That seems useful indeed! Using only 'coerce' requires too many type annotations. Would it make sense to generalize the type so it's clear that one argument is unused? (#.) :: Coercible b c => p b c -> (a -> b) -> (a -> c) (.#) :: Coercible a b => (b -> c) -> p a b -> (a -> c) Li-yao On 04/20/2018 03:56 PM, Daniel Cartwright wrote:
(#.) :: 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. "