Lazy type-class resolution

[Tomasz Zielonka]

I forgot to attach the code. Here it is.

Tom

-- 
.signature: Too many levels of symbolic links
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{-# OPTIONS -fglasgow-exts #-}
{-# OPTIONS -fallow-undecidable-instances #-}

module Comp where

newtype F a b = F { runF :: a -> b }

class Fun f a b | f -> a, f -> b where
    apply :: f -> a -> b

instance Fun (a -> b) a b where
    apply f x = f x

class MatchFun a b f | f -> a, f -> b where
    wrapF :: f -> F a b

instance MatchFun a b (a -> b) where
    wrapF f = F f

class MkComp a b where
    compose :: a -> b

instance MatchFun a b f => MkComp f (F a b) where
    compose f = wrapF f

instance ( Compose r f a b
	 , MkComp (a -> b) t
	 , Fun r b1 b
	 , Fun f a b1 )	=> MkComp r (f -> t)
  where
    compose r f = compose (comp r f)

class Compose t f a b | t f -> a, t f -> b where
    comp :: t -> f -> a -> b

instance (Fun t b c, Fun f a b) => Compose t f a c where
    comp t f x = apply t (apply f x)