21 Jan
2007
21 Jan
'07
12:42 p.m.
On Sun, Jan 21, 2007 at 06:21:34PM +0100, Hans van Thiel wrote:
class (Num a, Monoid a) => NumMon a where e = 0 add x y = x + y
What am I doing wrong? Many thanks in advance.
Nothing! What you are trying to do, declare a class B containing all members of a class A, is simply not supported in Haskell. This is the closest you'll get with Haskell98:
newtype Wrap a = Wrap (Wrap a) instance Num a => Monoid (Wrap a) where e = Wrap 0 add (Wrap x) (Wrap y) = Wrap (x + y)
This is the closest you'll get with GHC extensions:
{-# OPTIONS_GHC -fallow-undecidable-instances #-} instance Num a => Monoid a where e = 0 add = (+)
Hope this helps.