Add flipped fmap
wren ng thornton
wren at community.haskell.org
Thu Apr 8 17:21:07 EDT 2010
Bas van Dijk wrote:
> I would like to propose adding a flipped variant of <$> to Data.Functor:
> (<$$>) ∷ Functor f ⇒ f α → (α → β) → f β
> (<$$>) = flip (<$>)
> Please see the ticket for motivation and patch:
I'm also not in favor.
There is good motivation for having both of (>>=) and (=<<):
* the (=<<) variant provides an applicative style by giving a name for
* and the (>>=) variant provides an imperative style for those who like
that sort of thing.
There is good reason for having (<$>) as an alternative to fmap:
* It also serves to support the applicative style by giving a name for
There is good reason to have (<*>):
* It supports the use of (<$>) for multi-argument functions.
But I don't see much benefit to having (<**>) let alone (<$$>), when
they are easily defined by using flip. The other clutter of Applicative
((<*), (*>),...) can be motivated by performance concerns for certain
applicative combinator parsers, but I'm not a big fan of them either.
(<$$>) offers no performance benefit, does not offer a definition for a
perspicuous type, and does not seem to offer significant improvement
in code legibility which could not be obtained from a local definition.
 For example,
* (=<<) is perspicuous because it names Kleisli application. Given a
prefix name like 'star' it would also be perspicuous for point-free
style where it serves as the superscript star operator used in category
theory. Having both (=<<) and 'star' could even be justified for similar
reasons to having both 'fmap' and (<$>): supporting both applicative and
* forM_ is perspicuous because it captures an intuitive notion of
imperative style, even though mapM_ captures an applicative version of a
* (flip lookup) would be perspicuous for the same reasons as (=<<): it
supports the idea of transforming an assoc-list into the function that
list represents. Similar variations on this theme apply to indexing
functions for Data.Map where the (Map a b -> (a->b)) ordering supports
the idea of transforming the map into its associated function.
* (curry . (flip (uncurry foldr))), (curry . (flip (uncurry either))),
and (curry . (flip (uncurry maybe))) would be perspicuous for capturing
the transformation of concrete types into their Church encodings.
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