hi,

2009/1/23 Francesco Bochicchio <bieffe62@gmail.com>:

>
> 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 guess you mean:

a :: Num n => n

The problem whith your implementation of 'a'

a = 3 :: Integer

is that it provides too specific result. Its type signature says that
its result has to be of the type n for *any* instance of the class
Num. But your result is simply Integer that is just *one* specific
instance of Num. In other words it has to be possible to specialize
("retype") 'a' to any other instance of Num, which is no longer
possible because (3 :: Integer) is already specialized.

Sincerely,
 jan.