On Tue, Jun 22, 2004 at 01:29:02PM +0100, MR K P SCHUPKE wrote:

...

data E a b = L1 { r :: a } | L2 { r :: a } | L3 { r :: a } | L4 { r :: a } | R b deriving Show

How is this different from:

data E a b = L1 a | L2 a | R b

f g (R a) = R (g a) f _ other = other

Isn't this more or less what was in the original...

Nope. My f has type (b -> b1) -> E a b -> E a b1

The problem is the type:

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

this line: f _ other = other

says: Either String a == Either String b -- which it doesn't

Record update avoids this because of the way it is translated. Basically, the result of record update is a reconstructed record, but constructors not containing updated fields are not built/copied, so they don't constrain the resulting type. See Haskell 98 Report for details. Anyway, I doubt the OP can benefit from it.

Keean.

Best regards, Tom -- .signature: Too many levels of symbolic links