On Mon, 13 Oct 2008 23:32:30 -0400, Tommy M. McGuire
Andrew Coppin wrote:
Bulat Ziganshin wrote:
people that make critique on haskell type classes, don't take into account that it's unlike C++ templates, implemented via run-time dictionaries and other modules may define new instances
Personally, I have no clue how C++ templates work [yet]. (As in, I'm learning C++, but I haven't got to that chapter yet.)
Some guy told me that templates are "the best feature in the language", and proceeded to show me a huge chunk of highly complex-looking code which is approximately equivilent to
join :: Array x -> Array x -> Array x
I was unimpressed.
Actually, that's a lie. I was impressed that such a low-level language could manage even that much abstraction. But I still prefer the Haskell way...
C++ values have sizes:
class foo { int x; };
is half (ahem; make that "different from") the size of
class bar { int x; int y; };
As a result, doing parametric polymorphism requires duct taping something suspiciously similar to cpp macros to the type system. Hence, how C++ templates work: weirdly.
Java (and presumably C#) "generics" are very much like a weakened version of normal parametric polymorphism. C++ templates are an attempt at the same thing in a completely different landscape. I'd be willing to bet that Some Guy's code was very close to exactly equivalent to your join.
Now, as to what C++ templates have to do with Haskell type classes, I dunno...
In the next C++ standard, type checking capabilities are being added to templates---"concepts" specify a set of operations a templated type must support. See http://en.wikipedia.org/wiki/C%2B%2B0x#Concepts. Seems somewhat similar to Haskell typeclasses to me, but perhaps the similarity is merely superficial. :-) Regards, Brad Larsen