Re: [Haskell-beginners] Type classes are not like interfaces, after all

On 23 Jan 2009, at 14:37, Francesco Bochicchio wrote:

2009/1/23 Paul Visschers Hello,

It seems like you have some trouble with grasping the concept of polymorphism in this particular case.

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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.

Suppose I declare this constant: x :: Num a => a x = 3 :: Integer Now suppose I want to use that in a function. It's type signature says that x is *any* numeric type i want it to be, so I'm going to add it to another numeric value: y :: Complex Float y = x + (5.3 :+ 6.93) Unfortunately, x *can't* take the form of any Numeric type – it has to be an Integer, so I can't add it do Complex Floating point numbers. The type system is telling you "while Integers may imelement the numeric interface, the value 3 :: Integer is not a generic value – it can't take the form of *any* numeric value, only a specific type of numeric values". Bob