I had it pretty well worked out for single parameter type classes, but I
couldn't see any nice extension to multiple parameters.
On Dec 11, 2007 5:30 PM, 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 _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users