[Haskell] XML Serialization and type constraints

Simon D. Foster u1sf at dcs.shef.ac.uk
Wed Aug 25 11:14:48 EDT 2004

On Wed, 2004-08-25 at 14:00, Tomasz Zielonka wrote:
> On Wed, Aug 25, 2004 at 01:54:08PM +0100, Simon D. Foster wrote:
> > On Wed, 2004-08-25 at 08:39, Tomasz Zielonka wrote:
> > - snip -
> > 
> > > I hope that helps,
> > > 
> > > Best regards,
> > > Tom
> > 
> > That method works perfectly! Thank you so much! I assume there is no way
> > of achieving this without overlapping instances?

I think I jumped the gun it a bit; it almost works, but when I try to
declare a serializer for a type with several parts e.g.

data Person = Person PackedString PackedString Int

instance (Hook Person t) => Serializer Person t where ...

If I try to call encode on any of the attributes I get;

    Could not deduce (Hook PackedString t)
        from the context (Serializer Person t,
                          Hook Person t,
                          Hook Person t)

Thus, I have to add a (Hook x t) constraint for every type that is part
of the given data-type. For your example try;

data D = D Int  
instance (Mixin D t) => Serializer D t where
  encodePrim t (D n) = "(D " ++ encode t n ++ ")"

Is there anyway of getting around this?



Simon D. Foster <u1sf at dcs.shef.ac.uk>
Sheffield University

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