libraries/base/Data/Functor/Utils.hs¶
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.
