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

[Jan Jakubuv]

hi,

2009/1/23 Francesco Bochicchio <bieffe62 at 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.

> I also discovered that this (2) instead is legal:
>
> Num n => a :: n
> a = 3
>

It's fine because 3 has the type (Num t) => t::

Prelude> :t 3
3 :: (Num t) => t

> because it is implicitely translated into (3):
>
> Num n => a :: n
> a = fromInteger 3
>

also fine:

Prelude> :t fromInteger
fromInteger :: (Num a) => Integer -> a

Sincerely,
  jan.