Re: [Haskell-beginners] Real world example of Typeclasses > Interfaces

On Sat, Sep 04, 2010 at 01:04:43PM -0400, Alec Benzer wrote:

On Sat, Sep 4, 2010 at 12:29 PM, Brent Yorgey wrote:

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).

No, I really do mean that this comes up all the time in a nontrivial way. Here's an example I just pulled randomly from a library of mine: -- | Create a scale transformation. scaling :: (HasLinearMap v, HasLinearMap (Scalar v), Scalar (Scalar v) ~ Scalar v, Fractional (Scalar v)) => Scalar v -> Transformation v scaling s = fromLinear $ (s *^) <-> (^/ s) None of those class constraints implies any of the others (not even in theory), and they are all necessary.

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?

Sure, it's a hassle, but you were looking for examples of why type classes are better than interfaces, and I'm not sure that simple matters of convenience really count.

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?

No, you don't need type classes to use algebraic data types; see OCaml. Type classes were a major innovation at the time they were added to Haskell, but algebraic data types had been around for a while. -Brent