Re: [Haskell-beginners] Type classes vs Java interfaces

On Feb 1, 2013, at 5:25 AM, Bob Hutchison wrote:

On 2013-01-31, at 6:36 PM, Mateusz Kowalczyk wrote:

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Greetings,

I often wonder how one would explain type classes to someone coming from an environment such as Java. Whenever I think about type classes, I seem to think of them as Java interfaces. Bah, even the bottom of [1] states

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Haskell classes are roughly similar to a Java interface. Like an interface declaration, a Haskell class declaration defines a protocol for using an object rather than defining an object itself.

Is there more to this `roughly similar' statement? Syntax is an obvious difference but beyond that, I can't think of anything I can do with a Haskell type class that I wouldn't be able to do with similar amount of effort with a Java interface, except for the fact that the interface would look absolutely disgusting syntax wise.

Any insight appreciated.

Well I have limited insight but…

Type classes can provide default implementations, which is not possible in Java.

Type classes in a type signature describe or constrain a type and but are not themselves types. Among other things, this means in Haskell that collections must be homogeneous in their actual type, it's not sufficient to be "homogeneous in a type class". There are extensions to GHC that make this possible [1] but there are limitations and the usage has its detractors [2]. In Java you can have collections of objects that conform to a given interface even if they are of different classes. Personally, I find Haskell's restriction counter-intuitive but the sense of surprise and limitation is diminishing as I use the language.

As I understand them, typeclasses allow you to add a type to a class without modifying the type definition at all, so they are more flexible than Java interfaces in that way. I don't do much java, but I don't think you can define a class' implementation of an interface outside of the class. In Haskell, I can define a typeclass that suites my domain, and add another author's type as an instance, and the author doesn't have to know about it. Bryan Vicknair