id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
3962,Add flipped fmap,basvandijk,,"The Applicative apply {{{<*>}}} and Monadic bind {{{>>=}}} both have flipped variants: {{{<**>}}} and {{{=<<}}} respectively. The Functor {{{fmap}}} or {{{<$>}}} does not have such a flipped variant.

I would like to propose adding a flipped variant of {{{<$>}}} to {{{Data.Functor}}}:

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

infixl 4 <$>, <$$>
}}}
 
because of the following two reasons:

 * I often use {{{<$>}}} in the last expression of a do-expression to do a final transformation of the last monadic value. When the code of the transformation function is big and spans multiple lines it visually breaks the sequential nature of a do-expression:
{{{
do m1
   m2
   bigPieceOfCodeThatVisuallyBreaks
     theSequentialNatureOfADoExpression <$> m3 
}}}
I would rather like to see:
{{{
do m1
   m2
   m3 <$$> bigPieceOfCodeThatVisuallyBreaks
             theSequentialNatureOfADoExpression
}}}

 * Consistency. As already mentioned, Applicative and Monad have flipped variants but Functor does not. I like the consistency of:
{{{
(<$>)  ∷ Functor f ⇒ (α → β) → (f α → f β)
(<$$>) ∷ Functor f ⇒ f α → (α → β) → f β

(<*>)  ∷ Applicative f ⇒ f (α → β) → (f α → f β)
(<**>) ∷ Applicative f ⇒ f α → f (α → β) → f β

(>>=)  ∷ Monad f ⇒ f α → (α → f β) → f β
(=<<)  ∷ Monad f ⇒ (α → f β) → (f α → f β)
}}}

With regard to naming, I don't particularly like {{{<$$>}}} because it's undirectional. However so are {{{<$>}}}, {{{<*>}}} and {{{<**>}}}. I choose {{{<$$>}}} because it's consistent with {{{<**>}}}.",proposal,closed,normal,Not GHC,libraries/base,6.12.1,wontfix,,,Unknown/Multiple,Unknown/Multiple,None/Unknown,,,,,