[Haskell-cafe] Crazy idea: overloading function application notation

[David House]

On 31/05/07, Jon Harrop <jon at ffconsultancy.com> wrote:
> 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 at gmail.com