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

[Stephen Tetley]

On 20 May 2010 21:16, Evan Laforge <qdunkan at gmail.com> wrote:

> 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...


Hi Evan, hasn't EHC had something like this for a while with 'type
class directives'?