[Haskell-cafe] Polyvariadic composition

[Artyom Kazak]

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…