[Haskell-cafe] class default method proposal

[Simon Peyton-Jones]

| If it really would work ok we should get it fully specified and
| implemented so we can fix the most obvious class hierarchy problems in a
| nice backwards compatible way. Things are only supposed to be candidates
| for Haskell' if they're already implemented.

Getting it fully specified is the first thing.

Personally I am not keen about

a) coupling it to explicit import/export (independently-desirable though such a change might be)

b) having instance declarations silently spring into existence


Concerning (b) here's a suggestion.  As now, require that every instance requires an instance declaration.  So, in the main example of http://haskell.org/haskellwiki/Class_system_extension_proposal, for a new data type T you'd write
        instance Monad T where
          return = ...
          (>>=)  = ...

        instance Functor T
        instance Applicative T

The instance declaration for (Functor T) works just as usual (no explicit method, so use the default method) except for one thing: how the default method is found.  The change is this:
    Given "instance C T where ...", for any method 'm' not
    defined by "...":
        for every class D of which C is a superclass
        where there is an instance for (D T)
        see if the instance gives a binding for 'm'
    If this search finds exactly one binding, use it,
        otherwise behave as now

This formulation reduces the problem to a more manageable one: a search for the default method.

I'm not sure what is supposed to happen if the instance is for something more complicated (T a, say, or multi-parameter type class) but I bet you could work it out.

All these instances would need to be in the same module:
   - you can't define Functor T without Monad T, because you
        want to pick up the monad-specific default method
   - you can't define Monad T without Functor T, because
        the latter is a superclass of the former

It still sounds a bit complicated.

Simon