Proposal: Export (#.) and (.#) from Data.Coerce (Libraries Digest, Vol 176, Issue 8)

[Li-yao Xia]

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.
> "
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