+1 for me on (<$!>). I have inlined it in multiple packages. I'm neutral on the other combinator. -Edward On Sat, Apr 26, 2014 at 3:14 PM, David Luposchainsky < dluposchainsky@googlemail.com> wrote:
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(HTML-rendered version of the text below see [1])
Strictifying Monadic values ===========================
Half a year ago, Johan Tibell proposed adding a strict equivalent to fmap [2]. This discussion got lost in details, it was reopened, it ended in bikeshedding again. This is a third attempt to get this through, with individual options and no alternatives proposed, compiled from the two more popular responses in the other threads.
Please do not propose alternative implementations here, we've been through this twice already. Vote ±1 on each point to show (dis)agreement. I think this functionality should be in the standard libraries in one way or another, regardless of how it's named in the end.
1. Add a strict version of <$>, 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 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.14 (GNU/Linux) Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/
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