Proposal: Add (<$>) to Prelude as part of the AMP in GHC 7.10-RC3

[S. Doaitse Swierstra]

They definitely should be in both, since the default implementations of <$> and <$

f <$> x = pure f <*> x
f <$   x  = const f <*> x

may be less efficient than a special implementation having more knowledge about applicative.

 Doaitse

PS: for that reason they are for almost 20 years now member of the class IsParser in http://hackage.haskell.org/package/uulib-0.9.16/docs/UU-Parsing-Interface.html <http://hackage.haskell.org/package/uulib-0.9.16/docs/UU-Parsing-Interface.html> , which inspired the introduction of Applicative and Alternative in the first place.






> On 27 Feb 2015, at 5:45 , Edward Kmett <ekmett at gmail.com> wrote:
> 
> On Thu, Feb 26, 2015 at 6:37 PM, Greg Fitzgerald <garious at gmail.com <mailto:garious at gmail.com>> wrote:
> Those operators are for Power Users - why put them in the Prelude? 
> 
> .. because without them that whole Applicative thing that we're bringing in as a superclass of Monad with the AMP is cut off at the knees.
> 
> Applicative sugar is typically used like
> 
> both f (x,y) = (,) <$> x <*> y
> 
> or
> 
> traverse f (x:xs) = (:) <$> f x <*> traverse f xs
> traverse f [] = pure []
> 
> Without (<$>), (<*>) from Applicative is quite difficult to use. 
> 
> both f (x,y) = pure (,) <*> x <*> y
> 
> is much less efficient and
> 
> both f (x,y) = fmap (,) x <*> y
> 
> is painfully far from idiomatic.
> 
> -Edward
> 
> -Greg
> 
> 
> > Dne út 24. 2. 2015 16:39 uživatel Edward Kmett <ekmett at gmail.com <mailto:ekmett at gmail.com>> napsal:
> >>
> >> We have a couple of weeks until the third release candidate for GHC 7.10
> >> goes out the door.
> >>
> >> Along the way with the last couple of release candidates folks have found
> >> some problems with the way we implemented the AMP. [1][2]
> >>
> >> Most notably, we failed to include (<$>) in the Prelude, so the standard
> >> idiom of
> >>
> >> foo <$> bar <*> baz <*> quux
> >>
> >> doesn't work out of the box!
> >>
> >> I'd like to include (<$>) in the Prelude in RC3.
> >>
> >> I'd also like to invite discussion about whether folks believe we should
> >> include (<$) out of the box.
> >>
> >> (<$) has been a member of Functor for a long time, which is only visible
> >> if you import it from Data.Functor or bring in Control.Applicative. There is
> >> an idiom that you use (<*) and (<$) to point to the parts of the structure
> >> that you want to keep the answers from when building longer such Applicative
> >> chains.
> >>
> >> Discussion Period: 2 weeks
> >>
> >> Thank you,
> >> -Edward Kmett
> >>
> >> [1]
> >> http://www.reddit.com/r/haskell/comments/2wzixa/shouldnt_be_in_prelude/ <http://www.reddit.com/r/haskell/comments/2wzixa/shouldnt_be_in_prelude/>
> >> [2] https://plus.google.com/115504368969270249241/posts/URzeDWd7qMp <https://plus.google.com/115504368969270249241/posts/URzeDWd7qMp>
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