[Haskell-cafe] Polymorphic algebraic type constructors

[MR K P SCHUPKE]

>(I still think it's a bug though:-)

It is definitely not a bug... you cannot assert that the types:

Either String a
Either String b

are both equal ant not equal at the same time. You either mean:

f :: (a->a) -> Either String a -> Either String a

Or you mean

f :: (a->b) -> Either String a -> Either String b

actually the second form a and b can be the same but they cannot
be unified inside the closure.


One way of doing this is to group all the string types on one side:

so instead of:

data A a = A a | B String | C String | D String

you do:

data A a = A a | B Strings
data Strings = S1 String | S2 String | S3 String

you can then write:

 f :: (a->b) -> A a -> A b
 f g (A a) = (A $ g a)
 f g (B a) = (B a)

and use (S1 . B), (S2 . B), (S3 . B) to access each string.

Keean.