I'm sorry this was a typo. Here's the correct one:

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}

class Add a b where
    add :: a -> b

instance Num a => Add a a where
    add = id

instance (Num a, Add a b) => Add a (a -> b) where
    add x y = add (x + y)

I'm trying to use the printf trick to achieve this.

On Tue, Mar 13, 2012 at 9:30 PM, Johannes Waldmann <waldmann@imn.htwk-leipzig.de> wrote:

> Can someone tell me why this is not working

that "someone" is actually ghci:

Prelude> instance (Num a, Add a b) => Add (a -> b) where add x y = add (x + y)

<interactive>:8:30:
Expecting one more argument to `Add (a -> b)'
In the instance declaration for `Add (a -> b)'

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