Strictifying monadic values, take 3
David Luposchainsky
dluposchainsky at googlemail.com
Sat Apr 26 19:14:46 UTC 2014
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(HTML-rendered version of the text below see [1])
Strictifying Monadic values
===========================
Half a year ago, Johan Tibell proposed adding a strict
equivalent to fmap [2]. 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 <$>, named <$!>.
```haskell
infixl 4 <$!>
(<$!>) :: Monad m => (a -> b) -> m a -> m b
f <$!> m = do x <- m
return $! f x
```
This is closely related to fmap in terms of functionality,
but requires a Monad constraint.
This would allow defining
```haskell
seqM m = id <$!> m
```
if such a function is needed.
2. Add a seqM function that evaluates the "contents" of a
monadic action to WHNF.
```haskell
seqM :: Monad m => m a -> m a
seqM m = do x <- m
return $! x
```
This is less close to fmap, but allows building other
strict operations (locally and as needed) based on it
easier, for example
```haskell
f <$!> 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
reinvented) very often by different people, reconsider
adding them as well.
A voting period of two weeks (until 10-05-2014) should be
sufficient to allow everyone to join in.
Greetings,
David
Links:
[1]: https://github.com/quchen/articles/blob/master/seqm_proposal.md
[2]: http://www.haskell.org/pipermail/libraries/2013-November/021728.html
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