add map back to Functor

[Ryan Reich]

This pragma needs to have a type inference guarantee that the old signature
`map :: (a -> b) -> [a] -> [b]` is applied even after subsituting `fmap ::
(Functor f) => (a -> b) -> f a -> f b`.  There must be approximately one
zillion ad-hoc functions out there written without type signatures that
implicitly depend on inferring a list.

On Fri, Feb 8, 2019 at 11:33 AM Carter Schonwald <carter.schonwald at gmail.com>
wrote:

> You make a good point, we should cook up this desired magic pragma!
>
> Could you sketch out 1-2 examples of what you’d want this pragma to do?
>
> Something like
> {-# method_synonym_of fmap map #-}
> ?
> This would sort of be like a sort of duplicate record field for how names
> are mapped to definitions?
> This example would be “fmap is an alias of map”
>
>
> On Fri, Feb 8, 2019 at 1:46 PM Edward Kmett <ekmett at gmail.com> wrote:
>
>> The real problem with this sketch of a proposal is the lack of a nice
>> migration plan.
>>
>> If it is top level and just calls fmap, then the distinction requires
>> random rote memorization of which one you can define.
>>
>> On the other hand, say you put it in Functor today as an extra member.
>> Currently everybody has definitions in terms of `fmap`, so you'd need to
>> make the two definitions mutual. This would mean enlarging the dictionaries
>> for one of the most common structures in Haskell for a rather far flung
>> removal that breaks everyone.
>>
>> If we had a way (say a pragma or other syntactic form) to say that fmap
>> and map were somehow the 'same name' or something and so that defining one
>> was the same as defining the other, so that this tax didn't exist, I could
>> see how we might get there.
>>
>> That sort of "magic" might be useful for making migration plans to get
>> sequenceA to be sequence and mapM and traverse without requiring folks to
>> memorize which one is in the class and which is a top level definition.
>>
>> Without something like that, I'd remain somewhat inclined against
>> concocting a complicated migration plan to save one letter.
>>
>> -Edward
>>
>> On Fri, Feb 8, 2019 at 12:07 PM Carter Schonwald <
>> carter.schonwald at gmail.com> wrote:
>>
>>> harkening back to the old Haskell 1.5 days, Lets have map back in
>>> functor. it "F"-  strange to not have our lovely maps be functorial by
>>> default.
>>>
>>> I think it would be a genuine boon for old and new haskellers alike, and
>>> those who disagree with this change already use alt-preludes for pedagogy
>>> reasons anyways
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