[Haskell-cafe] Inferred type not most general?

I'm curious about a type inference oddity. In the code below, if I leave off the type signature for tmap, both GHC and Hugs infer that tmap has type... tmap :: (b -> a, b -> a) -> Twist b b -> Twist a a ...I'm wondering why they couldn't infer the more general... tmap :: (a -> b, c -> d) -> Twist a c -> Twist b d

data Twist a b = Nil | Cons a (Twist b a) deriving Show

x = (Cons "foo" (Cons 1 (Cons "bar" (Cons 2 Nil))))

tmap :: (a->b,c->d) -> Twist a c -> Twist b d tmap _ Nil = Nil tmap (f,g) (Cons x rest) = Cons (f x) (tmap (g,f) rest)

Thanks, Greg Buchholz