On 19/01/06, Johannes Waldmann
PS (I know this is heresy but - can we please in Haskell-0X rename "class" to "interface" so that the non-Haskell world knows what we're talking about. You know if the mountain's not gonna walk to the prophet, then ...) (note that I'm currently not proposing to rename "data" to "class". But looking at GADT data definitions still makes me wonder...)
Typeclasses are about as much unlike Java interfaces as they are unlike Java classes. Typeclasses are not types, like interfaces are in Java (at least, when I learned Java, I'll admit I haven't been following progress too closely). Typeclasses (at least in modern Haskell), are allowed to have arbitrarily many type parameters as well as functional dependencies between those parameters, whereas interfaces in Java are something that each class is permitted to implement, but they do not tie multiple classes together. Further, functions in Haskell are permitted to require that some types involved satisfy any number of classes, whereas with interfaces, classes implement some interfaces, but only one interface at a time can be mentioned when it comes to using them. For example, in Haskell, I can write: f :: (Ord a, Read a) => String -> String -> a f xs ys = max (read xs) (read ys) Whereas in Java, this kind of requirement that a type implement multiple interfaces seems harder to specify (correct me if I'm wrong, it's been a few years since I wrote any Java code). If I recall correctly, interfaces in Java are not permitted to provide default implementations of methods which they declare (and that's one of the major things separating them from classes), whereas in Haskell, we are permitted to define default method implementations in classes, which can be overruled in the instances. In any event, I think it's dangerous when coming to a new language to expect that the terms mean the same thing at all. "Class" means something different to a Haskell programmer than a Java programmer, and that's fine. So do most other words. :) Haskell isn't claiming to be an OO language, so we don't have to align with OO terminology. That said, 'interface' or perhaps 'predicate' or 'relation' seem like slightly better words than 'class' here, purely from an English language perspective, but the term 'class' does have a justification, and I think it's already blatantly obvious to new Haskell programmers that Haskell classes are not too much like OO classes, even if the terminology is similar. Is the small benefit really worth the trouble of altering all the literature and code to change the name? - Cale