[Haskell-cafe] Polymorphic algebraic type constructors

[Graham Klyne]

Well, I can live with it well enough, but I wanted to make sure I wasn't 
overlooking something simple (which it appears I'm not).

#g
--

At 13:38 22/06/04 +0100, Adrian Hey wrote:
>I think this was the topic of my very first post to Haskell
>mailing list, many years ago..
>
>  http://www.dcs.gla.ac.uk/mail-www/haskell/msg00452.html
>
>I think the answer is no. Apparently this is feature
>(I still think it's a bug though:-)
>
>Regards
>--
>Adrian Hey
>
>On Monday 21 Jun 2004 6:03 pm, Graham Klyne wrote:
> > If I have a polymorphic algebraic type (T a) with several type
> > constructors, only one of which actually references the type parameter, is
> > there any way to express type conversion for the type-parameter-independent
> > constructors without actually mentioning all the constructors?
> >
> > Here's a simple example based on Either:
> >
> > [[
> > data A = A String deriving (Show, Eq)
> > data B = B String deriving (Show, Eq)
> >
> > f :: (a->b) -> Either String a -> Either String b
> > f g (Right a) = (Right $ g a)
> > f g (Left  s) = (Left s)
> > -- f g (s) = (s) -- doesn't work
> >
> > a2b (A s) = (B s)
> >
> > t1 = f a2b (Left "x")
> > t2 = f a2b (Right (A "y"))
> > ]]
> >
> > The second case for 'f' throws a type error when the constructor 'Left' is
> > omitted, because the type of 's' is fixed to be Either String A when the
> > required result (in this case, because of a2b) is Either String B.
> >
> > #g
> >
> >
> > ------------
> > Graham Klyne
> > For email:
> > http://www.ninebynine.org/#Contact
> >
> > _______________________________________________
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> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe

------------
Graham Klyne
For email:
http://www.ninebynine.org/#Contact