The problem seems equivalent to the following: http://pobox.com/~oleg/ftp/Haskell/typecast.html#local-fd That is, the inferred type is too general to chose the appropriate instance. The solution is also the same: either add type annotations to restrict the inferred type (and so make it _match_ the desired instance) -- or use local type inference. Here's the working code of your Haskell embedding of the Forth-like stack language:
{-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-}
main = print $ test ()
test = square . 4
dup (a, b) = (a, (a, b)) mult (a, (b, c)) = (b*a, c) square = mult . dup
instance Num c => Eq (a -> (c, b)) instance Num c => Show (a -> (c, b)) instance (Num c, TypeCast a b) => Num (a -> (c, b)) where fromInteger val stack = (fromInteger val,typeCast stack) [TypeCast elided]