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 , which inspired the introduction of Applicative and Alternative in the first place.

On 27 Feb 2015, at 5:45 , Edward Kmett <ekmett@gmail.com> wrote:

On Thu, Feb 26, 2015 at 6:37 PM, Greg Fitzgerald <garious@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@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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