[Haskell-cafe] problem with type equality constraints

Manuel M T Chakravarty chak at cse.unsw.edu.au
Mon Mar 17 23:13:48 EDT 2008

Ganesh Sittampalam:
> On Mon, 17 Mar 2008, Manuel M T Chakravarty wrote:
>> Your are completely right.  Unfortunately, superclass equalities  
>> (ie, the Id a ~ ida in the class declaration of Foo) aren't fully  
>> implemented yet.
> OK, thanks. Is there any rough idea of when they will be?

That's a bit difficult to say.  The main problem is that the current  
handling of evidence for classes and access to superclass evidence  
doesn't mix well with the fact that the evidence for equalities are  
coercions (ie, type-level things, rather than value level things).

It's high up on the list, but some other things like the interaction  
between GADTs and type families (basically done now) were higher up.   
If its important to you, I'd try to get to it earlier than if nobody  
is really waiting for it.


More information about the Haskell-Cafe mailing list