[Haskell-cafe] FGL instance constraint

[Ivan Lazar Miljenovic]

Sebastian Fischer <sebf at informatik.uni-kiel.de> writes:

>> Furthermore, as I said earlier,
>> it doesn't make sense to constrain the label type just to make an
>> instance of a type class.
>>
>> (Now, if we had other functions in there which _might_ depend on the
>> label types, this _would_ make sense; as it stands however, it
>> doesn't.)
>>
>> You'll notice that my empty does depend on a having a Cls instance
>> because it will fail to compile. [...] I'm not understanding your
>> point, and I suspect you're not understanding mine :)
>
> Let's assume he did understand your point. I think Ivan doubt's that
> there is any real need for the change because (while defining the set
> monad may make sense) "it does not make sense" to wish for being able
> to use additional constraints when defining the specific functions
> that are currently in the Graph class.

My objections were that there are no ways any such Graph instance
would/should use any of the label values when defining definitions for
the various methods; the labels are just meant to be extra things
_attached_ to the nodes and edges.

> I'm not sure if I agree. It would be interesting to see whether the
> real graph behind the original problem is an example where such
> additional constraints are really necessary and make sense.

Well, yes, having Kevin actually responding back would help ;-)

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