Jason Dagit
On Sat, May 1, 2010 at 12:23 AM, Ivan Lazar Miljenovic < ivan.miljenovic@gmail.com> wrote:
Hmmm.... this is an interesting way of doing it, but I would argue that it's pointless: the fact that you're using MPTCs doesn't give you anything extra that the original class. 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.)
Try removing "Cls a" from the instance. You'll notice that my empty does depend on a having a Cls instance because it will fail to compile. In other words, I don't understand what you're talking about. I did need the constraint to define my instance.
Except that example is bogus: "isEmpty empty" returns False.
And if that example gets boring, try making an instance of Set for Monad.
My understanding was that Set couldn't be a Monad specifically why you can't make it one: Monads shouldn't constrain the value of the type contained within.
Then re-read the article I linked from Oleg's website. I'm not understanding your point, and I suspect you're not understanding mine :)
My point was that Kevin was doing it wrong and didn't need a constraint there; what's yours? -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com