2009/1/23 Paul Visschers
Hello,
It seems like you have some trouble with grasping the concept of polymorphism in this particular case.
<...>
I think I get the polymorphism. What I don't get is why a specialized type cannot replace a more generic type, since the specialized type implements the interface defined in the generic type. As I tried to explain, I'm probably misled by the comparison with OOP languages, where polymorphism is implemented via interfaces (Java) or abstract classes/methods (C++). For instance in Java you can write: AbstractInterface a = new ConcreteClass(); if ConcreteClass implements AbstractInterface. The complier will handle a as an instance of AbstractInterface, ignoring anything that is not specifed in its declaration. This make the substitution safe: for instance calling a.AbstractInterfaceMethod() is fine, while calling a.ConcreteClassMethod() gives you an error. Now, I understand that this is not the case in haskell, and I understand the formal reason you give for this. What I am trying to understand is the _actual_ reason behind the formal reason: usually a programming language place limits to avoid ambiguity or misuse of language feature. <...> The problem comes from the extra type annotation: a = 3 :: Integer Which says that instead of being of any numeric type, a is of type Integer. This is less general, since you can't use it when you need an Int or a Double for example. This is what I don't get : why yielding an Integer is not good enough for a function that promises to yield a num? What is missing in Integer?
I hope I explained it clearly. If not please reply.
Paul
You have been clear. I'm probably still too bound to OOP world. Thanks. Ciao ------- FB