Add flipped fmap

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 <**>.

Trac metadata

Trac field	Value
Version	6.12.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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