Here's a type that fits:
d :: forall b a t c. (F t c b, F t a c) => t -> a -> b
from the following code:
>-# OPTIONS -fglasgow-exts #-} >module Main where > >main :: IO () >main = putStrLn "OK" > >data ID = ID >data HEAD = HEAD >data FST = FST > >class F t a b | t a -> b where > f :: t -> a -> b >instance F ID a a where > f _ a = a >instance F HEAD [a] a where > f _ a = head a >instance F FST (a,b) a where > f _ a = fst a > >d :: (F t a c, F t c b) => t -> a -> b >d t = f t . f t > >t0 a = d ID a >t1 a = d HEAD a >t2 a = d FST a
Keean.
Jim Apple wrote:
Is there a type we can give to
y f = f . f
y id y head y fst
are all typeable?
Jim Apple
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