[Haskell] XML Serialization and type constraints

[Tomasz Zielonka]

On Wed, Aug 25, 2004 at 04:14:48PM +0100, Simon D. Foster wrote:
> 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?

Perhaps you could just 'encode Id' that parts?

Best regards,
Tom

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