[Haskell-cafe] FGL instance constraint

[Jason Dagit]

On Fri, Apr 30, 2010 at 11:30 PM, Jason Dagit <dagit at codersbase.com> wrote:

>
>
> On Fri, Apr 30, 2010 at 11:08 PM, Ivan Lazar Miljenovic <
> ivan.miljenovic at gmail.com> wrote:
>>
>>
>> You're putting the constraint in the wrong places: put the "(Cls a) => "
>> in the actual functions where you need it.
>>
>
> That's solid advice in general, but it's still not going to work here if
> any of the functions needed for the instance of Graph require the type class
> constraint.
>
> The same problem comes up some times with the Monad class.  For example, if
> you wanted to make a monad instance for Set a, you need "(Ord a) => " in the
> instance functions.
>
> I think associated types can work around this, if Graph used them for the a
> and b types, but I've never tried that so I don't actually know how it
> works.
>
> A solution to the monad problem I just mentioned is outlined here as
> 'restricted monads':
> http://okmij.org/ftp/Haskell/types.html#restricted-datatypes
>

Looking over this real quick, I think the Graph class should be changed to
mention a and b:

class Graph (gr a b)  where ...

Then your instances would be able to mention constraints:

instance Cls a => Graph (GrB a b) where ...

Why wasn't the Graph class designed this way?  My guess:  It was probably a
decision that predated multiparameter type classes.

Jason
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20100501/590cd504/attachment.html