[Haskell-cafe] Polyvariadic composition

[MigMit]

Works here.

GHC 7.4.2

On Jul 30, 2012, at 11:32 PM, Artyom Kazak <artyom.kazak at gmail.com> wrote:

> 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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