How do I derive a type class for different type parameters?

[Ben_Yu at asc.aon.com]

Hi there,

I'm having a problem with type classes.

I have a class called PrettyPrintable,

For data type:

data Generic a = ......

I need to provide instance of Generic a for type class PrettyPrintable.

The parameter type a could be either PrettyPrintable or Show.

If I write the following, it won't compile:

instance Show a => PrettyPrintable (Generic a) where
  ......

instance PrettyPrintable a => PrettyPrintable (Generic a) where
  ...


How do I achieve this?

Another alternative that I think may work is if I can say "all Show a will
be PrettyPrintable a in terms of this implementation". But it seems that
the following code won't compile either:

instance Show a => PrettyPrintable a where
  ......


Am I doing the design totally wrong? Or there is actually some clever
scheme in Haskell that can do this elegantly?

Thanks.

Ben.




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