Can you not define functor like Hughes defines a restricted monad
(section 3 in the paper)...
Keean
Arjun Guha wrote:
One way to do roughly what you want is to pass the dictionary yourself:
>data EqDict a = EqDict {
>leq :: a -> a -> Bool }
>
>data EqList a = EqList (EqDict a) [
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
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
>
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
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