[Haskell-cafe] Polymorphic algebraic type constructors

[Tomasz Zielonka]

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