On Jan 10, 2008 1:36 PM, Bulat Ziganshin
Hello Mark,
Thursday, January 10, 2008, 4:25:20 PM, you wrote:
"instance Num a => A a"
Mean the same thing as
"instance A (forall a.Num a=>a)"
programmers going from OOP world always forget that classes in Haskell doesn't the same as classes in C++. *implementation* of this instance require to pass dictionary of Num class along with type. now imagine the following code:
f :: A a => a -> a
f cannot use your instance because it doesn't receive Num dictionary of type `a`. it is unlike OOP situation where every object carries the generic VMT which includes methods for every class/interface that object supports
I'm not sure that's a good argument. It doesn't need a Num dictionary, it only needs an A dictionary. That's what it says. You only need a Num dictionary in order to construct an A dictionary, which seems perfectly reasonable. Luke