Hello, I have accidentally written my version of polyvariadic composition combinator, `mcomp`. It differs from Oleg’s version ( http://okmij.org/ftp/Haskell/polyvariadic.html#polyvar-comp ) in three aspects: a) it is simpler, b) it works without enumerating basic cases (all existing types, in other words), and c) it needs more type extensions.
{-# LANGUAGE MultiParamTypeClasses , FunctionalDependencies , FlexibleInstances , UndecidableInstances , TypeFamilies , OverlappingInstances #-}
class Mcomp a ar b br | a br -> b where mcomp :: a -> (ar -> br) -> b
instance (a ~ ar, b ~ br) => Mcomp a ar b br where mcomp a f = f a
instance (Mcomp a ar b br) => Mcomp (x -> a) ar (x -> b) br where mcomp a f = \x -> mcomp (a x) f
My question is: why doesn’t it work when I replace instance (a ~ ar, b ~ br) => Mcomp a ar b br with instance Mcomp a a b b ? I thought that equal letters mean equal types…