[m-dev.] Existentially quantified data constructors

[Ralph Becket]

Mark Brown, Tuesday, 15 April 2008:
> The fact that you are required to use `=>' rather than `<=' is meant to
> be a hint that you are specifying one kind of constraint over another.
> Perhaps it isn't strong enough, but the error message *does* try to draw
> your attention to this.  It's also covered in section 11.2 of the reference
> manual.

I'm not saying the Reference Manual is unclear, just that it tells you
what, not why.  Perhaps the problem is just with me, but I reckon I'm not
alone: the picture I have when I'm programming is that a type class
specifies an interface implemented by a vector of types.  Where things
start to get fuzzy is with the restrictions.  I remember a certain list
of "do"s and "don't"s (don't use equivalence types in instance
declarations, don't repeat variables in instance declarations, instance
variable arguments must be of the form typector(typevar, typevar, ...),
don't mix existentially and universally quantified variables in the same
constraint, etc.), but I'm damned if I can remember why any of those
restrictions are necessary.  It would be Very Helpful if for each
restriction we could provide a simple example somewhere of *why*
violating it leads to ambiguity/unsoundness/madness/divorce.  Whether
this document would belong in the RM or not is neither here nor there.

-- Ralph
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