[Haskell-cafe] Proposal to solve Haskell's MPTC dilemma

[Evan Laforge]

> I wonder: Of cases where overload resolution via available instances
> would be reasonable, how many would also make sense as a closed type
> class? By comparison, it seems that many uses of OverlappingInstances
> are really just trying to express a closed type class with one or more
> default instances, akin to functions with _ patterns. I think, though
> I'm not certain, that both would be straightforward and non-fragile
> for a closed type class.

Someone recently described the HASP project, at
http://hasp.cs.pdx.edu/.  It describes "habit", a haskell like
language with some additions and subtractions.  There are a couple
interesting extensions to 'instance' declarations:

-- explicitly declare that there is no instance, halting the compiler's search
instance xyz fails

-- declares instances along with search order
instance abc ...
else def

The result is that if you put 'fails' at the end, you can make a
closed typeclass.  Presumably you could also make typeclasses open but
only in a restricted way.  Also presumably the compiler would then be
able to make better decisions about overlapping instances and you
could avoid a lot of overlapping problems.

Of course, it's just a paper with no compiler, so it's all
"presumably" for the moment...