[Haskell-cafe] class default method proposal
simonpj at microsoft.com
Tue Dec 11 12:30:21 EST 2007
| 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.
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