[Haskell-cafe] instance Monad m => Functor m

[Hans Aberg]

On 9 Apr 2008, at 15:23, Henning Thielemann wrote:
> I also recognized that problem in the past, but didn't know how to  
> solve it. In Haskell 98, methods are resolved using the types of  
> the operands. How would the compiler find out which implementation  
> of (+) to choose for an expression like x+y using your approach?

I might describe the idea via mangling. So if one has
   class Magma (a; unit, mult) where
     unit :: a
     mult :: a -> a -> a
then instances
   Monoid (a; 0; (+))
   Monoid (a; 1; (*))
should logically equivalent to
   Monoid_0_+ (a)
     0 :: a
     (+) :: a -> a -> a

   Monoid_1_* (a)
     1 :: a
     (*) :: a -> a -> a
or whatever internal mangling that ensures that the names Monoid_0_+  
and Monoid_1_* are different.

Would this not work? - They code should be essentially a shortcut for  
defining new classes.

   Hans