[Syntax]Pipe operator (reverse application)

[yannham]

Close #455. Add a reverse application operator to nicely write a sequence of transformation as something like a Unix pipeline.

Usually you want this operator to not take precedence too much, to avoid parentheses around the initial expression such as x * 2 |> nums.pow 2 and such that it can also serve as a Haskell's $, just reversed. I put it at a similar precedence level as in OCaml, where it takes higher precedence than boolean operators like && and ||. However Haskell's $ and Elm |> take the lowest precedence of all operators, even than boolean ones. I'm not sure how much it matters: should foo && bar |> stuff |> otherstuff be interpreted as foo && (bar |> stuff |> otherstuff) or (foo && bar) |> stuff |> otherstuff? I don't have a strong opinion, although I arbitrarily prefer the first option, as is implemented in this PR.

[thufschmitt]

should foo && bar |> stuff |> otherstuff be interpreted as foo && (bar |> stuff |> otherstuff) or (foo && bar) |> stuff |> otherstuff?

I don’t have a strong opinion either, I think either case can make sense or be utterly confusing depending on the line-breaks and indentation. So let’s stick with the first one.

(If anything, the bit where I’d like this to have a very weak precedence is wrt | and :, so that we can write

x
  |> lists.map (fun x => x - 1) : Num -> Num
  |> lists.filter (fun x => x > 2) : Num -> Bool

but that sounds like a false good idea, so let’s forget about it)