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

[Edward Kmett]

On Thu, Feb 26, 2015 at 6:37 PM, Greg Fitzgerald <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>
> 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/
> >> [2] https://plus.google.com/115504368969270249241/posts/URzeDWd7qMp
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> >
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