[Haskell-cafe] class default method proposal
Lennart Augustsson
lennart at augustsson.net
Wed Dec 12 16:47:01 EST 2007
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 <simonpj at microsoft.com> wrote:
> | 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
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>
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