[Haskell-beginners] Why is type "Integer -> Integer" and not "(Num a) => a -> a"?

[Felipe Lessa]

On Thu, Nov 12, 2009 at 7:06 AM, Shawn Willden
<shawn-haskell at willden.org> wrote:
> Hmm.  Would that also explain this?
>
> Prelude> let f1 x = x * 2
> Prelude> :type f1
> f1 :: (Num a) => a -> a
> Prelude> let f2 = \x -> f1 x
> Prelude> :type f2
> f2 :: Integer -> Integer

Yes, that's the same monomorphism restriction.  Also, note that you
are defaulting to Integer here:

Prelude> :s -Wall
Prelude> let f1 x = x * 2
Prelude> :t f1
f1 :: (Num a) => a -> a
Prelude> let f2 = \x -> f1 x

<interactive>:1:15:
    Warning: Defaulting the following constraint(s) to type `Integer'
             `Num a' arising from a use of `f1' at <interactive>:1:15-18
    In the expression: f1 x
    In the expression: \ x -> f1 x
    In the definition of `f2': f2 = \ x -> f1 x
Prelude> let f3 :: Int -> Int; f3 = \x -> f1 x
Prelude> let f4 :: Num a => a -> a; f4 = \x -> f1 x

I find -Wall very useful. Relatively few times it gets annoying.

HTH,

-- 
Felipe.