0:02:24.20,Default,,0,0,0,,因为在设计你自己的特定用途语言时
Dialogue: 0,0:02:24.72,0:02:28.68,Default,,0,0,0,,这种简单的技术非常重要
Dialogue: 0,0:02:30.79,0:02:32.08,Default,,0,0,0,,我们试着先在Lisp中
Dialogue: 0,0:02:32.51,0:02:34.21,Default,,0,0,0,,添加一个非常简单的FEATURE
Dialogue: 0,0:02:40.64,0:02:44.37,Default,,0,0,0,,在这之前 我先来谈谈FEATURE吧
Dialogue: 0,0:02:49.56,0:02:52.17,Default,,0,0,0,,很多语言添加了大量的FEATURE
Dialogue: 0,0:02:53.05,0:02:54.91,Default,,0,0,0,,把它们本身搞得混乱不堪
Dialogue: 0,0:02:56.86,0:02:58.38,Default,,0,0,0,,计算机科学家有一个笑话
Dialogue: 0,0:02:59.28,0:03:02.52,Default,,0,0,0,,“这不是BUG 这是FEATURE”
Dialogue: 0,0:03:05.03,0:03:06.46,Default,,0,0,0,,而我宁愿认为
Dialogue: 0,0:03:08.91,0:03:11.44,Default,,0,0,0,,很多系统都在遭受着“功能蔓延”的影响
Dialogue: 0,0:03:12.82,0:03:13.44,Default,,0,0,0,,比方说
Dialogue: 0,0:03:14.94,0:03:18.16,Default,,0,0,0,,George希望系统中有某个FEATURE
Dialogue: 0,0:03:18.72,0:03:19.36,Default,,0,0,0,,他就加了进来
Dialogue: 0,0:03:20.17,0:03:22.14,Default,,0,0,0,,Harry也想着
Dialogue: 0,0:03:22.17,0:03:24.20,Default,,0,0,0,,这个系统现在也不是我喜欢的那个了
Dialogue: 0,0:03:24.24,0:03:25.92,Default,,0,0,0,,然后加入了自己最喜欢的FEATURE
Dialogue: 0,0:03:26.64,0:03:30.24,Default,,0,0,0,,Jim也这样做
Dialogue: 0,0:03:30.83,0:03:31.79,Default,,0,0,0,,一段时间过后
Dialogue: 0,0:03:31.80,0:03:34.81,Default,,0,0,0,,操作手册就多达500页
Dialogue: 0,0:03:35.15,0:03:36.51,Default,,0,0,0,,以至于没人能看得懂
Dialogue: 0,0:03:37.79,0:03:39.32,Default,,0,0,0,,有时候也可能只是
Dialogue: 0,0:03:39.90,0:03:41.37,Default,,0,0,0,,同一个人在添加FEATURE
Dialogue: 0,0:03:41.39,0:03:43.23,Default,,0,0,0,,也会导致同样糟糕的结果
Dialogue: 0,0:03:44.14,0:03:46.09,Default,,0,0,0,,很多情况下 比如编辑器
Dialogue: 0,0:03:47.37,0:03:49.12,Default,,0,0,0,,具有很多FEATURE就很合理
Dialogue: 0,0:03:50.92,0:03:52.65,Default,,0,0,0,,因为你想要能够完成
Dialogue: 0,0:03:52.68,0:03:53.76,Default,,0,0,0,,各种不同的事情
Dialogue: 0,0:03:56.11,0:03:57.29,Default,,0,0,0,,但对计算机语言来说
Dialogue: 0,0:03:57.85,0:03:58.91,Default,,0,0,0,,我认为太多的FEATURE
Dialogue: 0,0:04:00.01,0:04:01.29,Default,,0,0,0,,是一个灾难
Dialogue: 0,0:04:04.03,0:04:08.00,Default,,0,0,0,,另外 系统也可能变成某种“尖叫的怪物”
Dialogue: 0,0:04:09.52,0:04:11.39,Default,,0,0,0,,假设你有一个盒子
Dialogue: 0,0:04:11.80,0:04:15.29,Default,,0,0,0,,它有一个鼠标和花哨的显示器
Dialogue: 0,0:04:15.95,0:04:20.04,Default,,0,0,0,,这些花哨的IO带来了各种各样的复杂性
Dialogue: 0,0:04:21.01,0:04:22.80,Default,,0,0,0,,你的程序语言就变成了
Dialogue: 0,0:04:23.34,0:04:25.37,Default,,0,0,0,,阴暗无用的小玩意儿
Dialogue: 0,0:04:25.40,0:04:27.90,Default,,0,0,0,,这是由计算机上的视窗系统进行的
Dialogue: 0,0:04:28.09,0:04:29.36,Default,,0,0,0,,内存换页和磁盘抖动所导致的
Dialogue: 0,0:04:30.08,0:04:31.82,Default,,0,0,0,,每当你使用计算机的时候
Dialogue: 0,0:04:31.93,0:04:33.45,Default,,0,0,0,,鼠标处理进程就会唤醒
Dialogue: 0,0:04:33.85,0:04:35.95,Default,,0,0,0,,你有什么事情要我做的吗？
Dialogue: 0,0:04:36.14,0:04:37.23,Default,,0,0,0,,然后又回去休眠
Dialogue: 0,0:04:37.44,0:04:39.44,Default,,0,0,0,,如果你的胳膊肘不小心碰到了鼠标
Dialogue: 0,0:04:39.61,0:04:42.32,Default,,0,0,0,,一大堆烟雾就会从你的电脑出来 类似于这样
Dialogue: 0,0:04:42.94,0:04:45.29,Default,,0,0,0,,这就是由于添加FEATURE
Dialogue: 0,0:04:45.55,0:04:47.21,Default,,0,0,0,,导致系统不能用的两种典型情况
Dialogue: 0,0:04:47.50,0:04:49.73,Default,,0,0,0,,现在我们要添加的是 一个非常简单的FEATURE
Dialogue: 0,0:04:52.60,0:04:53.77,Default,,0,0,0,,这个FEATURE非常好
Dialogue: 0,0:04:53.85,0:04:56.17,Default,,0,0,0,,事实上 Lisp中就有这个FEATURE
Dialogue: 0,0:04:57.25,0:04:58.17,Default,,0,0,0,,我们都知道
Dialogue: 0,0:04:59.29,0:05:03.13,Default,,0,0,0,,像+、*这样的过程
Dialogue: 0,0:05:03.37,0:05:04.89,Default,,0,0,0,,可以接受不定数目的参数
Dialogue: 0,0:05:05.43,0:05:06.44,Default,,0,0,0,,因此我们就可以写
Dialogue: 0,0:05:06.57,0:05:10.94,Default,,0,0,0,,(+ (* A X X)
Dialogue: 0,0:05:12.09,0:05:16.99,Default,,0,0,0,,(* B X) C)
Dialogue: 0,0:05:17.54,0:05:18.68,Default,,0,0,0,,这里可以看到
Dialogue: 0,0:05:18.92,0:05:21.76,Default,,0,0,0,,+有两到三个参数
Dialogue: 0,0:05:22.30,0:05:24.81,Default,,0,0,0,,*也有两到三个参数
Dialogue: 0,0:05:25.08,0:05:26.76,Default,,0,0,0,,不管多少个参数
Dialogue: 0,0:05:26.78,0:05:28.49,Default,,0,0,0,,都应该用同样的方式对待
Dialogue: 0,0:05:30.00,0:05:32.17,Default,,0,0,0,,支持不定数目的参数
Dialogue: 0,0:05:32.28,0:05:34.01,Default,,0,0,0,,这一点非常有用
Dialogue: 0,0:05:34.96,0:05:38.41,Default,,0,0,0,,而我上节课所讲的Lisp求值器
Dialogue: 0,0:05:39.23,0:05:41.85,Default,,0,0,0,,只能处理固定数目的参数
Dialogue: 0,0:05:42.62,0:05:45.28,Default,,0,0,0,,因为我要用BIND过程中的PAIR-UP过程
Dialogue: 0,0:05:45.63,0:05:47.92,Default,,0,0,0,,让形式参数与实际参数一一对应
Dialogue: 0,0:05:50.81,0:05:53.80,Default,,0,0,0,,假如我想能够定义像这样的过程
Dialogue: 0,0:05:54.89,0:05:57.32,Default,,0,0,0,,它们可以接收任意个数的参数
Dialogue: 0,0:05:58.75,0:06:00.40,Default,,0,0,0,,这个问题有好几部分
Dialogue: 0,0:06:01.34,0:06:04.81,Default,,0,0,0,,首先是挑选合适的语法描述
Dialogue: 0,0:06:05.72,0:06:11.21,Default,,0,0,0,,我们需要能够标注额外的参数
Dialogue: 0,0:06:12.17,0:06:13.63,Default,,0,0,0,,标注那些不知道个数的参数
Dialogue: 0,0:06:15.48,0:06:16.62,Default,,0,0,0,,另外就是
Dialogue: 0,0:06:17.10,0:06:18.70,Default,,0,0,0,,一旦我们标注出来后
Dialogue: 0,0:06:19.07,0:06:20.78,Default,,0,0,0,,我们怎样解释这些个记号
Dialogue: 0,0:06:21.74,0:06:23.10,Default,,0,0,0,,才能得到
Dialogue: 0,0:06:23.85,0:06:25.37,Default,,0,0,0,,正确的结果呢？
Dialogue: 0,0:06:26.98,0:06:28.80,Default,,0,0,0,,让我们来考虑一种
Dialogue: 0,0:06:28.84,0:06:30.27,Default,,0,0,0,,我们可能会遇到的情况
Dialogue: 0,0:06:33.07,0:06:34.51,Default,,0,0,0,,比如说
Dialogue: 0,0:06:35.42,0:06:37.34,Default,,0,0,0,,我想要定义这样的一个过程
Dialogue: 0,0:06:37.95,0:06:41.36,Default,,0,0,0,,它有一个必选参数X
Dialogue: 0,0:06:42.20,0:06:45.26,Default,,0,0,0,,还有个可选参数 -- 或者说一堆参数
Dialogue: 0,0:06:45.28,0:06:47.23,Default,,0,0,0,,我不知道它们的数目 就记作Y吧
Dialogue: 0,0:06:49.09,0:06:50.36,Default,,0,0,0,,X是必选的
Dialogue: 0,0:06:55.88,0:06:57.44,Default,,0,0,0,,然而有很多参数Y
Dialogue: 0,0:06:59.53,0:07:05.99,Default,,0,0,0,,这些参数形成的表 -- 我就记作Y
Dialogue: 0,0:07:14.48,0:07:16.06,Default,,0,0,0,,写好了参数表
Dialogue: 0,0:07:16.09,0:07:17.68,Default,,0,0,0,,我现在要这样定义过程体
Dialogue: 0,0:07:19.02,0:07:21.98,Default,,0,0,0,,我要对每一个元素都做同样的处理
Dialogue: 0,0:07:22.52,0:07:25.76,Default,,0,0,0,,(MAP (LAMBDA (U)
Dialogue: 0,0:07:27.00,0:07:34.54,Default,,0,0,0,,(* X U)) Y)
Dialogue: 0,0:07:36.89,0:07:38.04,Default,,0,0,0,,这里 我用了一个“点号”
Dialogue: 0,0:07:38.59,0:07:41.31,Default,,0,0,0,,来表明点号后面的东西
Dialogue: 0,0:07:42.19,0:07:44.30,Default,,0,0,0,,是剩下的所有参数构成的表
Dialogue: 0,0:07:46.30,0:07:48.12,Default,,0,0,0,,这就是一个语法规范
Dialogue: 0,0:07:53.32,0:07:54.64,Default,,0,0,0,,为什么这样来写呢？
Dialogue: 0,0:07:55.71,0:07:58.06,Default,,0,0,0,,这种语法规范之所以合理
Dialogue: 0,0:07:59.77,0:08:01.96,Default,,0,0,0,,是因为Lisp的源码读取器
Dialogue: 0,0:08:02.00,0:08:03.60,Default,,0,0,0,,刚好使用这种语法
Dialogue: 0,0:08:04.41,0:08:07.15,Default,,0,0,0,,来表示序对
Dialogue: 0,0:08:08.94,0:08:11.08,Default,,0,0,0,,我们之前没有介绍过
Dialogue: 0,0:08:11.08,0:08:12.78,Default,,0,0,0,,你在自己尝试的时候可能遇到过
Dialogue: 0,0:08:13.04,0:08:14.62,Default,,0,0,0,,当你调用(CONS X Y)时
Dialogue: 0,0:08:14.89,0:08:18.12,Default,,0,0,0,,你会得到 X . Y
Dialogue: 0,0:08:19.79,0:08:22.83,Default,,0,0,0,,准确来说是(X . Y)
Dialogue: 0,0:08:23.08,0:08:24.64,Default,,0,0,0,,两边还有括号
Dialogue: 0,0:08:26.98,0:08:28.16,Default,,0,0,0,,举例来说吧
Dialogue: 0,0:08:28.97,0:08:35.04,Default,,0,0,0,,这里的(X . Y)对应着一个序对
Dialogue: 0,0:08:36.33,0:08:39.29,Default,,0,0,0,,X是CAR部分 Y是CDR部分
Dialogue: 0,0:08:41.48,0:08:43.98,Default,,0,0,0,,你们目前为止见过的其它记号
Dialogue: 0,0:08:44.94,0:08:46.67,Default,,0,0,0,,是像
Dialogue: 0,0:08:46.92,0:08:55.24,Default,,0,0,0,,(LAMBDA (X Y Z) ...)这样的
Dialogue: 0,0:08:55.71,0:08:57.63,Default,,0,0,0,,它们则是像这样的
Dialogue: 0,0:09:02.00,0:09:03.61,Default,,0,0,0,,就拿形式参数表来说
Dialogue: 0,0:09:04.22,0:09:05.29,Default,,0,0,0,,它实际上是这样
Dialogue: 0,0:09:09.93,0:09:17.32,Default,,0,0,0,,这里分别是 X、Y、Z和空表
Dialogue: 0,0:09:18.28,0:09:21.08,Default,,0,0,0,,如果我有一个想要与之匹配的参数表的话
Dialogue: 0,0:09:22.60,0:09:25.60,Default,,0,0,0,,假设实际参数表是'(1 2 3)
Dialogue: 0,0:09:25.87,0:09:27.26,Default,,0,0,0,,我想把它们和形参相匹配
Dialogue: 0,0:09:28.38,0:09:37.10,Default,,0,0,0,,所以这里 可能有个三个元素的表
Dialogue: 0,0:09:42.44,0:09:46.94,Default,,0,0,0,,分别是1、2、3
Dialogue: 0,0:09:48.99,0:09:53.16,Default,,0,0,0,,用'(1 2 3)来匹配'(X Y Z)
Dialogue: 0,0:09:54.22,0:09:56.28,Default,,0,0,0,,很显然1和X相匹配
Dialogue: 0,0:09:56.32,0:09:58.01,Default,,0,0,0,,因为我可以顺着这个结构来
Dialogue: 0,0:09:58.86,0:10:01.56,Default,,0,0,0,,2和Y相匹配
Dialogue: 0,0:10:02.46,0:10:04.04,Default,,0,0,0,,3和Z相匹配
Dialogue: 0,0:10:05.48,0:10:09.53,Default,,0,0,0,,假设我现在要把这个(X . Y)
Dialogue: 0,0:10:09.55,0:10:11.84,Default,,0,0,0,,这个是(X . Y)
Dialogue: 0,0:10:12.51,0:10:16.91,Default,,0,0,0,,如果我想把它跟'(1 2 3)相匹配的话
Dialogue: 0,0:10:19.08,0:10:20.00,Default,,0,0,0,,我们再来看
Dialogue: 0,0:10:28.00,0:10:30.32,Default,,0,0,0,,这里是1、2、3
Dialogue: 0,0:10:30.86,0:10:32.88,Default,,0,0,0,,我可以沿着这里遍历
Dialogue: 0,0:10:32.99,0:10:35.50,Default,,0,0,0,,会发现 1和X相匹配
Dialogue: 0,0:10:37.56,0:10:41.84,Default,,0,0,0,,而Y和表'(2 3)相匹配
Dialogue: 0,0:10:43.74,0:10:46.22,Default,,0,0,0,,所以这里选用的表示法
Dialogue: 0,0:10:46.41,0:10:50.16,Default,,0,0,0,,对于Lisp来说是非常自然的
Dialogue: 0,0:10:52.66,0:10:54.14,Default,,0,0,0,,所以我就选择用这个记号
Dialogue: 0,0:10:54.17,0:10:55.80,Default,,0,0,0,,来表示数目不定的参数
Dialogue: 0,0:10:58.29,0:11:00.09,Default,,0,0,0,,还有一种可能性
Dialogue: 0,0:11:00.59,0:11:02.78,Default,,0,0,0,,如果我不是特别想命名某个参数
Dialogue: 0,0:11:03.00,0:11:05.00,Default,,0,0,0,,或者是命名某两个参数之类的
Dialogue: 0,0:11:06.54,0:11:07.56,Default,,0,0,0,,如果我不想那样的话
Dialogue: 0,0:11:08.78,0:11:10.44,Default,,0,0,0,,如果我想像+那样
Dialogue: 0,0:11:10.52,0:11:12.52,Default,,0,0,0,,一下子引用所有的参数
Dialogue: 0,0:11:13.88,0:11:17.96,Default,,0,0,0,,那么我就应该把参数表写成
Dialogue: 0,0:11:18.20,0:11:23.45,Default,,0,0,0,,(LAMBDA X ...)
Dialogue: 0,0:11:25.14,0:11:26.30,Default,,0,0,0,,举例来说
Dialogue: 0,0:11:26.81,0:11:27.96,Default,,0,0,0,,如果我定义一个过程
Dialogue: 0,0:11:28.06,0:11:30.44,Default,,0,0,0,,它把接收所有的参数
Dialogue: 0,0:11:31.12,0:11:32.70,Default,,0,0,0,,然后返回一个由它们组成的表X
Dialogue: 0,0:11:34.81,0:11:38.67,Default,,0,0,0,,返回的结果就是过程的参数表 明白吗？
Dialogue: 0,0:11:45.85,0:11:46.67,Default,,0,0,0,,这又是怎么回事呢？
Dialogue: 0,0:11:46.84,0:11:50.06,Default,,0,0,0,,实际上 无论我们的参数表是何种形式
Dialogue: 0,0:11:50.60,0:11:51.45,Default,,0,0,0,,无论是何种形式
Dialogue: 0,0:11:51.61,0:11:53.68,Default,,0,0,0,,都要与实际参数表相匹配
Dialogue: 0,0:11:55.14,0:11:57.14,Default,,0,0,0,,现在 这个符号就是所有的实际参数了
Dialogue: 0,0:12:01.49,0:12:05.13,Default,,0,0,0,,所以 我选择使用这个特定的语法规范
Dialogue: 0,0:12:05.64,0:12:07.63,Default,,0,0,0,,来描述那些
Dialogue: 0,0:12:08.04,0:12:10.56,Default,,0,0,0,,接收不定数目参数的过程
Dialogue: 0,0:12:13.45,0:12:14.60,Default,,0,0,0,,一共有两种情况
Dialogue: 0,0:12:15.40,0:12:16.35,Default,,0,0,0,,上面这种和下面这种
Dialogue: 0,0:12:17.44,0:12:18.36,Default,,0,0,0,,这两种都 --
Dialogue: 0,0:12:18.42,0:12:20.11,Default,,0,0,0,,当你们在制定语法规范时
Dialogue: 0,0:12:20.44,0:12:22.54,Default,,0,0,0,,千万注意不要有歧义
Dialogue: 0,0:12:23.56,0:12:27.36,Default,,0,0,0,,就比如说这里的两种情况
Dialogue: 0,0:12:27.66,0:12:31.20,Default,,0,0,0,,就不要与这里我们已有的这种混淆了
Dialogue: 0,0:12:33.61,0:12:35.82,Default,,0,0,0,,我总是可以区分出
Dialogue: 0,0:12:36.54,0:12:39.80,Default,,0,0,0,,过程的形式参数
Dialogue: 0,0:12:40.28,0:12:41.76,Default,,0,0,0,,是数目固定的具名参数
Dialogue: 0,0:12:42.64,0:12:43.13,Default,,0,0,0,,还是
Dialogue: 0,0:12:43.28,0:12:45.36,Default,,0,0,0,,既有数目固定的具名参数
Dialogue: 0,0:12:45.44,0:12:48.01,Default,,0,0,0,,又跟着数目可变的参数
Dialogue: 0,0:12:49.42,0:12:53.52,Default,,0,0,0,,又或者是所有参数组成的表
Dialogue: 0,0:12:53.68,0:12:56.52,Default,,0,0,0,,这个表会和这里的形式参数X相匹配
Dialogue: 0,0:12:56.99,0:12:58.84,Default,,0,0,0,,我都是可以从语法上区分它们
Dialogue: 0,0:13:02.25,0:13:04.62,Default,,0,0,0,,由于语言中存在语法歧义
Dialogue: 0,0:13:05.04,0:13:08.03,Default,,0,0,0,,整个待解释的程序被错误地分段
Dialogue: 0,0:13:08.64,0:13:13.92,Default,,0,0,0,,从而导致了可怕的错误
Dialogue: 0,0:13:14.56,0:13:16.67,Default,,0,0,0,,类Algol语言中就有些传统问题
Dialogue: 0,0:13:16.67,0:13:23.47,Default,,0,0,0,,就跟谓词部分的嵌套IF语句有关
Dialogue: 0,0:13:25.06,0:13:25.93,Default,,0,0,0,,总之
Dialogue: 0,0:13:27.52,0:13:29.44,Default,,0,0,0,,我现在已经把语法告诉你们了
Dialogue: 0,0:13:30.27,0:13:34.83,Default,,0,0,0,,我们要怎么来处理它的语义呢？
Dialogue: 0,0:13:35.25,0:13:36.11,Default,,0,0,0,,我们如何来解释它？
Dialogue: 0,0:13:36.59,0:13:37.96,Default,,0,0,0,,其实很简单
Dialogue: 0,0:13:38.44,0:13:42.57,Default,,0,0,0,,我修改一下元循环解释器就行
Dialogue: 0,0:13:43.71,0:13:44.76,Default,,0,0,0,,只需修改一行
Dialogue: 0,0:13:45.98,0:13:46.57,Default,,0,0,0,,在这里
Dialogue: 0,0:13:47.53,0:13:49.56,Default,,0,0,0,,我修改一下PAIR-UP过程
Dialogue: 0,0:13:50.81,0:13:54.19,Default,,0,0,0,,这里的PAIR-UP过程把
Dialogue: 0,0:13:56.76,0:14:02.03,Default,,0,0,0,,这是从上节课的元循环求值器中
Dialogue: 0,0:14:04.81,0:14:09.56,Default,,0,0,0,,摘录过来的PAIR-UP过程
Dialogue: 0,0:14:12.16,0:14:16.68,Default,,0,0,0,,它把形式参数与传递过来的实际参数匹配起来
Dialogue: 0,0:14:18.96,0:14:21.93,Default,,0,0,0,,大部分地方都和以前一样
Dialogue: 0,0:14:22.67,0:14:23.23,Default,,0,0,0,,也就是说
Dialogue: 0,0:14:23.31,0:14:25.07,Default,,0,0,0,,如果变量表为空
Dialogue: 0,0:14:25.52,0:14:27.31,Default,,0,0,0,,并且值表也为空
Dialogue: 0,0:14:27.45,0:14:29.61,Default,,0,0,0,,就返回空表
Dialogue: 0,0:14:31.05,0:14:33.00,Default,,0,0,0,,否则就是参数过多
Dialogue: 0,0:14:33.98,0:14:40.19,Default,,0,0,0,,如果变量表为空 但值表非空
Dialogue: 0,0:14:41.58,0:14:44.00,Default,,0,0,0,,如果值表为空
Dialogue: 0,0:14:44.96,0:14:47.47,Default,,0,0,0,,但是变量表又非空
Dialogue: 0,0:14:47.48,0:14:48.56,Default,,0,0,0,,那就是实际参数少了
Dialogue: 0,0:14:48.94,0:14:51.31,Default,,0,0,0,,然而如果我有一个变量是符号的话
Dialogue: 0,0:14:55.53,0:14:56.49,Default,,0,0,0,,这就有意思了
Dialogue: 0,0:14:58.30,0:15:04.40,Default,,0,0,0,,那么 我就认为遇到了特殊情况
Dialogue: 0,0:15:04.59,0:15:06.51,Default,,0,0,0,,也就是尾部分为符号的情况
Dialogue: 0,0:15:08.35,0:15:14.11,Default,,0,0,0,,情况就像这里的一样
Dialogue: 0,0:15:14.90,0:15:17.87,Default,,0,0,0,,这个尾部分就是一个符号Y
Dialogue: 0,0:15:18.63,0:15:19.39,Default,,0,0,0,,它不是NIL
Dialogue: 0,0:15:20.73,0:15:21.72,Default,,0,0,0,,不是个空表
Dialogue: 0,0:15:23.26,0:15:25.60,Default,,0,0,0,,而这个一开始就是个符号
Dialogue: 0,0:15:25.98,0:15:26.81,Default,,0,0,0,,就没有别的东西了
Dialogue: 0,0:15:27.79,0:15:28.72,Default,,0,0,0,,这种情况下
Dialogue: 0,0:15:29.96,0:15:37.20,Default,,0,0,0,,我就用这个符号去匹配所有的值
Dialogue: 0,0:15:38.03,0:15:42.52,Default,,0,0,0,,并把它们添加到要返回的结果中
Dialogue: 0,0:15:44.50,0:15:46.91,Default,,0,0,0,,否则的话 我就像正常情况那样
Dialogue: 0,0:15:47.15,0:15:48.52,Default,,0,0,0,,来创建所有的配对
Dialogue: 0,0:15:52.02,0:15:53.82,Default,,0,0,0,,我认为这很容易理解
Dialogue: 0,0:15:54.51,0:15:55.84,Default,,0,0,0,,就是这些
Dialogue: 0,0:15:57.08,0:15:58.33,Default,,0,0,0,,现在 答疑时间
Dialogue: 0,0:16:02.62,0:16:05.05,Default,,0,0,0,,有什么问题吗？
Dialogue: 0,0:16:06.60,0:16:06.94,Default,,0,0,0,,你说
Dialogue: 0,0:16:07.37,0:16:09.92,Default,,0,0,0,,学生：你能再解释一下第三种形式吗？
Dialogue: 0,0:16:09.98,0:16:12.12,Default,,0,0,0,,教授：第三种？这个？
Dialogue: 0,0:16:12.59,0:16:14.27,Default,,0,0,0,,或许你用表结构来思考
Dialogue: 0,0:16:14.30,0:16:16.24,Default,,0,0,0,,会更容易理解一些
Dialogue: 0,0:16:18.57,0:16:22.73,Default,,0,0,0,,这是一个过程 包含一个LAMBDA
Dialogue: 0,0:16:25.85,0:16:29.61,Default,,0,0,0,,我画出来的这个表结构就代表上面的这个
Dialogue: 0,0:16:31.26,0:16:32.44,Default,,0,0,0,,这里是X
Dialogue: 0,0:16:32.73,0:16:33.98,Default,,0,0,0,,这些是我们的符号
Dialogue: 0,0:16:37.41,0:16:39.58,Default,,0,0,0,,过程体就是X而已
Dialogue: 0,0:16:44.84,0:16:48.75,Default,,0,0,0,,如果我需要这个过程的形式参数表
Dialogue: 0,0:16:50.09,0:16:51.58,Default,,0,0,0,,我就取它的CADR部分
Dialogue: 0,0:16:52.14,0:16:53.16,Default,,0,0,0,,我会得到一个符号
Dialogue: 0,0:16:54.01,0:16:57.16,Default,,0,0,0,,所以我们的匹配器 -- 也就是我给你们展示的PAIR-UP过程
Dialogue: 0,0:16:58.24,0:17:00.44,Default,,0,0,0,,就会把这个符号对象
Dialogue: 0,0:17:01.56,0:17:04.40,Default,,0,0,0,,跟我们传递的实际参数表相匹配了
Dialogue: 0,0:17:05.76,0:17:09.55,Default,,0,0,0,,这个符号与实际参数表相绑定
Dialogue: 0,0:17:11.37,0:17:16.48,Default,,0,0,0,,而在这个例子中 如果我去取它的话
Dialogue: 0,0:17:16.92,0:17:20.97,Default,,0,0,0,,匹配器就会把它与变量表的这个部分相匹配
Dialogue: 0,0:17:24.14,0:17:26.14,Default,,0,0,0,,如果一个过程只是
Dialogue: 0,0:17:26.17,0:17:29.13,Default,,0,0,0,,直接返回得到的参数表的话 返回的就是一个表
Dialogue: 0,0:17:30.40,0:17:31.39,Default,,0,0,0,,这个过程就是这样的
Dialogue: 0,0:17:34.51,0:17:35.48,Default,,0,0,0,,好吧 谢谢大家
Dialogue: 0,0:17:36.14,0:17:37.28,Default,,0,0,0,,大家休息一下吧
Dialogue: 0,0:17:37.83,0:17:55.36,Default,,0,0,0,,[音乐]
Dialogue: 0,0:17:55.36,0:17:59.02,Declare,,0,0,0,,{\an2\fad(500,500)}《计算机程序的构造和解释》
Dialogue: 0,0:18:03.53,0:18:07.56,Declare,,0,0,0,,{\an2\fad(500,500)}讲师：哈罗德·艾伯森教授 及 格兰德·杰·萨斯曼教授
Dialogue: 0,0:18:07.56,0:18:11.69,Declare,,0,0,0,,{\an2\fad(500,500)}《计算机程序的构造和解释》
Dialogue: 0,0:18:12.25,0:18:16.11,Declare,,0,0,0,,{\an2\fad(500,500)}元循环求值器 II
Dialogue: 0,0:18:20.86,0:18:21.61,Default,,0,0,0,,教授：我们接着来看
Dialogue: 0,0:18:23.26,0:18:26.32,Default,,0,0,0,,现在 我将介绍一种相当重要的变种
Dialogue: 0,0:18:27.45,0:18:31.04,Default,,0,0,0,,这种变体非常有名
Dialogue: 0,0:18:31.60,0:18:36.80,Default,,0,0,0,,早期的很多Lisp都支持它
Dialogue: 0,0:18:38.25,0:18:40.06,Default,,0,0,0,,它被称为变量的动态绑定
Dialogue: 0,0:18:41.77,0:18:44.68,Default,,0,0,0,,我们现在来研究一下它
Dialogue: 0,0:18:47.62,0:18:50.16,Default,,0,0,0,,我先来介绍一下是什么导致
Dialogue: 0,0:18:50.35,0:18:52.36,Default,,0,0,0,,人们产生这样的想法
Dialogue: 0,0:18:53.74,0:18:55.23,Default,,0,0,0,,然而我并不会直接点明原因
Dialogue: 0,0:18:55.40,0:18:57.60,Default,,0,0,0,,我来举一个例子 你们来感受一下
Dialogue: 0,0:18:58.64,0:18:59.93,Default,,0,0,0,,假设
Dialogue: 0,0:19:00.75,0:19:02.59,Default,,0,0,0,,我们再来考察一下
Dialogue: 0,0:19:05.02,0:19:06.43,Default,,0,0,0,,计算一系列数之和的SUM过程
Dialogue: 0,0:19:08.14,0:19:09.47,Default,,0,0,0,,它的参数为
Dialogue: 0,0:19:09.60,0:19:10.78,Default,,0,0,0,,计算当前项的TERM
Dialogue: 0,0:19:13.04,0:19:14.41,Default,,0,0,0,,下界A
Dialogue: 0,0:19:15.24,0:19:17.04,Default,,0,0,0,,计算下一项索引的NEXT
Dialogue: 0,0:19:17.24,0:19:18.56,Default,,0,0,0,,上界B
Dialogue: 0,0:19:19.36,0:19:20.16,Default,,0,0,0,,过程体是
Dialogue: 0,0:19:23.16,0:19:26.94,Default,,0,0,0,,如果A>B
Dialogue: 0,0:19:27.15,0:19:28.64,Default,,0,0,0,,那么结果就是0
Dialogue: 0,0:19:30.24,0:19:31.08,Default,,0,0,0,,否则就是
Dialogue: 0,0:19:33.68,0:19:39.82,Default,,0,0,0,,(+ (TERM A)
Dialogue: 0,0:19:40.60,0:19:44.24,Default,,0,0,0,,(SUM TERM
Dialogue: 0,0:19:47.68,0:19:52.64,Default,,0,0,0,,(NEXT A)
Dialogue: 0,0:20:00.30,0:20:03.56,Default,,0,0,0,,这个NEXT过程直接传递过去
Dialogue: 0,0:20:06.40,0:20:08.25,Default,,0,0,0,,上界B也直接传递过去
Dialogue: 0,0:20:14.51,0:20:15.76,Default,,0,0,0,,（闭合括号中）
Dialogue: 0,0:20:17.82,0:20:21.45,Default,,0,0,0,,当我使用SUM过程的时候
Dialogue: 0,0:20:21.96,0:20:24.35,Default,,0,0,0,,我可以像这样来用
Dialogue: 0,0:20:25.45,0:20:38.04,Default,,0,0,0,,我们可以把SUM-POWERS过程定义为
Dialogue: 0,0:20:38.08,0:20:40.33,Default,,0,0,0,,这个函数是用来计算Σ(X^N)的
Dialogue: 0,0:20:41.10,0:20:45.93,Default,,0,0,0,,它的参数有A、B以及N
Dialogue: 0,0:20:45.95,0:20:47.69,Default,,0,0,0,,分别指下界、上界以及指数
Dialogue: 0,0:20:48.06,0:20:53.34,Default,,0,0,0,,它的定义是(SUM (LAMBDA (X)
Dialogue: 0,0:20:53.60,0:20:59.31,Default,,0,0,0,,这个参数为X的过程计算(EXPT X N)
Dialogue: 0,0:21:02.19,0:21:09.29,Default,,0,0,0,,我们还要传递A、1+还有B
Dialogue: 0,0:21:11.82,0:21:15.76,Default,,0,0,0,,因此 给定一系列X 我们计算Σ(X^N)的值
Dialogue: 0,0:21:16.14,0:21:19.74,Default,,0,0,0,,X从A到B取值 步长为1
Dialogue: 0,0:21:22.94,0:21:24.38,Default,,0,0,0,,我也可以定义--
Dialogue: 0,0:21:27.68,0:21:28.20,Default,,0,0,0,,好像这里有点问题
Dialogue: 0,0:21:29.78,0:21:31.02,Default,,0,0,0,,不好意思
Dialogue: 0,0:21:31.91,0:21:33.36,Default,,0,0,0,,这里应该是PRODUCT-POWERS
Dialogue: 0,0:21:38.08,0:21:39.12,Default,,0,0,0,,名字有点奇怪
Dialogue: 0,0:21:40.02,0:21:40.80,Default,,0,0,0,,还是不改了
Dialogue: 0,0:21:41.96,0:21:46.32,Default,,0,0,0,,有点怪 就按原来的吧
Dialogue: 0,0:21:49.34,0:21:50.19,Default,,0,0,0,,这回应该对了
Dialogue: 0,0:21:51.37,0:21:53.82,Default,,0,0,0,,而PRODUCT-POWERS的定义则是
Dialogue: 0,0:21:58.41,0:22:02.36,Default,,0,0,0,,（意义不明）
Dialogue: 0,0:22:03.00,0:22:06.81,Default,,0,0,0,,我可以用像SUM一样的过程
Dialogue: 0,0:22:06.81,0:22:08.22,Default,,0,0,0,,只不过是用来计算乘积的
Dialogue: 0,0:22:08.56,0:22:11.05,Default,,0,0,0,,但是很类似 就跟你们在那里见到的一样
Dialogue: 0,0:22:11.45,0:22:16.38,Default,,0,0,0,,它也是一个三参数的过程
Dialogue: 0,0:22:17.00,0:22:25.42,Default,,0,0,0,,求积的因数是通过构造而来
Dialogue: 0,0:22:25.66,0:22:31.60,Default,,0,0,0,,也就是(PRODUCT (LAMBDA (X) (EXPT X N))
Dialogue: 0,0:22:34.43,0:22:37.85,Default,,0,0,0,,下界是A 步长为1 上界为B
Dialogue: 0,0:22:41.53,0:22:41.88,Default,,0,0,0,,现在
Dialogue: 0,0:22:46.83,0:22:49.50,Default,,0,0,0,,你可能马上就意识到一些问题
Dialogue: 0,0:22:50.75,0:22:52.01,Default,,0,0,0,,它们看起来几乎一样
Dialogue: 0,0:22:53.18,0:22:55.20,Default,,0,0,0,,为什么要重复写代码呢？
Dialogue: 0,0:22:56.59,0:22:59.72,Default,,0,0,0,,现在就很像我们之前遇到的情况了
Dialogue: 0,0:23:01.00,0:23:03.15,Default,,0,0,0,,构建一个抽象不是更好吗？
Dialogue: 0,0:23:03.81,0:23:05.76,Default,,0,0,0,,如何构建良好的抽象呢？
Dialogue: 0,0:23:05.85,0:23:07.55,Default,,0,0,0,,我看到有一些完全相同的代码
Dialogue: 0,0:23:08.47,0:23:09.32,Default,,0,0,0,,这有一段
Dialogue: 0,0:23:09.98,0:23:11.08,Default,,0,0,0,,这是另一段
Dialogue: 0,0:23:14.45,0:23:16.22,Default,,0,0,0,,所以我应该把它们提取出来
Dialogue: 0,0:23:17.09,0:23:19.23,Default,,0,0,0,,我就会想
Dialogue: 0,0:23:20.51,0:23:22.67,Default,,0,0,0,,SUM-POWERS可以用
Dialogue: 0,0:23:22.88,0:23:24.52,Default,,0,0,0,,NTH-POWERS的过程来编写
Dialogue: 0,0:23:25.71,0:23:27.40,Default,,0,0,0,,假如有人想写一个
Dialogue: 0,0:23:27.74,0:23:30.03,Default,,0,0,0,,稍微不同的过程 就像这个一样
Dialogue: 0,0:23:37.63,0:23:45.18,Default,,0,0,0,,(DEFINE SUM-POWERS
Dialogue: 0,0:23:46.44,0:23:48.46,Default,,0,0,0,,(LAMBDA (A B N)
Dialogue: 0,0:23:48.75,0:23:52.27,Default,,0,0,0,,(SUM (NTH-POWER
Dialogue: 0,0:23:53.88,0:23:55.42,Default,,0,0,0,,我们调用过程NTH-POWER
Dialogue: 0,0:23:58.35,0:24:02.27,Default,,0,0,0,,下界为A 步长为1 上界为B
Dialogue: 0,0:24:05.74,0:24:06.91,Default,,0,0,0,,类似地
Dialogue: 0,0:24:10.65,0:24:12.76,Default,,0,0,0,,我想用这种方式来重写PRODUCT-POWERS
Dialogue: 0,0:24:12.89,0:24:15.24,Default,,0,0,0,,把求幂指数从这里抽象出来
Dialogue: 0,0:24:16.27,0:24:17.37,Default,,0,0,0,,可以这样写
Dialogue: 0,0:24:22.10,0:24:23.02,Default,,0,0,0,,(DEFINE PRODUCT-POWERS
Dialogue: 0,0:24:29.48,0:24:34.94,Default,,0,0,0,,(LAMBDA (A B N)
Dialogue: 0,0:24:35.31,0:24:42.33,Default,,0,0,0,,(PRODUCT NTH-POWERS
Dialogue: 0,0:24:46.44,0:24:50.30,Default,,0,0,0,,A 1+ B)))
Dialogue: 0,0:24:53.50,0:24:57.56,Default,,0,0,0,,把NTH-POWER的结果作为PRODUCT的参数
Dialogue: 0,0:24:58.38,0:25:00.24,Default,,0,0,0,,我们还需要定义
Dialogue: 0,0:25:02.04,0:25:03.88,Default,,0,0,0,,还需要定义过程NTH-POWERS
Dialogue: 0,0:25:04.89,0:25:05.93,Default,,0,0,0,,我把它写在这边
Dialogue: 0,0:25:12.22,0:25:12.99,Default,,0,0,0,,写在上面
Dialogue: 0,0:25:25.41,0:25:29.04,Default,,0,0,0,,它是一个参数为X的过程
Dialogue: 0,0:25:29.60,0:25:34.56,Default,,0,0,0,,计算(EXPT X N)
Dialogue: 0,0:25:35.93,0:25:36.96,Default,,0,0,0,,但是我遇到一个问题
Dialogue: 0,0:25:38.64,0:25:39.93,Default,,0,0,0,,我们使用的环境模型
Dialogue: 0,0:25:40.57,0:25:43.23,Default,,0,0,0,,我们用来解释
Dialogue: 0,0:25:44.00,0:25:45.95,Default,,0,0,0,,目前所定义的语言的这种手段
Dialogue: 0,0:25:46.27,0:25:48.81,Default,,0,0,0,,并没有给我说明这个N的值
Dialogue: 0,0:25:52.76,0:25:59.26,Default,,0,0,0,,因为 正如大家所知
Dialogue: 0,0:26:00.76,0:26:04.25,Default,,0,0,0,,在这个过程中 N是自由变量
Dialogue: 0,0:26:06.41,0:26:07.98,Default,,0,0,0,,环境模型告诉我们
Dialogue: 0,0:26:08.60,0:26:10.20,Default,,0,0,0,,自由变量的值
Dialogue: 0,0:26:11.21,0:26:14.99,Default,,0,0,0,,取决于过程被定义时所在的环境
Dialogue: 0,0:26:16.64,0:26:17.47,Default,,0,0,0,,在我编写它们的时候
Dialogue: 0,0:26:17.48,0:26:19.84,Default,,0,0,0,,就假设它们已经在黑板上被定义了
Dialogue: 0,0:26:21.64,0:26:23.63,Default,,0,0,0,,NTH-POWER是定义在全局环境下的
Dialogue: 0,0:26:24.06,0:26:25.15,Default,,0,0,0,,其中没有N的定义
Dialogue: 0,0:26:25.93,0:26:27.63,Default,,0,0,0,,因此 N是未绑定的变量
Dialogue: 0,0:26:28.72,0:26:31.66,Default,,0,0,0,,但对我们来说
Dialogue: 0,0:26:32.60,0:26:36.32,Default,,0,0,0,,我们明确希望它是这里和这里的N
Dialogue: 0,0:26:38.99,0:26:42.67,Default,,0,0,0,,另外一方面
Dialogue: 0,0:26:42.84,0:26:44.28,Default,,0,0,0,,当然我们要十分小心地确保
Dialogue: 0,0:26:44.56,0:26:46.06,Default,,0,0,0,,这里的N是这里的N
Dialogue: 0,0:26:48.96,0:26:52.83,Default,,0,0,0,,还有这里的这个 要跟这里的一致
Dialogue: 0,0:26:57.39,0:26:59.74,Default,,0,0,0,,这种想法造就了
Dialogue: 0,0:27:00.67,0:27:02.72,Default,,0,0,0,,一个非常著名的BUG
Dialogue: 0,0:27:04.65,0:27:06.04,Default,,0,0,0,,我来细说下这个BUG
Dialogue: 0,0:27:07.15,0:27:09.40,Default,,0,0,0,,请看这张幻灯片
Dialogue: 0,0:27:10.66,0:27:12.70,Default,,0,0,0,,这种想法被称作“动态绑定”
Dialogue: 0,0:27:13.99,0:27:16.91,Default,,0,0,0,,在这种情况下 自由变量不再被
Dialogue: 0,0:27:17.76,0:27:21.23,Default,,0,0,0,,定义过程时的环境所解释
Dialogue: 0,0:27:22.40,0:27:25.16,Default,,0,0,0,,这种情况下 自由变量的值
Dialogue: 0,0:27:25.44,0:27:29.31,Default,,0,0,0,,就像是存储在过程调用者的环境中一样
Dialogue: 0,0:27:31.85,0:27:34.84,Default,,0,0,0,,所以在这个系统中
Dialogue: 0,0:27:34.86,0:27:39.68,Default,,0,0,0,,你需要不断地搜索调用过程的调用者的环境
Dialogue: 0,0:27:40.43,0:27:42.65,Default,,0,0,0,,当然 在本例中
Dialogue: 0,0:27:42.84,0:27:44.30,Default,,0,0,0,,无论NTH-POWER在何处定义
Dialogue: 0,0:27:44.33,0:27:45.98,Default,,0,0,0,,它都是在PRODUCT过程中被调用的
Dialogue: 0,0:27:46.41,0:27:48.68,Default,,0,0,0,,我就需要在SUM过程中再编写一个类似的过程
Dialogue: 0,0:27:50.51,0:27:54.92,Default,,0,0,0,,而PRODUCT又是被PRODUCT-POWERS所调用
Dialogue: 0,0:27:55.13,0:27:56.14,Default,,0,0,0,,就是你们在这里看到的
Dialogue: 0,0:27:56.83,0:27:59.37,Default,,0,0,0,,由于PRODUCT-POWERS过程绑定了变量N
Dialogue: 0,0:28:00.09,0:28:04.09,Default,,0,0,0,,因此NTH-POWER中的N会从这个链中派生出来
Dialogue: 0,0:28:08.14,0:28:09.64,Default,,0,0,0,,相似地 这个N
Dialogue: 0,0:28:10.11,0:28:12.01,Default,,0,0,0,,NTH-POWER中的N在这种情况下
Dialogue: 0,0:28:12.32,0:28:15.80,Default,,0,0,0,,可能是来自于这里SUM过程的调用
Dialogue: 0,0:28:15.80,0:28:18.27,Default,,0,0,0,,你们可以从这里看到 它在SUM内部被调用
Dialogue: 0,0:28:20.73,0:28:21.69,Default,,0,0,0,,对应这里的TERM
Dialogue: 0,0:28:22.90,0:28:25.72,Default,,0,0,0,,而SUM是在SUM-POWERS的内部被调用
Dialogue: 0,0:28:26.94,0:28:27.96,Default,,0,0,0,,后者绑定了N
Dialogue: 0,0:28:28.93,0:28:30.65,Default,,0,0,0,,因此这里就有一个N
Dialogue: 0,0:28:32.75,0:28:36.11,Default,,0,0,0,,可供NTH-POWER中的N取值
Dialogue: 0,0:28:37.95,0:28:39.24,Default,,0,0,0,,这就是动态 --
Dialogue: 0,0:28:39.28,0:28:43.10,Default,,0,0,0,,这条白线以下的东西 再加上这部分
Dialogue: 0,0:28:43.31,0:28:46.04,Default,,0,0,0,,就是我们所谓的动态绑定
Dialogue: 0,0:28:46.59,0:28:49.00,Default,,0,0,0,,用动态绑定的角度来解释 就可以正常运行
Dialogue: 0,0:28:50.85,0:28:52.65,Default,,0,0,0,,现在 让我们来看一个例子
Dialogue: 0,0:28:54.54,0:28:55.99,Default,,0,0,0,,要怎么实现这个功能
Dialogue: 0,0:28:55.99,0:28:56.96,Default,,0,0,0,,非常简单
Dialogue: 0,0:28:57.48,0:28:59.34,Default,,0,0,0,,事实上 最早的Lisp实现
Dialogue: 0,0:29:00.01,0:29:02.52,Default,,0,0,0,,对自由变量有各种形式的解释
Dialogue: 0,0:29:03.31,0:29:05.98,Default,,0,0,0,,包括用动态绑定来解释自由变量
Dialogue: 0,0:29:06.40,0:29:10.14,Default,,0,0,0,,APL也是用动态绑定来解释自由变量的
Dialogue: 0,0:29:11.68,0:29:14.32,Default,,0,0,0,,而不是词法绑定 或者说静态绑定
Dialogue: 0,0:29:15.22,0:29:17.00,Default,,0,0,0,,当然 要从EVAL开始修改
Dialogue: 0,0:29:19.31,0:29:20.59,Default,,0,0,0,,只需修改两个地方就行
Dialogue: 0,0:29:22.78,0:29:25.61,Default,,0,0,0,,首先我们会发现
Dialogue: 0,0:29:26.52,0:29:28.49,Default,,0,0,0,,事情变得更简单了
Dialogue: 0,0:29:29.39,0:29:33.63,Default,,0,0,0,,如果我不需要
Dialogue: 0,0:29:33.64,0:29:36.20,Default,,0,0,0,,在定义过程的那个环境中求值
Dialogue: 0,0:29:36.44,0:29:38.04,Default,,0,0,0,,过程在定义的时候就无需
Dialogue: 0,0:29:38.43,0:29:40.17,Default,,0,0,0,,捕获当时的环境了
Dialogue: 0,0:29:42.03,0:29:44.96,Default,,0,0,0,,所以我们可以在幻灯片的这里看到
Dialogue: 0,0:29:45.84,0:29:50.08,Default,,0,0,0,,这条用于判断是否为LAMBDA表达式的子句
Dialogue: 0,0:29:50.73,0:29:52.43,Default,,0,0,0,,过程就是在这个时候创建的
Dialogue: 0,0:29:53.92,0:29:56.73,Default,,0,0,0,,就不会返回一个带有类型标签
Dialogue: 0,0:29:56.75,0:30:01.05,Default,,0,0,0,,和环境结构的过程对象了
Dialogue: 0,0:30:01.29,0:30:02.54,Default,,0,0,0,,就是EXP本身
Dialogue: 0,0:30:02.54,0:30:04.76,Default,,0,0,0,,而我们会在其它地方用某种方式来解耦
Dialogue: 0,0:30:06.44,0:30:09.40,Default,,0,0,0,,另外一处修改就是组合式的应用
Dialogue: 0,0:30:10.36,0:30:13.69,Default,,0,0,0,,应用的时候必须要取得调用者的环境
Dialogue: 0,0:30:14.29,0:30:17.24,Default,,0,0,0,,调用者的环境就在这里
Dialogue: 0,0:30:17.26,0:30:19.45,Default,,0,0,0,,如果表达式是一个过程应用--
Dialogue: 0,0:30:19.56,0:30:21.63,Default,,0,0,0,,如果我们求值的是一个组合式
Dialogue: 0,0:30:21.79,0:30:23.71,Default,,0,0,0,,我们就会调用一个组合式
Dialogue: 0,0:30:23.93,0:30:25.50,Default,,0,0,0,,调用一个过程
Dialogue: 0,0:30:25.66,0:30:27.37,Default,,0,0,0,,来取得运算符的值
Dialogue: 0,0:30:29.84,0:30:31.44,Default,,0,0,0,,调用者的环境
Dialogue: 0,0:30:31.98,0:30:34.51,Default,,0,0,0,,就是我们当前的环境
Dialogue: 0,0:30:35.89,0:30:40.72,Default,,0,0,0,,所以 我只需要要把这个环境传递给APPLY
Dialogue: 0,0:30:41.49,0:30:42.75,Default,,0,0,0,,我们再来看看APPLY
Dialogue: 0,0:30:43.58,0:30:44.97,Default,,0,0,0,,我们只需要
Dialogue: 0,0:30:45.71,0:30:48.41,Default,,0,0,0,,把参数列表加上一个环境ENV
Dialogue: 0,0:30:48.78,0:30:55.68,Default,,0,0,0,,然后用这个环境来扩展环境
Dialogue: 0,0:30:56.67,0:30:59.02,Default,,0,0,0,,扩展把形式参数
Dialogue: 0,0:30:59.02,0:31:01.37,Default,,0,0,0,,和传递过来的实际参数绑定在一起的环境
Dialogue: 0,0:31:03.08,0:31:05.98,Default,,0,0,0,,而不再是之前由过程捕获的环境了
Dialogue: 0,0:31:06.81,0:31:09.45,Default,,0,0,0,,最早的Lisp偶然地采用了
Dialogue: 0,0:31:09.66,0:31:11.96,Default,,0,0,0,,这种最显然的方式实现
Dialogue: 0,0:31:14.13,0:31:16.68,Default,,0,0,0,,当然 像往常一样 人们习惯了并喜欢上了它
Dialogue: 0,0:31:17.25,0:31:18.27,Default,,0,0,0,,因此就有一些人说
Dialogue: 0,0:31:18.40,0:31:19.50,Default,,0,0,0,,“就应该这么来做”
Dialogue: 0,0:31:21.59,0:31:24.09,Default,,0,0,0,,不幸的是 这导致一些严重的问题
Dialogue: 0,0:31:25.40,0:31:27.24,Default,,0,0,0,,最严重的一点是
Dialogue: 0,0:31:27.53,0:31:29.84,Default,,0,0,0,,采用动态绑定
Dialogue: 0,0:31:31.00,0:31:33.56,Default,,0,0,0,,破坏了模块性
Dialogue: 0,0:31:35.46,0:31:37.66,Default,,0,0,0,,如果有两个人在一个大型系统上协同工作
Dialogue: 0,0:31:38.57,0:31:40.01,Default,,0,0,0,,那么一个重要的原则就是
Dialogue: 0,0:31:40.35,0:31:42.19,Default,,0,0,0,,每个人所使用的名字
Dialogue: 0,0:31:42.99,0:31:44.58,Default,,0,0,0,,都不应该干扰到对方的名字
Dialogue: 0,0:31:47.93,0:31:50.78,Default,,0,0,0,,如果我写了一段代码
Dialogue: 0,0:31:51.07,0:31:53.13,Default,,0,0,0,,别人就不能通过在他代码内部
Dialogue: 0,0:31:53.88,0:31:56.57,Default,,0,0,0,,使用我代码中的名字来破坏我的代码
Dialogue: 0,0:31:56.75,0:31:57.71,Default,,0,0,0,,这一点很重要
Dialogue: 0,0:31:59.85,0:32:00.46,Default,,0,0,0,,然而
Dialogue: 0,0:32:01.04,0:32:05.18,Default,,0,0,0,,动态绑定明显地违背了这种特定的模块化约束
Dialogue: 0,0:32:06.67,0:32:08.08,Default,,0,0,0,,我们考虑一下
Dialogue: 0,0:32:09.18,0:32:10.35,Default,,0,0,0,,这段代码会有什么效果？
Dialogue: 0,0:32:12.54,0:32:13.79,Default,,0,0,0,,假设我想要把
Dialogue: 0,0:32:15.47,0:32:19.81,Default,,0,0,0,,我想要把变量NEXT换个名字
Dialogue: 0,0:32:19.81,0:32:24.41,Default,,0,0,0,,假设某个人要编写SUM过程
Dialogue: 0,0:32:25.10,0:32:26.68,Default,,0,0,0,,而别人则会使用这个SUM过程
Dialogue: 0,0:32:28.97,0:32:30.32,Default,,0,0,0,,编写SUM的那个人
Dialogue: 0,0:32:30.49,0:32:32.30,Default,,0,0,0,,可以选择他想要使用的名字
Dialogue: 0,0:32:33.66,0:32:34.84,Default,,0,0,0,,假设我就是那个编写者
Dialogue: 0,0:32:36.83,0:32:39.30,Default,,0,0,0,,刚巧 这里我不想用NEXT来表示
Dialogue: 0,0:32:39.30,0:32:40.09,Default,,0,0,0,,而是用N来表示
Dialogue: 0,0:32:41.74,0:32:43.10,Default,,0,0,0,,所以我把所有出现NEXT的地方
Dialogue: 0,0:32:44.28,0:32:45.26,Default,,0,0,0,,都换成N
Dialogue: 0,0:32:48.14,0:32:48.48,Default,,0,0,0,,哎呀
Dialogue: 0,0:32:49.94,0:32:52.22,Default,,0,0,0,,我没有改变这个程序的规范
Dialogue: 0,0:32:53.32,0:32:54.86,Default,,0,0,0,,但是整个程序就崩溃了
Dialogue: 0,0:32:56.11,0:32:57.96,Default,,0,0,0,,不仅如此 这边也出现了问题
Dialogue: 0,0:32:59.50,0:33:01.40,Default,,0,0,0,,为什么会这样？
Dialogue: 0,0:33:02.26,0:33:03.24,Default,,0,0,0,,答案非常明显
Dialogue: 0,0:33:04.48,0:33:09.29,Default,,0,0,0,,NTH-POWER中的变量N的值
Dialogue: 0,0:33:09.31,0:33:13.72,Default,,0,0,0,,也就是这个N和这个N
Dialogue: 0,0:33:14.97,0:33:17.16,Default,,0,0,0,,根据环境模型的定义
Dialogue: 0,0:33:17.20,0:33:19.58,Default,,0,0,0,,这两个N总是相关的
Dialogue: 0,0:33:19.87,0:33:21.48,Default,,0,0,0,,如果是根据环境模型的定义的话
Dialogue: 0,0:33:21.55,0:33:23.63,Default,,0,0,0,,因为N在这里被绑定
Dialogue: 0,0:33:24.37,0:33:26.25,Default,,0,0,0,,这个LAMBDA表达式是在
Dialogue: 0,0:33:26.59,0:33:28.59,Default,,0,0,0,,N被绑定的环境中执行的
Dialogue: 0,0:33:30.70,0:33:31.84,Default,,0,0,0,,如果不用环境模型的话
Dialogue: 0,0:33:32.01,0:33:33.68,Default,,0,0,0,,我必须追踪过程的调用链
Dialogue: 0,0:33:34.78,0:33:36.27,Default,,0,0,0,,那么就会发生糟糕的事儿
Dialogue: 0,0:33:37.32,0:33:41.18,Default,,0,0,0,,在SUM内部 这个是作为TERM调用的
Dialogue: 0,0:33:41.76,0:33:42.38,Default,,0,0,0,,这里的(TERM A)
Dialogue: 0,0:33:44.78,0:33:46.19,Default,,0,0,0,,这时再来查找N的值
Dialogue: 0,0:33:47.35,0:33:48.40,Default,,0,0,0,,我得到的不知这个值
Dialogue: 0,0:33:48.84,0:33:49.76,Default,,0,0,0,,而是这个值
Dialogue: 0,0:33:50.70,0:33:52.54,Default,,0,0,0,,因此 只是这个程序的内部做了修改
Dialogue: 0,0:33:52.86,0:33:54.09,Default,,0,0,0,,这个程序却崩溃了
Dialogue: 0,0:33:56.77,0:34:00.08,Default,,0,0,0,,LAMBDA就不再像我以前说得那样是个量词了
Dialogue: 0,0:34:01.12,0:34:05.13,Default,,0,0,0,,LAMBDA应该是一个量词
Dialogue: 0,0:34:05.43,0:34:06.70,Default,,0,0,0,,量词有一个性质
Dialogue: 0,0:34:06.89,0:34:11.42,Default,,0,0,0,,被它绑定的名字都不重要
Dialogue: 0,0:34:12.65,0:34:15.71,Default,,0,0,0,,只要我用不在过程体中的新名字
Dialogue: 0,0:34:16.92,0:34:19.98,Default,,0,0,0,,统一地在过程体中代换旧名字
Dialogue: 0,0:34:20.94,0:34:23.16,Default,,0,0,0,,就不会改变表达式的语义
Dialogue: 0,0:34:24.04,0:34:25.50,Default,,0,0,0,,而我刚才却通过修改一个名字
Dialogue: 0,0:34:25.53,0:34:27.20,Default,,0,0,0,,改变了表达式的语义
Dialogue: 0,0:34:28.69,0:34:30.89,Default,,0,0,0,,因此LAMBDA就不再是一个良好定义的量词了
Dialogue: 0,0:34:32.17,0:34:33.37,Default,,0,0,0,,这个问题非常严重
Dialogue: 0,0:34:34.55,0:34:35.55,Default,,0,0,0,,正是因为这个原因
Dialogue: 0,0:34:36.64,0:34:42.51,Default,,0,0,0,,我和同事放弃了这种抽象方法
Dialogue: 0,0:34:43.13,0:34:44.36,Default,,0,0,0,,相对的 我更喜欢
Dialogue: 0,0:34:45.61,0:34:47.50,Default,,0,0,0,,模块化原则
Dialogue: 0,0:34:48.09,0:34:50.20,Default,,0,0,0,,如果你愿意探索解释器
Dialogue: 0,0:34:51.96,0:34:53.68,Default,,0,0,0,,那就非常值得做这类实验
Dialogue: 0,0:34:54.83,0:34:56.91,Default,,0,0,0,,你可以尝试多种设计
Dialogue: 0,0:34:58.11,0:35:00.25,Default,,0,0,0,,探索更优雅的语言设计
Dialogue: 0,0:35:02.68,0:35:04.49,Default,,0,0,0,,这是元循环求值器非常重要的功能
Dialogue: 0,0:35:04.99,0:35:06.68,Default,,0,0,0,,现在 我也想讲一讲
Dialogue: 0,0:35:06.72,0:35:08.49,Default,,0,0,0,,这种情况下的正确做法
Dialogue: 0,0:35:09.32,0:35:12.91,Default,,0,0,0,,我又如何来获得这种
Dialogue: 0,0:35:13.04,0:35:15.34,Default,,0,0,0,,词法作用域的能力呢？
Dialogue: 0,0:35:16.28,0:35:17.39,Default,,0,0,0,,当然 实际情况是
Dialogue: 0,0:35:17.55,0:35:20.03,Default,,0,0,0,,在这里我想要的是
Dialogue: 0,0:35:20.68,0:35:22.60,Default,,0,0,0,,针对特定N的求指数函数
Dialogue: 0,0:35:23.69,0:35:24.28,Default,,0,0,0,,给定一个N
Dialogue: 0,0:35:24.32,0:35:25.66,Default,,0,0,0,,它会返回给我一个特定的求指数过程
Dialogue: 0,0:35:26.28,0:35:27.40,Default,,0,0,0,,这非常简单
Dialogue: 0,0:35:28.17,0:35:30.57,Default,,0,0,0,,换言之 我可以这样来写
Dialogue: 0,0:35:35.84,0:35:37.84,Default,,0,0,0,,我要定义一个过程PGEN
Dialogue: 0,0:35:40.25,0:35:42.54,Default,,0,0,0,,它有一个参数N
Dialogue: 0,0:35:43.16,0:35:45.95,Default,,0,0,0,,返回一个指数过程
Dialogue: 0,0:35:50.24,0:35:51.23,Default,,0,0,0,,计算X^N
Dialogue: 0,0:35:56.80,0:35:57.98,Default,,0,0,0,,有了这个以后
Dialogue: 0,0:35:58.59,0:36:00.88,Default,,0,0,0,,我就可以进行想要的那种抽象
Dialogue: 0,0:36:01.42,0:36:03.93,Default,,0,0,0,,甚至于现在的封装方法还要更好一些
Dialogue: 0,0:36:04.09,0:36:06.60,Default,,0,0,0,,因为系统现在不会因改名而崩溃了
Dialogue: 0,0:36:07.89,0:36:12.35,Default,,0,0,0,,(DEFINE SUM-POWERS
Dialogue: 0,0:36:17.28,0:36:20.70,Default,,0,0,0,,(LAMBDA (A B N)
Dialogue: 0,0:36:21.61,0:36:26.83,Default,,0,0,0,,(SUM
Dialogue: 0,0:36:26.88,0:36:32.32,Default,,0,0,0,,(PGEN N)
Dialogue: 0,0:36:34.40,0:36:38.01,Default,,0,0,0,,A 1+ B)))
Dialogue: 0,0:36:42.49,0:36:47.95,Default,,0,0,0,,(DEFINE PRODUCT-POWERS
Dialogue: 0,0:36:54.11,0:36:58.84,Default,,0,0,0,,(LAMBDA (A B N)
Dialogue: 0,0:36:59.80,0:37:09.96,Default,,0,0,0,,(PRODUCT (PGEN N) A 1+ B)))
Dialogue: 0,0:37:11.28,0:37:13.28,Default,,0,0,0,,当然 这只是一个非常简单的例子
Dialogue: 0,0:37:13.60,0:37:16.35,Default,,0,0,0,,这里 我想要抽象的对象也十分简单
Dialogue: 0,0:37:17.28,0:37:18.83,Default,,0,0,0,,但它也有可能是长达100行的代码
Dialogue: 0,0:37:20.10,0:37:23.67,Default,,0,0,0,,我这么写是为了保持简单
Dialogue: 0,0:37:23.67,0:37:24.57,Default,,0,0,0,,我给它命了名
Dialogue: 0,0:37:24.73,0:37:26.94,Default,,0,0,0,,这里它只是一个参数化的名字
Dialogue: 0,0:37:28.20,0:37:30.27,Default,,0,0,0,,这个名字显式地依赖于
Dialogue: 0,0:37:30.49,0:37:33.63,Default,,0,0,0,,词法作用域下N的值
Dialogue: 0,0:37:37.13,0:37:38.59,Default,,0,0,0,,因此可以把它看做一个很长的名字
Dialogue: 0,0:37:40.21,0:37:41.58,Default,,0,0,0,,这里 我是通过
Dialogue: 0,0:37:41.76,0:37:45.82,Default,,0,0,0,,为计算TERM的过程命名
Dialogue: 0,0:37:46.12,0:37:49.22,Default,,0,0,0,,来解决问题的
Dialogue: 0,0:37:55.08,0:37:55.87,Default,,0,0,0,,有什么问题吗？
Dialogue: 0,0:37:57.00,0:37:58.38,Default,,0,0,0,,David 你说
Dialogue: 0,0:37:58.57,0:38:02.27,Default,,0,0,0,,学生：刚才那个问题
Dialogue: 0,0:38:03.07,0:38:06.46,Default,,0,0,0,,只能通过新建一个过程来解决吗？
Dialogue: 0,0:38:06.47,0:38:08.92,Default,,0,0,0,,换句话说 是不是必须要语言能够
Dialogue: 0,0:38:08.99,0:38:11.56,Default,,0,0,0,,把对象定义为过程？
Dialogue: 0,0:38:12.41,0:38:13.76,Default,,0,0,0,,教授：我明白了
Dialogue: 0,0:38:15.90,0:38:19.74,Default,,0,0,0,,我构建抽象的这种方法
Dialogue: 0,0:38:20.14,0:38:22.86,Default,,0,0,0,,需要过程能够返回或者导出一个过程
Dialogue: 0,0:38:23.26,0:38:26.81,Default,,0,0,0,,以便我不想让过程体中包含特定过程
Dialogue: 0,0:38:27.04,0:38:27.24,Default,,0,0,0,,学生：没错
Dialogue: 0,0:38:28.19,0:38:28.88,Default,,0,0,0,,教授：是这样的
Dialogue: 0,0:38:29.53,0:38:31.52,Default,,0,0,0,,如果我不能这么做的话
Dialogue: 0,0:38:32.24,0:38:35.13,Default,,0,0,0,,那么我就无法去构造一个抽象
Dialogue: 0,0:38:35.53,0:38:41.77,Default,,0,0,0,,使得符号之间不会出现冲突
Dialogue: 0,0:38:43.00,0:38:43.48,Default,,0,0,0,,你说得对
Dialogue: 0,0:38:44.14,0:38:46.51,Default,,0,0,0,,我认为
Dialogue: 0,0:38:46.54,0:38:48.91,Default,,0,0,0,,能够把过程作为返回值
Dialogue: 0,0:38:49.20,0:38:58.28,Default,,0,0,0,,更一般地说是支持“第一级过程”
Dialogue: 0,0:38:59.13,0:39:02.46,Default,,0,0,0,,是模块化程序程序设计所必须的
Dialogue: 0,0:39:03.70,0:39:06.43,Default,,0,0,0,,有很多种方式来解决这个问题
Dialogue: 0,0:39:07.44,0:39:09.16,Default,,0,0,0,,你可以的做的就是
Dialogue: 0,0:39:09.18,0:39:11.84,Default,,0,0,0,,针对你所需要关心的每一种糟糕情况
Dialogue: 0,0:39:12.27,0:39:15.20,Default,,0,0,0,,你可以添加一个特殊的FEATURE来解决它
Dialogue: 0,0:39:15.84,0:39:17.12,Default,,0,0,0,,你可以做一个包系统
Dialogue: 0,0:39:17.74,0:39:21.16,Default,,0,0,0,,或者像Ada中的模块系统 等等
Dialogue: 0,0:39:22.24,0:39:24.88,Default,,0,0,0,,这些都可以 可能区别只是解决的程度不一
Dialogue: 0,0:39:26.44,0:39:28.38,Default,,0,0,0,,而能够把过程作为返回值
Dialogue: 0,0:39:28.41,0:39:29.74,Default,,0,0,0,,可以解决这所有的问题
Dialogue: 0,0:39:32.68,0:39:34.60,Default,,0,0,0,,这种最简单的机制
Dialogue: 0,0:39:35.58,0:39:37.79,Default,,0,0,0,,却可以给予你最好的模块性
Dialogue: 0,0:39:39.21,0:39:41.31,Default,,0,0,0,,它赋予你所有已知的模块机制
Dialogue: 0,0:39:45.59,0:39:48.24,Default,,0,0,0,,好的 该休息一会儿了 谢谢大家
Dialogue: 0,0:39:48.24,0:40:01.08,Default,,0,0,0,,[音乐]
Dialogue: 0,0:40:01.28,0:40:04.75,Declare,,0,0,0,,{\an2\fad(500,500)}《计算机程序的构造和解释》
Dialogue: 0,0:40:25.69,0:40:29.42,Declare,,0,0,0,,{\an2\fad(500,500)}讲师：哈罗德·艾伯森教授 及 格兰德·杰·萨斯曼教授
Dialogue: 0,0:40:30.01,0:40:33.28,Declare,,0,0,0,,{\an2\fad(500,500)}《计算机程序的构造和解释》
Dialogue: 0,0:40:34.17,0:40:37.61,Declare,,0,0,0,,{\an2\fad(500,500)}元循环求值器 II
Dialogue: 0,0:40:42.32,0:40:44.28,Default,,0,0,0,,教授：昨天你们学习流的时候
Dialogue: 0,0:40:46.01,0:40:51.16,Default,,0,0,0,,Hal教授告诉了你们求值顺序
Dialogue: 0,0:40:51.95,0:40:53.87,Default,,0,0,0,,以及过程的延迟求值
Dialogue: 0,0:40:55.61,0:40:58.30,Default,,0,0,0,,昨天在讲流的时候 我们说
Dialogue: 0,0:41:00.25,0:41:04.22,Default,,0,0,0,,调用者和被调者都应该认同
Dialogue: 0,0:41:05.77,0:41:08.84,Default,,0,0,0,,参数是被延迟了的
Dialogue: 0,0:41:09.42,0:41:13.44,Default,,0,0,0,,如果被调者需要结果 就需要对参数FORCE
Dialogue: 0,0:41:15.13,0:41:17.87,Default,,0,0,0,,因此在过程的设计者和使用者之间
Dialogue: 0,0:41:18.17,0:41:24.32,Default,,0,0,0,,就有很多关于延时求值的握手
Dialogue: 0,0:41:26.36,0:41:28.72,Default,,0,0,0,,当然 这看起来相当糟糕
Dialogue: 0,0:41:29.48,0:41:30.96,Default,,0,0,0,,虽说对于流没什么不妥
Dialogue: 0,0:41:31.74,0:41:32.86,Default,,0,0,0,,但作为一般性的原则来说
Dialogue: 0,0:41:32.92,0:41:36.32,Default,,0,0,0,,我们希望能有个地方
Dialogue: 0,0:41:36.46,0:41:38.49,Default,,0,0,0,,能够把我们的设计考虑
Dialogue: 0,0:41:38.89,0:41:41.28,Default,,0,0,0,,显式地、清晰地
Dialogue: 0,0:41:41.63,0:41:43.93,Default,,0,0,0,,标注出来
Dialogue: 0,0:41:45.88,0:41:49.28,Default,,0,0,0,,因此就不必在过程编写者
Dialogue: 0,0:41:50.46,0:41:54.89,Default,,0,0,0,,和使用者之间达成共识
Dialogue: 0,0:41:55.08,0:41:57.98,Default,,0,0,0,,有关于参数求值
Dialogue: 0,0:41:58.43,0:41:59.50,Default,,0,0,0,,以及求值顺序等细节
Dialogue: 0,0:41:59.50,0:42:00.75,Default,,0,0,0,,虽然 这也不是太糟糕
Dialogue: 0,0:42:01.02,0:42:03.95,Default,,0,0,0,,我的意思是 可能还有像“输入是一个数字”这样的共识
Dialogue: 0,0:42:05.20,0:42:06.08,Default,,0,0,0,,但是
Dialogue: 0,0:42:06.35,0:42:09.20,Default,,0,0,0,,如果其中一个人可以全权负责 就再好不过了
Dialogue: 0,0:42:11.02,0:42:13.31,Default,,0,0,0,,这个想法已经不算新潮了
Dialogue: 0,0:42:15.51,0:42:21.16,Default,,0,0,0,,Algol60就支持两种不同的过程调用方法
Dialogue: 0,0:42:22.02,0:42:24.28,Default,,0,0,0,,参数可以按名或按值传递
Dialogue: 0,0:42:25.59,0:42:27.48,Default,,0,0,0,,按名传递就意味着
Dialogue: 0,0:42:27.63,0:42:29.72,Default,,0,0,0,,参数会延时求值
Dialogue: 0,0:42:31.11,0:42:32.84,Default,,0,0,0,,当你按名传递一个参数时
Dialogue: 0,0:42:33.64,0:42:36.52,Default,,0,0,0,,只有你去取它的值的时候
Dialogue: 0,0:42:36.96,0:42:39.55,Default,,0,0,0,,它的值才会被计算出来
Dialogue: 0,0:42:42.29,0:42:44.20,Default,,0,0,0,,所以 现在我就要
Dialogue: 0,0:42:44.43,0:42:46.96,Default,,0,0,0,,像之前那样
Dialogue: 0,0:42:46.99,0:42:48.65,Default,,0,0,0,,对语言做出一些小小的修改
Dialogue: 0,0:42:50.32,0:42:51.79,Default,,0,0,0,,这里 我们再添加一个新的FEATURE
Dialogue: 0,0:42:53.37,0:42:55.05,Default,,0,0,0,,我们要添加的FEATURE是
Dialogue: 0,0:42:55.36,0:42:58.73,Default,,0,0,0,,“按名传递参数” 或者可以叫做“延迟求值参数”
Dialogue: 0,0:43:00.43,0:43:04.41,Default,,0,0,0,,因为事实上 Lisp系统中默认
Dialogue: 0,0:43:04.76,0:43:06.60,Default,,0,0,0,,传递的是一个指针
Dialogue: 0,0:43:08.22,0:43:09.15,Default,,0,0,0,,指针被复制了一份
Dialogue: 0,0:43:09.15,0:43:10.91,Default,,0,0,0,,但所指的数据结构却没有被复制
Dialogue: 0,0:43:13.41,0:43:14.84,Default,,0,0,0,,现在我要告诉你们
Dialogue: 0,0:43:15.04,0:43:18.38,Default,,0,0,0,,如何来添加按名传递参数
Dialogue: 0,0:43:19.99,0:43:22.12,Default,,0,0,0,,为什么我们需要这样的FEATURE呢？
Dialogue: 0,0:43:23.10,0:43:24.72,Default,,0,0,0,,假设我们想要开发
Dialogue: 0,0:43:25.24,0:43:28.44,Default,,0,0,0,,像是某种特殊形式的功能
Dialogue: 0,0:43:28.73,0:43:29.72,Default,,0,0,0,,类似于“保留字”
Dialogue: 0,0:43:29.72,0:43:31.48,Default,,0,0,0,,但不是用保留字的方式来实现
Dialogue: 0,0:43:32.18,0:43:34.76,Default,,0,0,0,,我想用过程来实现类似IF的效果
Dialogue: 0,0:43:36.36,0:43:39.42,Default,,0,0,0,,无论是IF还是COND 都是特殊形式
Dialogue: 0,0:43:39.42,0:43:40.43,Default,,0,0,0,,它俩是一样的
Dialogue: 0,0:43:40.59,0:43:42.86,Default,,0,0,0,,这个特殊形式用于
Dialogue: 0,0:43:42.92,0:43:45.02,Default,,0,0,0,,根据谓词返回真假
Dialogue: 0,0:43:46.22,0:43:49.76,Default,,0,0,0,,决定求值真子句还是假子句
Dialogue: 0,0:43:50.84,0:43:53.12,Default,,0,0,0,,它们都是根据某个值
Dialogue: 0,0:43:53.44,0:43:55.36,Default,,0,0,0,,来决定是否去做另外的某件事
Dialogue: 0,0:43:57.27,0:43:58.88,Default,,0,0,0,,然而像+之类的过程
Dialogue: 0,0:43:59.15,0:44:01.20,Default,,0,0,0,,也就是那些我们现在可以定义的过程
Dialogue: 0,0:44:01.42,0:44:06.56,Default,,0,0,0,,是在应用前就求值所有的参数
Dialogue: 0,0:44:08.67,0:44:09.64,Default,,0,0,0,,因此 举例来说
Dialogue: 0,0:44:10.46,0:44:12.41,Default,,0,0,0,,假设我想定义一个过程
Dialogue: 0,0:44:15.39,0:44:18.75,Default,,0,0,0,,用IF来实现与IF相反的效果
Dialogue: 0,0:44:19.85,0:44:20.70,Default,,0,0,0,,我叫它UNLESS
Dialogue: 0,0:44:24.89,0:44:27.47,Default,,0,0,0,,参数是 谓词P、真子句C和假子句A
Dialogue: 0,0:44:28.67,0:44:30.44,Default,,0,0,0,,接下来 我想
Dialogue: 0,0:44:30.46,0:44:32.08,Default,,0,0,0,,用COND来实现
Dialogue: 0,0:44:32.64,0:44:36.72,Default,,0,0,0,,(COND ((NOT P)
Dialogue: 0,0:44:38.96,0:44:40.32,Default,,0,0,0,,结果就是真子句C
Dialogue: 0,0:44:41.58,0:44:45.63,Default,,0,0,0,,否则就是假子句A
Dialogue: 0,0:44:51.29,0:44:52.76,Default,,0,0,0,,我定义这个过程是为了
Dialogue: 0,0:44:53.32,0:44:55.40,Default,,0,0,0,,请考虑下面这种情况
Dialogue: 0,0:44:56.92,0:45:04.12,Default,,0,0,0,,(UNLESS (= 1 0)
Dialogue: 0,0:45:05.08,0:45:06.64,Default,,0,0,0,,那么结果就是2
Dialogue: 0,0:45:07.90,0:45:11.35,Default,,0,0,0,,否则 结果就是(/ 1 0)
Dialogue: 0,0:45:15.92,0:45:18.91,Default,,0,0,0,,这段代码相当于进行这样的代换：
Dialogue: 0,0:45:20.00,0:45:23.26,Default,,0,0,0,,用(= 1 0)、2和(/ 1 0)
Dialogue: 0,0:45:23.66,0:45:24.76,Default,,0,0,0,,分别代换上面的P、C以及A
Dialogue: 0,0:45:25.58,0:45:27.58,Default,,0,0,0,,这样很有趣
Dialogue: 0,0:45:28.11,0:45:30.33,Default,,0,0,0,,代换后就变成了
Dialogue: 0,0:45:30.75,0:45:38.44,Default,,0,0,0,,(COND ((NOT (= 1 0))
Dialogue: 0,0:45:40.62,0:45:42.54,Default,,0,0,0,,结果就是2
Dialogue: 0,0:45:44.28,0:45:45.10,Default,,0,0,0,,否则就是
Dialogue: 0,0:45:48.22,0:45:51.16,Default,,0,0,0,,(/ 1 0)
Dialogue: 0,0:45:54.48,0:45:56.48,Default,,0,0,0,,你们也知道 如果向Lisp中输入这段代码
Dialogue: 0,0:45:57.74,0:45:58.59,Default,,0,0,0,,结果会是2
Dialogue: 0,0:45:59.97,0:46:01.32,Default,,0,0,0,,这没问题
Dialogue: 0,0:46:02.91,0:46:04.64,Default,,0,0,0,,但如果我输入的是这段代码
Dialogue: 0,0:46:05.28,0:46:07.79,Default,,0,0,0,,由于参数会在过程调用前求值
Dialogue: 0,0:46:09.12,0:46:10.73,Default,,0,0,0,,那么这段代码就会报错
Dialogue: 0,0:46:13.38,0:46:15.61,Default,,0,0,0,,当然 如果成功进行代换的话
Dialogue: 0,0:46:16.03,0:46:16.88,Default,,0,0,0,,我可以得到正确的结果
Dialogue: 0,0:46:16.88,0:46:20.16,Default,,0,0,0,,但是这里这种情况 代换并不能进行
Dialogue: 0,0:46:22.17,0:46:23.86,Default,,0,0,0,,我连错误的结果都无法得到
Dialogue: 0,0:46:23.86,0:46:24.67,Default,,0,0,0,,没有结果
Dialogue: 0,0:46:24.80,0:46:25.60,Default,,0,0,0,,只能得到错误
Dialogue: 0,0:46:28.42,0:46:31.21,Default,,0,0,0,,现在 我要想办法
Dialogue: 0,0:46:31.61,0:46:32.99,Default,,0,0,0,,使这样的定义可以成功运行
Dialogue: 0,0:46:34.48,0:46:36.51,Default,,0,0,0,,但我想标注出
Dialogue: 0,0:46:36.70,0:46:38.76,Default,,0,0,0,,C和A是特殊的东西
Dialogue: 0,0:46:39.93,0:46:43.15,Default,,0,0,0,,我想使它们自动地延时求值
Dialogue: 0,0:46:44.27,0:46:48.08,Default,,0,0,0,,我不想它们在我调用过程的时候
Dialogue: 0,0:46:48.52,0:46:49.74,Default,,0,0,0,,就被求值
Dialogue: 0,0:46:51.52,0:46:52.72,Default,,0,0,0,,所以我得先制定一种声明
Dialogue: 0,0:46:52.75,0:46:55.32,Default,,0,0,0,,然后再考虑如何实现此种声明
Dialogue: 0,0:46:55.60,0:46:57.63,Default,,0,0,0,,再次强调 希望你们能够提醒自己
Dialogue: 0,0:46:57.79,0:47:00.25,Default,,0,0,0,,我们这里添加的是临时组件
Dialogue: 0,0:47:00.76,0:47:02.16,Default,,0,0,0,,必须要知道
Dialogue: 0,0:47:02.25,0:47:04.72,Default,,0,0,0,,滥用临时组件会造成大混乱
Dialogue: 0,0:47:05.75,0:47:09.79,Default,,0,0,0,,还会破坏一些已有的东西
Dialogue: 0,0:47:10.12,0:47:12.70,Default,,0,0,0,,首先 它会造成语法歧义性么？
Dialogue: 0,0:47:13.86,0:47:15.50,Default,,0,0,0,,就我们目前已有的语法来说
Dialogue: 0,0:47:15.71,0:47:16.91,Default,,0,0,0,,它不会造成什么歧义
Dialogue: 0,0:47:17.84,0:47:20.76,Default,,0,0,0,,但接下来要做的却可能招来麻烦
Dialogue: 0,0:47:21.67,0:47:24.67,Default,,0,0,0,,我要添加的东西可能会跟
Dialogue: 0,0:47:25.15,0:47:27.10,Default,,0,0,0,,我以后添加的类型声明冲突
Dialogue: 0,0:47:28.19,0:47:31.08,Default,,0,0,0,,类型系统通过提供已知的类型信息
Dialogue: 0,0:47:31.21,0:47:33.66,Default,,0,0,0,,使得语言系统或者编译器可以做出优化
Dialogue: 0,0:47:34.75,0:47:36.97,Default,,0,0,0,,当然也会与我想添加的形式参数的
Dialogue: 0,0:47:37.00,0:47:39.71,Default,,0,0,0,,其它类型的声明相冲突
Dialogue: 0,0:47:40.57,0:47:42.56,Default,,0,0,0,,所以这里我并不打算做一个一般性的机制
Dialogue: 0,0:47:43.77,0:47:45.24,Default,,0,0,0,,使得我可以添加声明
Dialogue: 0,0:47:45.28,0:47:46.54,Default,,0,0,0,,虽然我很想那么做
Dialogue: 0,0:47:46.89,0:47:48.81,Default,,0,0,0,,但现在并不打算这么做
Dialogue: 0,0:47:51.01,0:47:53.88,Default,,0,0,0,,接下来 我要添加某种临时的解决方法
Dialogue: 0,0:47:57.56,0:48:08.38,Default,,0,0,0,,(DEFINE (UNLESS P
Dialogue: 0,0:48:08.81,0:48:10.27,Default,,0,0,0,,后面的参数都是按名调用
Dialogue: 0,0:48:12.78,0:48:15.28,Default,,0,0,0,,分别记作(NAME C)和(NAME A)
Dialogue: 0,0:48:19.85,0:48:25.28,Default,,0,0,0,,哈 哈 卡在黑板边了
Dialogue: 0,0:48:31.76,0:48:35.61,Default,,0,0,0,,(COND ((NOT P) C)
Dialogue: 0,0:48:36.80,0:48:41.16,Default,,0,0,0,,(ELSE A)))
Dialogue: 0,0:48:44.67,0:48:46.88,Default,,0,0,0,,我可以显式地声明
Dialogue: 0,0:48:47.55,0:48:51.65,Default,,0,0,0,,哪些参数按名称传递或延时求值
Dialogue: 0,0:48:55.60,0:48:58.48,Default,,0,0,0,,对解释器的这个修改并不简单
Dialogue: 0,0:48:58.70,0:48:59.77,Default,,0,0,0,,反而相当复杂
Dialogue: 0,0:49:00.45,0:49:03.10,Default,,0,0,0,,我们之前介绍的动态绑定
Dialogue: 0,0:49:03.40,0:49:06.89,Default,,0,0,0,,或者让过程支持不定数目的参数
Dialogue: 0,0:49:07.50,0:49:08.52,Default,,0,0,0,,都相对简单
Dialogue: 0,0:49:09.28,0:49:11.28,Default,,0,0,0,,这次的修改涉及基本策略
Dialogue: 0,0:49:12.32,0:49:13.39,Default,,0,0,0,,这里的问题是
Dialogue: 0,0:49:13.96,0:49:17.63,Default,,0,0,0,,我们的解释器 就如代码所写的那样
Dialogue: 0,0:49:17.96,0:49:23.40,Default,,0,0,0,,在求值组合式时
Dialogue: 0,0:49:24.24,0:49:25.92,Default,,0,0,0,,先通过求值运算符取得过程
Dialogue: 0,0:49:26.20,0:49:30.35,Default,,0,0,0,,然后再求值运算对象得到参数
Dialogue: 0,0:49:30.76,0:49:35.26,Default,,0,0,0,,再把过程应用到参数上
Dialogue: 0,0:49:36.38,0:49:37.07,Default,,0,0,0,,然而这里
Dialogue: 0,0:49:37.36,0:49:41.48,Default,,0,0,0,,直到我检查了整个过程
Dialogue: 0,0:49:41.74,0:49:43.66,Default,,0,0,0,,确定了程序的声明
Dialogue: 0,0:49:44.62,0:49:46.86,Default,,0,0,0,,才会去求值程序的参数
Dialogue: 0,0:49:49.59,0:49:50.59,Default,,0,0,0,,我们来看这个
Dialogue: 0,0:49:52.68,0:49:56.54,Default,,0,0,0,,这是修改后的求值器
Dialogue: 0,0:49:57.48,0:50:01.15,Default,,0,0,0,,我是基于那个最简单的词法作用域求值器
Dialogue: 0,0:50:01.72,0:50:02.65,Default,,0,0,0,,不是动态绑定的那个
Dialogue: 0,0:50:04.14,0:50:08.20,Default,,0,0,0,,但是却要做一些类似于动态绑定的修改
Dialogue: 0,0:50:09.75,0:50:11.45,Default,,0,0,0,,这是因为
Dialogue: 0,0:50:11.90,0:50:13.34,Default,,0,0,0,,如果我延时一个过程 --
Dialogue: 0,0:50:13.66,0:50:15.15,Default,,0,0,0,,哦说错了 -- 延时一个过程的参数
Dialogue: 0,0:50:15.40,0:50:17.52,Default,,0,0,0,,就必须把当前的环境和参数关联在一起
Dialogue: 0,0:50:19.36,0:50:21.55,Default,,0,0,0,,还记得Hal教授如何实现DELAY的吧？
Dialogue: 0,0:50:23.38,0:50:25.44,Default,,0,0,0,,Hal教授把DELAY实现为
Dialogue: 0,0:50:25.50,0:50:27.47,Default,,0,0,0,,一个无参过程
Dialogue: 0,0:50:28.56,0:50:30.52,Default,,0,0,0,,用来执行某些表达式
Dialogue: 0,0:50:31.18,0:50:36.94,Default,,0,0,0,,就是这样让表达式延迟求值的
Dialogue: 0,0:50:39.29,0:50:40.99,Default,,0,0,0,,(DELAY E)实际上是这个
Dialogue: 0,0:50:44.52,0:50:46.92,Default,,0,0,0,,然而 如果我求值这个LAMBDA表达式
Dialogue: 0,0:50:47.42,0:50:49.20,Default,,0,0,0,,我就必须得捕获当前环境
Dialogue: 0,0:50:51.41,0:50:53.45,Default,,0,0,0,,这是因为
Dialogue: 0,0:50:54.60,0:50:56.32,Default,,0,0,0,,我想让这其中的变量的值
Dialogue: 0,0:50:57.02,0:51:00.83,Default,,0,0,0,,取决于它们被定义时的上下文
Dialogue: 0,0:51:04.01,0:51:05.76,Default,,0,0,0,,这也就是为什么要用LAMBDA表达式
Dialogue: 0,0:51:06.62,0:51:07.50,Default,,0,0,0,,这才是正确的
Dialogue: 0,0:51:08.07,0:51:15.12,Default,,0,0,0,,(FORCE E)则相当于
Dialogue: 0,0:51:16.52,0:51:20.08,Default,,0,0,0,,无参地调用这个过程
Dialogue: 0,0:51:21.09,0:51:22.28,Default,,0,0,0,,恰恰和上面相对
Dialogue: 0,0:51:24.10,0:51:26.94,Default,,0,0,0,,这个调用产生的环境则是
Dialogue: 0,0:51:27.36,0:51:29.90,Default,,0,0,0,,定义这个过程时的环境
Dialogue: 0,0:51:30.81,0:51:32.36,Default,,0,0,0,,额外加上一个空框架
Dialogue: 0,0:51:33.23,0:51:34.41,Default,,0,0,0,,我并不在意它
Dialogue: 0,0:51:36.24,0:51:39.40,Default,,0,0,0,,我们再来看这张幻灯片
Dialogue: 0,0:51:40.99,0:51:43.72,Default,,0,0,0,,仔细观察一会儿
Dialogue: 0,0:51:44.14,0:51:46.12,Default,,0,0,0,,会发现大部分跟以前相同
Dialogue: 0,0:51:46.35,0:51:50.65,Default,,0,0,0,,只是对应用或组合式的处理不同
Dialogue: 0,0:51:51.98,0:51:53.71,Default,,0,0,0,,处理组合式分两步
Dialogue: 0,0:51:54.68,0:51:57.79,Default,,0,0,0,,首先要求值这个过程
Dialogue: 0,0:51:57.92,0:51:59.88,Default,,0,0,0,,我就必须通过求值运算符来得到对应过程
Dialogue: 0,0:52:00.70,0:52:01.69,Default,,0,0,0,,也就是这一部分
Dialogue: 0,0:52:02.38,0:52:04.35,Default,,0,0,0,,我得这个值是计算求出的现值
Dialogue: 0,0:52:04.46,0:52:05.76,Default,,0,0,0,,而不是一个延时对象
Dialogue: 0,0:52:06.36,0:52:09.85,Default,,0,0,0,,也就要求值它在被延时前的表达式
Dialogue: 0,0:52:10.73,0:52:12.08,Default,,0,0,0,,接下来我就要
Dialogue: 0,0:52:12.24,0:52:17.32,Default,,0,0,0,,把它应用于运算对象
Dialogue: 0,0:52:18.03,0:52:19.61,Default,,0,0,0,,但我仍然要保持这个环境
Dialogue: 0,0:52:19.63,0:52:20.92,Default,,0,0,0,,并将其传递过去
Dialogue: 0,0:52:21.53,0:52:23.71,Default,,0,0,0,,如果有一些运算对象是延时了的
Dialogue: 0,0:52:23.71,0:52:27.53,Default,,0,0,0,,我就需要为这些运算对象附上相应的环境
Dialogue: 0,0:52:29.66,0:52:31.52,Default,,0,0,0,,这里的处理相当复杂
Dialogue: 0,0:52:32.99,0:52:34.24,Default,,0,0,0,,来看看APPLY中对应的部分
Dialogue: 0,0:52:36.40,0:52:38.72,Default,,0,0,0,,APPLY这一部分处理基本过程
Dialogue: 0,0:52:39.36,0:52:40.60,Default,,0,0,0,,这和之前一样
Dialogue: 0,0:52:42.61,0:52:44.68,Default,,0,0,0,,但复合过程部分就比较有意思了
Dialogue: 0,0:52:47.25,0:52:49.52,Default,,0,0,0,,和之前一样 我需要求值过程体
Dialogue: 0,0:52:50.48,0:52:51.98,Default,,0,0,0,,基于的环境是
Dialogue: 0,0:52:52.28,0:52:54.97,Default,,0,0,0,,把形式参数和
Dialogue: 0,0:52:55.61,0:53:00.29,Default,,0,0,0,,实际参数绑定在一起的结果
Dialogue: 0,0:53:00.29,0:53:01.07,Default,,0,0,0,,是这样的
Dialogue: 0,0:53:01.53,0:53:03.82,Default,,0,0,0,,环境来自于过程对象
Dialogue: 0,0:53:03.82,0:53:06.65,Default,,0,0,0,,因为我们的语言是词法作用域、静态绑定的
Dialogue: 0,0:53:08.04,0:53:11.82,Default,,0,0,0,,然而 我还需要去掉NAME声明
Dialogue: 0,0:53:11.84,0:53:12.84,Default,,0,0,0,,获得变量的实际名字
Dialogue: 0,0:53:12.84,0:53:15.20,Default,,0,0,0,,这是由VNAMES过程完成的
Dialogue: 0,0:53:15.45,0:53:16.67,Default,,0,0,0,,然后要做的就是
Dialogue: 0,0:53:16.97,0:53:18.86,Default,,0,0,0,,处理这些声明
Dialogue: 0,0:53:19.13,0:53:21.52,Default,,0,0,0,,决定这些运算对象中
Dialogue: 0,0:53:21.76,0:53:23.92,Default,,0,0,0,,现在它们还是形式参数 而非实际参数
Dialogue: 0,0:53:24.09,0:53:25.87,Default,,0,0,0,,哪些运算对象需要立即求值
Dialogue: 0,0:53:26.62,0:53:30.20,Default,,0,0,0,,而哪些运算对象又要
Dialogue: 0,0:53:30.99,0:53:33.77,Default,,0,0,0,,用某种方式封装为延时对象
Dialogue: 0,0:53:37.28,0:53:40.08,Default,,0,0,0,,另外 在处理基本过程这里
Dialogue: 0,0:53:40.60,0:53:42.38,Default,,0,0,0,,当遇到像+这样的基本过程
Dialogue: 0,0:53:42.68,0:53:45.58,Default,,0,0,0,,它们的参数最好立即求值
Dialogue: 0,0:53:45.82,0:53:47.39,Default,,0,0,0,,也就我们需要是这里FORCE这些表达式
Dialogue: 0,0:53:47.92,0:53:50.38,Default,,0,0,0,,EVLIST中完成了很多FORCE操作
Dialogue: 0,0:53:51.34,0:53:52.78,Default,,0,0,0,,现在 我们有了两种不同的EVLIST
Dialogue: 0,0:53:52.78,0:53:54.09,Default,,0,0,0,,EVLIST和GEVLIST
Dialogue: 0,0:53:54.52,0:53:57.16,Default,,0,0,0,,GEVLIST封装延迟参数
Dialogue: 0,0:53:57.18,0:53:59.74,Default,,0,0,0,,而对另外的参数立即求值
Dialogue: 0,0:53:59.87,0:54:05.85,Default,,0,0,0,,而EVLIST则会FORCE所有的表达式
Dialogue: 0,0:54:07.90,0:54:09.16,Default,,0,0,0,,简单地看下EVLIST的代码
Dialogue: 0,0:54:09.69,0:54:11.98,Default,,0,0,0,,课后你们一定要亲自上手试试
Dialogue: 0,0:54:12.25,0:54:14.67,Default,,0,0,0,,光是听我在这里讲课
Dialogue: 0,0:54:14.72,0:54:18.20,Default,,0,0,0,,可不能够学到求值器的不同变种
Dialogue: 0,0:54:19.52,0:54:21.24,Default,,0,0,0,,你们需要上手亲自实践一下。
Dialogue: 0,0:54:21.37,0:54:23.84,Default,,0,0,0,,你试验过后 对它们有了感悟
Dialogue: 0,0:54:24.22,0:54:27.02,Default,,0,0,0,,你才能理解各种可能的设计决策
Dialogue: 0,0:54:27.77,0:54:29.16,Default,,0,0,0,,才能清楚它们如何相互关联
Dialogue: 0,0:54:29.93,0:54:32.38,Default,,0,0,0,,了解求值器描述的是何种语言
Dialogue: 0,0:54:33.16,0:54:34.64,Default,,0,0,0,,以及构建一门合理的语言
Dialogue: 0,0:54:34.94,0:54:36.32,Default,,0,0,0,,需要哪些一致性集合
Dialogue: 0,0:54:37.20,0:54:40.06,Default,,0,0,0,,哪些临时方案又是复杂而无用
Dialogue: 0,0:54:41.85,0:54:44.68,Default,,0,0,0,,就和我说得一样 这里的EVLIST
Dialogue: 0,0:54:44.81,0:54:46.03,Default,,0,0,0,,参数之一为运算对象表
Dialogue: 0,0:54:46.70,0:54:50.28,Default,,0,0,0,,表中的元素会在求值之后被取消延时
Dialogue: 0,0:54:50.75,0:54:51.90,Default,,0,0,0,,它们都会被FORCE
Dialogue: 0,0:54:53.28,0:54:54.44,Default,,0,0,0,,无论它们是否为延时对象
Dialogue: 0,0:54:56.05,0:54:58.51,Default,,0,0,0,,下一个 GEVLIST
Dialogue: 0,0:55:01.26,0:55:01.85,Default,,0,0,0,,谢谢
Dialogue: 0,0:55:04.04,0:55:06.35,Default,,0,0,0,,我们在这里会发现
Dialogue: 0,0:55:07.80,0:55:09.61,Default,,0,0,0,,这里面有多种可能
Dialogue: 0,0:55:09.81,0:55:11.52,Default,,0,0,0,,要么是普通的情况
Dialogue: 0,0:55:12.48,0:55:13.69,Default,,0,0,0,,比如元素直接是一个符号
Dialogue: 0,0:55:13.74,0:55:16.20,Default,,0,0,0,,就像UNLESS中的参数P那样
Dialogue: 0,0:55:17.64,0:55:18.81,Default,,0,0,0,,对应这一部分代码
Dialogue: 0,0:55:19.39,0:55:22.49,Default,,0,0,0,,在这种情况下 我们就用应用序来求值
Dialogue: 0,0:55:23.34,0:55:25.45,Default,,0,0,0,,基本上就像以前一样
Dialogue: 0,0:55:25.63,0:55:28.84,Default,,0,0,0,,就是将EVAL映射在这个表上
Dialogue: 0,0:55:29.95,0:55:32.14,Default,,0,0,0,,换言之 就是先求值第一个表达式
Dialogue: 0,0:55:32.65,0:55:37.36,Default,,0,0,0,,然后在ENV中 求值(GEVLIST (CDR EXPRS))
Dialogue: 0,0:55:37.93,0:55:43.20,Default,,0,0,0,,然而 我们也可能遇到按名传递的参数
Dialogue: 0,0:55:44.00,0:55:45.05,Default,,0,0,0,,如果参数是按名传递
Dialogue: 0,0:55:45.20,0:55:46.59,Default,,0,0,0,,我就需要给它包裹上一个DELAY
Dialogue: 0,0:55:47.00,0:55:50.97,Default,,0,0,0,,DELAY里面就是我想按名调用的表达式
Dialogue: 0,0:55:52.14,0:55:57.74,Default,,0,0,0,,还要附上定义过程时的环境
Dialogue: 0,0:55:59.05,0:56:00.59,Default,,0,0,0,,把它们作为实际参数
Dialogue: 0,0:56:02.79,0:56:05.04,Default,,0,0,0,,然后像这样继续递归处理
Dialogue: 0,0:56:09.07,0:56:11.31,Default,,0,0,0,,这个解释器中另外一个有意思的地方
Dialogue: 0,0:56:11.37,0:56:13.53,Default,,0,0,0,,就在于COND
Dialogue: 0,0:56:14.70,0:56:15.92,Default,,0,0,0,,人们可能就这么来写
Dialogue: 0,0:56:15.93,0:56:17.24,Default,,0,0,0,,然后就不管了
Dialogue: 0,0:56:18.55,0:56:19.98,Default,,0,0,0,,你需要在一处FORCE
Dialogue: 0,0:56:20.51,0:56:23.10,Default,,0,0,0,,COND表达式需要知道
Dialogue: 0,0:56:24.20,0:56:25.90,Default,,0,0,0,,谓词判定结果的真假
Dialogue: 0,0:56:25.99,0:56:26.83,Default,,0,0,0,,就像基本过程那样
Dialogue: 0,0:56:28.55,0:56:30.56,Default,,0,0,0,,求值COND语句时 需要FORCE
Dialogue: 0,0:56:31.72,0:56:33.95,Default,,0,0,0,,剩下的细节就没什么特别的了
Dialogue: 0,0:56:34.62,0:56:36.28,Default,,0,0,0,,就先不深究了
Dialogue: 0,0:56:36.75,0:56:38.99,Default,,0,0,0,,剩下的就是如何实现MAKE-DELAY
Dialogue: 0,0:56:38.99,0:56:40.91,Default,,0,0,0,,延时对象是一种数据结构
Dialogue: 0,0:56:41.31,0:56:44.75,Default,,0,0,0,,它包括：类型标识、表达式以及环境
Dialogue: 0,0:56:44.84,0:56:46.36,Default,,0,0,0,,它的类型标识是'THUNK
Dialogue: 0,0:56:46.96,0:56:48.46,Default,,0,0,0,,这个术语来自于Algol语言
Dialogue: 0,0:56:49.07,0:56:50.81,Default,,0,0,0,,据说这是个拟声词
Dialogue: 0,0:56:50.83,0:56:52.06,Default,,0,0,0,,是把东西压栈的声音
Dialogue: 0,0:56:52.97,0:56:53.41,Default,,0,0,0,,我不太清楚
Dialogue: 0,0:56:53.41,0:56:57.12,Default,,0,0,0,,我既不是Algol学家 又不是Algol程序员
Dialogue: 0,0:56:57.60,0:56:58.38,Default,,0,0,0,,所以我不太清楚
Dialogue: 0,0:56:58.74,0:56:59.64,Default,,0,0,0,,但据说它是那样的
Dialogue: 0,0:57:00.27,0:57:01.56,Default,,0,0,0,,而UNDELAY的定义则是
Dialogue: 0,0:57:01.77,0:57:03.66,Default,,0,0,0,,递归地UNDELAY这些THUNK
Dialogue: 0,0:57:03.69,0:57:06.00,Default,,0,0,0,,直到得到一个非THUNK对象
Dialogue: 0,0:57:07.72,0:57:10.94,Default,,0,0,0,,这就是如何实现Algol中的按名调用
Dialogue: 0,0:57:12.05,0:57:13.76,Default,,0,0,0,,差不多就是这样了
Dialogue: 0,0:57:15.21,0:57:16.25,Default,,0,0,0,,有什么问题吗？
Dialogue: 0,0:57:26.68,0:57:27.52,Default,,0,0,0,,学生：Gerry？
Dialogue: 0,0:57:28.09,0:57:28.80,Default,,0,0,0,,教授：你说 Vesko
Dialogue: 0,0:57:30.03,0:57:32.99,Default,,0,0,0,,学生：我注意到 对于基本过程
Dialogue: 0,0:57:33.44,0:57:34.89,Default,,0,0,0,,你是避免按名调用的
Dialogue: 0,0:57:36.41,0:57:38.38,Default,,0,0,0,,我很想知道 你为什么要这样？
Dialogue: 0,0:57:38.41,0:57:39.21,Default,,0,0,0,,需要这样吗？
Dialogue: 0,0:57:40.07,0:57:41.61,Default,,0,0,0,,教授：Vesko想问的是
Dialogue: 0,0:57:42.06,0:57:46.00,Default,,0,0,0,,基本过程也按名调用是否合理？
Dialogue: 0,0:57:47.14,0:57:48.70,Default,,0,0,0,,答案是：是的
Dialogue: 0,0:57:49.27,0:57:52.32,Default,,0,0,0,,有一种情况下是可以的 实际上是两种
Dialogue: 0,0:57:55.53,0:57:58.27,Default,,0,0,0,,比如用CONS来构造一个数据结构
Dialogue: 0,0:57:59.02,0:58:02.00,Default,,0,0,0,,构建一个元素个数不定的数组时
Dialogue: 0,0:58:03.26,0:58:07.44,Default,,0,0,0,,就没必要求值参数
Dialogue: 0,0:58:07.44,0:58:08.83,Default,,0,0,0,,你只需要创建一些PROMISE
Dialogue: 0,0:58:09.10,0:58:10.81,Default,,0,0,0,,在确实需要时才来求值这些参数
Dialogue: 0,0:58:11.50,0:58:15.08,Default,,0,0,0,,如果我把两个对象CONS起来
Dialogue: 0,0:58:16.24,0:58:17.77,Default,,0,0,0,,那么我CONS这些PROMISE
Dialogue: 0,0:58:17.80,0:58:19.93,Default,,0,0,0,,就和CONS这些对象一样容易
Dialogue: 0,0:58:21.15,0:58:23.37,Default,,0,0,0,,甚至在对它们进行CAR CDR的时候
Dialogue: 0,0:58:23.39,0:58:24.30,Default,,0,0,0,,也不用进行实际的计算
Dialogue: 0,0:58:24.84,0:58:26.97,Default,,0,0,0,,取出PROMISE 并直接传递给其它人
Dialogue: 0,0:58:28.26,0:58:30.51,Default,,0,0,0,,这也就是为什么Alonzo Church用LAMBDA演算
Dialogue: 0,0:58:30.57,0:58:34.03,Default,,0,0,0,,定义的CAR、CDR和CONS说得通的原因
Dialogue: 0,0:58:34.42,0:58:36.32,Default,,0,0,0,,这是因为CAR、CDR以及CONS并没有执行计算
Dialogue: 0,0:58:36.38,0:58:40.06,Default,,0,0,0,,你们可以认为它是在重组数据而已
Dialogue: 0,0:58:40.99,0:58:42.20,Default,,0,0,0,,然而像 + 这样的过程
Dialogue: 0,0:58:42.24,0:58:43.84,Default,,0,0,0,,必须要了解参数是什么
Dialogue: 0,0:58:45.28,0:58:46.91,Default,,0,0,0,,它们需要确认
Dialogue: 0,0:58:47.12,0:58:48.30,Default,,0,0,0,,构成这些数字的比特
Dialogue: 0,0:58:48.32,0:58:50.44,Default,,0,0,0,,除非它们处理的是LAMBDA演算中的数字
Dialogue: 0,0:58:50.44,0:58:51.88,Default,,0,0,0,,这就是另外一码事了
Dialogue: 0,0:58:52.43,0:58:53.58,Default,,0,0,0,,为了运算加法
Dialogue: 0,0:58:53.77,0:58:55.53,Default,,0,0,0,,它需要知道构成数字的比特
Dialogue: 0,0:58:59.21,0:58:59.92,Default,,0,0,0,,因此 实际上
Dialogue: 0,0:59:00.19,0:59:02.78,Default,,0,0,0,,数据的构造过程和选择过程
Dialogue: 0,0:59:03.24,0:59:05.50,Default,,0,0,0,,以及具有副作用的数据对象
Dialogue: 0,0:59:06.27,0:59:09.76,Default,,0,0,0,,在最极端的惰性解释器中
Dialogue: 0,0:59:11.34,0:59:13.39,Default,,0,0,0,,也不需要被FORCE
Dialogue: 0,0:59:16.46,0:59:16.99,Default,,0,0,0,,另外一方面
Dialogue: 0,0:59:17.02,0:59:18.70,Default,,0,0,0,,针对数据结构的谓词需要被FORCE
Dialogue: 0,0:59:19.61,0:59:22.65,Default,,0,0,0,,如果你想判断 这是一个序对吗？
Dialogue: 0,0:59:23.56,0:59:24.40,Default,,0,0,0,,或者是一个符号？
Dialogue: 0,0:59:24.64,0:59:26.57,Default,,0,0,0,,最好搞清楚是什么
Dialogue: 0,0:59:30.30,0:59:31.18,Default,,0,0,0,,还有问题吗？
Dialogue: 0,0:59:40.05,0:59:41.61,Default,,0,0,0,,那好吧 下课
Dialogue: 0,0:59:42.10,1:00:04.56,Declare,,0,0,0,,{\fad(500,500)}MIT OpenCourseWare\Nhttp://ocw.mit.edu
Dialogue: 0,0:59:42.10,1:00:04.56,Declare,,0,0,0,,{\an2\fad(500,500)}本项目主页\Nhttps://github.com/DeathKing/Learning-SICP