seqm_proposal.md

Edit: This proposal was implemented via <$!> in May 2014 (Trac ticket).

Strictifying Monadic values

Half a year ago, Johan Tibell proposed adding a strict equivalent to fmap (link). This discussion got lost in details, it was reopened, it ended in bikeshedding again. This is a third attempt to get this through, with individual options and no alternatives proposed, compiled from the two more popular responses in the other threads.

Please do not propose alternative implementations here, we've been through this twice already. Vote ±1 on each point to show (dis)agreement. I think this functionality should be in the standard libraries in one way or another, regardless of how it's named in the end.

1. Add a strict version of <$>, named <$!>.

infixl 4 <$!>

(<$!>) :: Monad m => (a -> b) -> m a -> m b
f <$!> m = do x <- m
              return $! f x

This is closely related to fmap in terms of functionality, but requires a Monad constraint.

This would allow defining

seqM m = id <$!> m

if such a function is needed.

2. Add a seqM function that evaluates the "contents" of a monadic action to WHNF.

seqM :: Monad m => m a -> m a
seqM m = do x <- m
            return $! x

This is less close to fmap, but allows building other strict operations (locally and as needed) based on it easier, for example

 f <$!>  x = seqM ( f <$>  x)
mf <*!> mx = seqM (mf <*> mx)
mf <*!  mx = seqM (mf <*  mx)
mf  *!> mx = seqM (mf  *> mx)

If these operators then turn out to be used (and locally reinvanted) very often by different people, reconsider adding them as well.