You can do this like:

data TTrue = TTrue data TFalse = TFalse

data Nil = Nil data Cons a l = Cons a l

class Constrain c a b | c a -> b where constrain :: c -> a -> b

data ZConstraint = ZConstraint instance Z a b => Constrain ZConstraint a b

class List c l instance List c Nil instance (Constrain c a TTrue,List c l) => List c (Cons a l)

Now define a class 'Z' using a fundep such that 'b' becomes TTrue if 'a' is in Z.

class Z a b | a -> b where instance (TypeEq a Int b,TypeEq a Float b,TOr a b c) => Z a c -- Int and Float are members of Z, TypeEq and TOr come from the -- HList library.

Finally you can constrain the list:

f :: List ZConstraint l -> List ZConstraint l f x = x

Regards, Keean. Bo Herlin wrote:

Hi everyone. Being new to Haskell I wonder how I can make a list contain different types but all implementing the same class, like this:

data X = X data Y = Y

class Z a where f :: a -> Int

instance Z X where f x = 0 instance Z Y where f y = 1

test1 :: Z a => [a] test1 = [X,Y]

test2 = map f test1

Is it possible to make this work?

/Bo Herlin _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

On Fri, 8 Apr 2005, Bo Herlin wrote: (snip)

Is it possible to make this work?

This is an extension beyond the 1998 standard, but http://haskell.org/hawiki/ExistentialTypes may be interesting to you. -- Mark