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

[Francesco Bochicchio]

Hi all.
It's about a month I'm trying to learn haskell in my spare time ( and, I
should add, with my spare neuron :-).
I made progress, but more slowly than I expected :-(. I started hasking
question on comp.lang.haskell
(I like newsgroups more than mailing lists), but then realized that this may
be a better places for my newbie
questions. So, here comes the first ...

As many  beginners coming from OOP, i made the mental equation 'type class
== interface'.
It worked well ... unitil yesterday.

Then I discovered that this piece of code  (1) is illegal in askell (ghc
gives the 'rigid type variable' error)

Num n => a :: n
a = 3 :: Integer

I also discovered that this (2) instead is legal:

Num n => a :: n
a = 3

because it is implicitely translated into (3):

Num n => a :: n
a = fromInteger 3

But I don't understand the difference between (1) and (3).  I could not find
the implementation of
fromInteger, but I suspect that it goes like this:

instance Num Integer where
    fromInteger x -> x


To me, it looks like (1) and (3) are equal, but that the compiler needs a
sort of 'formal reassurance' that
the value yielded by 'a' is of Num type. But since Integer is an instance of
Num, I expected that
(1) was already giving this reassurance. At least, it works this way with
interfaces in Java and with class pointers in C++.

But obviously there is something I don't get here ...

Anybody here can explain?

Ciao
-----
FB
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