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…
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