Generics and type classes

[Simon Peyton-Jones]

What you ask is not easy.  When you ask for
	everywhere special
you ask to apply special to each node of the tree -- and special
requires the MyClass dictionary.  But the library code for 'gmapT' and
'everywhere' don't know about MyClass.  In particular, to do (gmapT
special), poor old gmapT has to find a MyClass dictionary to pass to
each call to special -- and it has no way to do that. 

I bet that you could do what you want by replacing the 'instance MyClass
ExampleType1' by mkTs for special, so special uses run-time type
dispatch to implement the operations in MyClass.  I guess that's what
Keeane is suggesting.

Simon

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org
[mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of MR K P SCHUPKE
| Sent: 02 February 2004 11:48
| To: doc at drjava.de; glasgow-haskell-users at haskell.org
| Subject: Re: Generics and type classes
| 
| Because the 'cast' operator used in generics, works on having a
concrete
| type to cast to. What you need to do is:
| 
| module TypeTest where
| 
| import Data.Generics
| 
| class Data a => MyClass a
| 
| instance MyClass ExampleType1
| instance MyClass ExampleType2
| 
| special :: ExampleType1 -> ExampleType1
| special = ...
| 
| special2 :: ExampleType2 -> ExampleType2
| special2 = ...
| 
| generic :: MyClass a => a -> a
| generic = everywhere (mkT special `extT` special2 ...)
| 
| 	Regards,
| 	Keean.
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