In article <[EMAIL PROTECTED]>, [EMAIL PROTECTED] (Aahz) wrote: > In article <[EMAIL PROTECTED]>, > Alex Martelli <[EMAIL PROTECTED]> wrote: > > > >Right, you can get good genericity with Haskell's typeclasses (I've > >posted about that often in the past, and desperately and so far > >unsuccessfully tried to convince Guido to use something close to > >typeclasses rather than "interfaces" for such purposes as PEP 246 > >[protocol adaptation]); it's the state of _templates_ in Haskell, > >specifically, which I was rather dubious about (it may be that I just > >haven't dug into them deep enough yet, but they do seem not a little > >"convoluted" to me, so far). > > Hrm. I don't recall anything about typeclasses, so my suspicion is that > you were writing something lengthy and above my head. Can you write > something reasonably short about it? (I'm asking partly for your > benefit, because if it makes sense to me, that will likely go a long way > toward making sense to Guido -- we seem to think similarly in certain > ways.)
Couple of points to hopefully augment Alex's response - A type class may very well provide no implementation at all. They certainly can, but they are still useful when they don't, and indeed I think it's fair to say that isn't their main purpose. - Doesn't hurt to recall that Haskell is a functional language. Type classes have a superficial and maybe useful similarity to object classes, but in Haskell, if data objects have methods, they aren't aware of them, so to speak. If I define a new type class for some reason, I may make instances for every data type I can think of -- Int, Bool, (), whatever. Now do these types suddenly sprout new methods? Not really! I just have a set of functions that are now defined over all these types. Outside the scope of these declarations, of course these functions are unknown, and the data types are the same in either case. - If type classes are very commonly written in Haskell applications, it's news to me. The need to define functions over a set of data types in this way is relatively unusual - extremely useful in the core language where you get Eq, Ord, Show and such basic properties, but I think less needed higher up in the application hierarchy. That might be an interesting philosophical question, as a contrast between the basic world views of FP versus OOP, but of course you'd want to check it with someone with a lot more Haskell than I have. Donn Cave, [EMAIL PROTECTED] -- http://mail.python.org/mailman/listinfo/python-list
