Add flipped fmap

[Bas van Dijk]

Lets sum up the discussion so far for adding

(<$$>) ∷ Functor f ⇒ f α → (α → β) → f β
(<$$>) = flip (<$>)

to Data.Functor:


+1 Bas van Dijk:

  + Adds symmetry to the existing API.

  + Useful for not breaking the sequential nature of do-expressions.

  + Likes that the name <$$> is consistent with <**>.

  - Doesn't like the name that much because its "undirectional".


+1 Nicolas Puillard

  + Likes the consistent name.


+1 Felipe Lessa

  + Has needed <$$> many times.

  + "<$$> will be as concise and useful as <$>, maybe even more,
because `flip fmap` isn't valid."

  + Adds symmetry to the existing API.


+1 Edward Kmett

  + Complexity budget is hardly breaked because the function is added
to a single module.

  + "Explaining away its absence is harder than justifying its inclusion."


-1 Johan Tibell:

  - Makes the library more complex.

  - Fears a trend of adding flipped variants for every operator.

  - More operators make user code harder to understand.

  - "(<**>) and (=<<) shouldn't have been added in the first place....
should we really add flipped versions of everything because these
exists?"


-1 Jesper Louis Andersen:

  - "Personally, I wouldn't add it".

  + "But it cannot hurt since only 0.1% of packages are affected".


-1 Isaac Dupree:

  - Disagrees with the consistency argument.


0 Brent Yorhey:

 0 (<**>) is NOT defined as flip (<*>)...(<**>) performs effects in
the opposite order from flip (<*>).

 0 I'm personally fairly neutral on the proposal


? Brian Brunswick


? Heinrich Apfelmus


So +4 and -3 so far.

I must say that Brent Yorheys comment about (<**>) not being flip
(<*>) has made the 'symmetry' argument less convincing and has moved
me a bit closer to the - camp. However, I'm still for adding it
because it can make code nicer.

It would help if some more people can give their +1s or -1s so we will
have some bigger numbers.

Thanks,

Bas