[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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