Carlos Camarao wrote:
Sorry, I think my sentence: "To define (+) as an overloaded operator in Haskell, you have to define and use a type class." is not quite correct. I meant that to define any operator in Haskell you have to have a type class defined with that operator as member.
No. Operators and type classes are entirely orthogonal in Haskell. For example, the list concatenation operator (++) is not defined in any type class. It could be. Either the `mplus` of MonadPlus or the `mappend` of Monoid would make sense. But it happens not to be.
Yes, but the requirement of using the "predefined" (+) is an extra requirement (I would call (+) in Haskell not a predefined operator, but an operator whose type is defined in a class (Num) which is in the Prelude). A Haskell programmer can still define versions of (+) where the arguments are of two different types and the result is a third (he cannot though use the two type classes, and thus neither instances of these two type classes, in a program).
I wish we could argue over semantics instead of vocabulary. By calling the (+) of Num "predefined" I meant nothing other than "it is _defined_ in the Haskell report before (_pre_) you or I add any code of our own". We agree on the facts. I don't call it an "extra" requirement. The original context was very clearly that in C++ where you have int+int, int+double, double+int, double+double, char*+int, int+char* and so on all predefined, you can *also* add your own date+period *without* hiding the predefined versions. And _that_ is overloading. If the question is whether Haskell can do overloading, _that_ is what has to be achieved: you can add a *new* interface date+period *without* hiding the ones that were already defined before you started coding. The interesting challenge here is that we should have Date + Period -> Date Date - Period -> Date Period + Date -> Date Period - Date -> ILLEGAL Period + Period -> Deriod Period - Period -> Period Date + Date -> ILLEGAL Date - Date -> Date and _also_ (remember we are trying to beat C++ here) Int +/- Int -> Int. I suspect that this can be done using type-level programming (so that Date + Date and Period - Date _begin_ to type check but then violate a type constraint) but that's where my Haskell skills are most risible.