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

2009/1/23 Paul Visschers <mail@paulvisschers.net>

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.

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