In ghc 6.10.1 the ~ constraint is working:
{-# LANGUAGE TypeFamilies #-}{-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} module D where instance (Num a, Num b, a ~ b) => Num (a,b) where (x,y) * (u,v) = (x*u-y*v, x*v+y*u) test1 = (1,1) * (2,2) test2 = (1,1.0)*(2,2)
With ghci: *D> test1 test1 (0,4) *D> test2 test2 (0.0,4.0) *D> :t test1 :t test1 test1 :: (Integer, Integer) *D> :t test2 :t test2 test2 :: (Double, Double) -- Chris _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
