[Haskell-beginners] Type classes vs Java interfaces

[Bryan Vicknair]

On Feb 1, 2013, at 5:25 AM, Bob Hutchison <hutch-lists at recursive.ca> wrote:

> 
> On 2013-01-31, at 6:36 PM, Mateusz Kowalczyk <fuuzetsu at fuuzetsu.co.uk> wrote:
> 
>> 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
>>> 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