On 31/05/07, Jon Harrop
Is it possible to implement this in Haskell using type classes? Is there any way this could actually be practicable?
I had a go but didn't manage to quite get it. Here's my attempt, and the error it produces: {-# OPTIONS_GHC -fglasgow-exts #-} type Endo a = a -> a -- Dummy instances to satisfy the (Show n, Eq n) => Num n constraints instance Show (Endo a) where show _ = "<function>" instance Eq (Endo a) where _ == _ = False instance Num a => Num (Endo a) where fromInteger x = (fromInteger x *) x + y = \z -> x z + y z x * y = \z -> x z * y z abs x = error "Aaargh." signum x = error "Aaargh." main = print (2 3) /home/david/hs/sandbox/application-multiplication.hs:15:14: No instance for (Num (t -> a)) arising from the literal `2' at /home/david/hs/sandbox/application-multiplication.hs:15:14-16 Possible fix: add an instance declaration for (Num (t -> a)) In the first argument of `print', namely `(2 3)' In the expression: print (2 3) In the definition of `main': main = print (2 3) Failed, modules loaded: none. It seems to be wanting a more general instance than the one I'm providing, for whatever reason. Using print ((2 :: Endo Integer) 3) works, but that's hardly satisfactory. -- -David House, dmhouse@gmail.com