Superclass Equality constraints cp FunDeps

[Anthony Clayden]

Now that I've got the bit between my teeth ...

Superclass constraints are not subject to the Paterson
conditions.
IOW I can write superclass constraints
that are not permitted as instance constraints.
(Superclass constraints are required to be
 non-cyclic, which ensures they're terminating.)

Is that worth adding to the docos?

Something like this is OK:

> class (F a b ~ b) => C a b ...
> -- equivalently
> class (D a b b) => C a b ...

Can I think of a use for that? Maybe ...

Sometimes even though you have a type function,
you can use knowledge of the result
to 'improve' the parameters.

The classic case is adding type-level Naturals.

Maybe even type-level Boolean And:
- if the result is True, so must be the params.
- if the result is False,
  and you know one param is True,
  the other must be False.
- but that can't be a function, because
  if the result is False
  and you know one param is False,
  that tells nothing about the other param.


AntC

> > On Sun Apr 30 19:45:34 UTC 2017, Richard Eisenberg
> wrote:
> 
> > Documentation is just about always suboptimal -- but the
> > best people to suggest concrete improvements are those
> > who were confused to begin with. So, by all means,
> > submit patches!
> 
> OK. Done. See #13657.
>