deriving...

[Andres Loeh]

> Simon Peyton-Jones wrote:
> >  derive( Typeable (T a) )
> >
> >But that means adding 'derive' as a keyword.  Other possibilities:
> >  
> >  deriving( Typeable (T a) )
> >	-- (B) Re-use 'deriving' keyword
> >
> >The trouble with (B) is that the thing inside the parens is different in 
> >this situation than in a data type declaration. 
> >Any other ideas?
> 
>   instance Typeable (T a) deriving

Why not even simply

instance Typeable (T a)

In other words, derivable classes define default
implementations for all their methods.

Advantages:
  (1) no syntax change at all required
  (2) derived class instances can be partially
      redefined by the user

Disadvantages:
  (1) Slightly more work in some cases because
      a complete instance declaration is required.
      Example:

      instance Eq a => Eq (T a)


Cheers,
  Andres