[Haskell] What is the best way to write adapters?

[Ben_Yu at asc.aon.com]

Hi,

I have a class:

class Sig a where
  getName :: a -> Id
  getParents :: a -> [TypeExp]
  getMethods :: a -> [MethodDef]
  getFields :: a -> [FieldDef]

and a few data structures that are instances of Sig.

They are ClassDef, ProtocolDef, SignatureDef, etc.

Now I have a type Def defined as:

data Def = DefClass ClassDef | DefProtocol ProtocolDef | DefSignature
Signature

And I want to make Def also an instance of Sig.

The code is currently like this:

instance Sig Def where
  getName (DefClass c) = getName c
  getName(DefProtocol p) getName p
  getName(DefSignature s) = getName s
  getParents(DefClass c) = getParents c
  getParents(DefProtocol p) = getParents p
blah blah blah...

But this seems very annoying.

If I have 4 different constructors in Def, and 5 methods of class Sig,
(Please bear with me if I'm using some OO terminology because I'm still a
new FP programmer), I'll have to write 4*5=20 forwarding functions.


I was trying to write something like:
getSig:: Sig a => Def -> a
getSig (DefClass c) = c
getSig (DefProtocol p) = p
...

But this does not compile.


So, is there any nice way to save me from typing the x*y forwarding
functions?

Thanks!

Ben.


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