Hello Alexander, Monday, January 21, 2008, 7:36:18 PM, you wrote:
How does caller choose which particular instance of Num they want?
In object-oriented language If function return type is an interface it means that it can return any implementation of this interface, but caller can't choose which particular inplementation they want.
but type class isn't an interface! it's just like interface in one concrete area - it includes method specifications, but not includes data fields the type that should ìó returned by function is passed by means of so-called dictionary and which type should be returned is defined by type inference process. for example main = print (length [] + f 1) here f should return Int because length return Int and you can't add values of different types (without explicit type conversion). you should also read something about two-way type inference but i don't know any good source please note that in modern OOP languages (latest C# versions, C++ 0x) support for *one-way* type inference was only added, i.e. they only can deduce type of expression from types of operands, while Haskell deduces types in both directions -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com