On Sat, Sep 4, 2010 at 12:29 PM, Brent Yorgey
On Fri, Sep 03, 2010 at 10:51:59PM -0400, Alec Benzer wrote:
But there's no real way of ensuring an argument implements two interfaces, whereas in Haskell you can make sure that an argument belongs to two (or however many) typeclasses.
doSomething :: (Ord a, Eq a, OtherTypeClass a) => a -> Int ...
But as I thought about this, I couldn't seem to think of a practical case where this would be useful.
This is quite useful, and it comes up all the time. I can't think of a particular example off the top of my head, but I know I have written lots of code with multiple type class constraints on the same type.
I know there are cases where this comes up in Haskell, but in certain cases the needs arises only because of the way Haskell handles typeclasses. What I mean is, if you had a function: something n = [1..n], and your type was a -> [a], a needs to be declared as (Num a, Enum a), even though this is sort of redundant, since you can't really have a number that isn't also enumerable. I'm talking specifically about cases where you'd have (TypeClass1 a, TypeClass2 a), and TypeClass1 doesn't imply TypeClass2 or visa versa (Brandon's example might have served that purpose, though I'm very new to Haskell and haven't gotten a full grasp on monads yet).
However, this isn't actually an advantage over Java, since Java supports multiple interface inheritance. You can just declare a new interface that extends several other interfaces, and then use that new interface instead of having to list multiple ones.
Ya but isn't that a pretty big hassle, especially assuming there are many useful cases where you require a type to belong to multiple typeclasses? In Java you'd have to declare a whole new interface for every single different set of interfaces you want your arguments to conform to, whereas in Haskell you can just specify them in a list. If cases come up enough where you do need to make sure your arguments conform to/belong to multiple interfaces/typeclasses, I'd see Haskell's system as a pretty major improvement, though this is what prompted me to ask the question in the first place, do such situations come up enough for this to be as big a benefit as it might seem to be.
I don't think I'm qualified to comment on the precise differences between interfaces and type classes, since I haven't thought about it all that much. Perhaps others can chime in with some differences. There is this page on the Haskell wiki:
Read through some of that (will probably read through more later), but as I did it also occurred to me that I guess the typeclass system might simply be a necessity to work with the rest of Haskell's type system? Ie, the point of them isn't their benefits over interfaces, it's that it's just the way things need to work in order to effectively connect with algebraic types?
-Brent _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners