(#.) :: Coercible b c => (b -> c) -> (a -> b) -> (a -> c)
(#.) _ = coerce
{-# INLINE (#.) #-}
(.#) :: Coercible a b => (b -> c) -> (a -> b) -> (a -> c)
(.#) f _ = coerce f
{-# INLINE (.#) #-}
The first of these is exported from Data.Functor.Util, and used in many
places inside of base for efficiency over '.' (compose), However no module
in base actually exports these. I have recently been using Data.Coerce more
frequently and think it would be useful to go ahead and export these from
somewhere in base.
For convenience, I will paste the note about (#.) from Data.Functor.Util:
"Note [Function coercion]
~~~~~~~~~~~~~~~~~~~~~~~
Several functions here use (#.) instead of (.) to avoid potential efficiency
problems relating to #7542. The problem, in a nutshell:
If N is a newtype constructor, then N x will always have the same
representation as x (something similar applies for a newtype deconstructor).
However, if f is a function,
N . f = \x -> N (f x)
This looks almost the same as f, but the eta expansion lifts it--the lhs
could
be _|_, but the rhs never is. This can lead to very inefficient code. Thus
we
steal a technique from Shachaf and Edward Kmett and adapt it to the current
(rather clean) setting. Instead of using N . f, we use N #. f, which is
just
coerce f `asTypeOf` (N . f)
That is, we just *pretend* that f has the right type, and thanks to the
safety
of coerce, the type checker guarantees that nothing really goes wrong. We
still
have to be a bit careful, though: remember that #. completely ignores the
*value* of its left operand.
"
