#3962: Add flipped fmap
---------------------------------+------------------------------------------
Reporter: basvandijk | Owner:
Type: proposal | Status: new
Priority: normal | Component: libraries/base
Version: 6.12.1 | Keywords:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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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 {{{<**>}}}.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3962>
GHC <http://www.haskell.org/ghc/>
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