[Haskell-cafe] FGL instance constraint

[Ivan Lazar Miljenovic]

Jason Dagit <dagit at codersbase.com> writes:

> On Sat, May 1, 2010 at 12:49 AM, Ivan Lazar Miljenovic <
> I thought we were discussing how expressive the Graph typeclass is, not
> whether I made a sensible implementation.  I mean, I could pretty easily fix
> that "problem", but I think that's not the important topic.  For example,
> change empty to this:
>   empty = GrB (B ([] `asTypeOf` [boring]))

But what is the point of that?  That doesn't give you anything extra
than specifying the type when you use it.

Anyway, a better reason that I just thought of is that for some reason
the type can only accept values of a certain type and thus mkGraph needs
the constraint.  However, I believe this goes against "the spirit" of
the class (and would cause problem with nmap, emap, etc.).

A better solution would be my still-so-far-mostly-vapourware generic
graph class set where there is no requirement that the label types be
mappable (which means I can't just scrap those classes when Louis and I
start working on the new-and-improved FGL :s).

> I'm not a category theorist, but others seem to have looked into that issue
> in depth:
> http://www.randomhacks.net/articles/2007/03/15/data-set-monad-haskell-macros

Hmmm... his code samples using the list monad seem to be non-existant
for me, so I can't comment on them nor his rationales why this would be
a good thing.

Anyway, this split looks like the Applicative/etc. split that people
have proposed.  However, I still seem to recall someone telling me on
#haskell that Monads, etc. are specifically _not_ meant to constrain the
value type.


> I completely agreed that having the constraint in the data declaration was a
> bad idea.  Then I took it a step further to assume it may still come up in
> the definition of the Graph instance for Kevin or others.  So then I made up
> a silly example where it does matter.  Hopefully Kevin will post back
> letting us know what worked (or didn't) for him.

-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
IvanMiljenovic.wordpress.com