defaults

[Malcolm Wallace]

Simon Peyton-Jones <simonpj at microsoft.com> wrote:

> remember that defaulting does not happen class-by-class at the moment.

Yes, and this is exactly the point of the proposal (#2).  It makes sense
(to me at least) to change the rules so that defaulting happens
per-class.  At the moment, it _can_ occur for any class, provided only
that the type is _also_ constrained to be an instance of Num.  It is a
very strange rule, and one that I guess very few people understand
fully, yet I hope to show that one can subsume it entirely by
introducing a different but more general rule, that is easier to
understand.

> If you have
>         (Eq a, Fractional a)
> where 'a' is unconstrained, the current rules pick the first type T in
> the defaulting list that satisfies all the constraints (Eq T, Fractional T).
> I don't know what would that would mean if Eq had a different
> defaulting list than Fractional, as you propose.

Well, for one thing I don't like the idea of a "list" of potential
defaults.  My suggestion is that there should be a single default type
per class.  If more than one class is involved, then either their
defaults should agree, or only one of them in fact declares a default.

I claim that this would be sufficient for every extant use case.  But I
would quite like to have some evidence to back up this claim!  If anyone
actually uses explicit defaults in their programs, I would be interested
in seeing some real data on this.  Feel free to submit such example
decls to my email address.  I suspect that almost no-one uses defaults
except for the original motivating case of literal numbers.  If so, then
even quite a big change in this area would hurt very few people, provided it
was backward compatible with the literal number thing.

> Another issue that Haskell' needs to address is what defaulting means
> when you have multi-parameter type classes.

Yes, I was going to mention that myself on the wiki, but it slipped my
mind again.  So here is one idea.  If you have a single default type per
class in the SPTC scenario, then you would have a single default
_relation_ of types in the MPTC case.  The default decl could be made to
look rather like an instance decl:

    default Num Integer
    default Fractional Double

    class Dual d c where ...
    default Dual Binary [Bool]

except, unlike instances, you are permitted only one of them.

Regards,
    Malcolm