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

[Nathan M. Holden]

This is just a theory, but in my (limited) experience, GHCi is willing to 
guess at the type of values, where at functions (explicitly or implicitly 
typed) it can't guess.

In your original function, it multiplied a number (Num a) => a by an Int, 
therefor it must be Int -> Int (because you can't multiply a Double by an Int, 
don't be crazy.

In the new function, though, it can infer the type of the 2, and therefor it 
can infer it to whatever numerical type you send it.

I've run into this problem a few times, but for whatever reason I can't get 
myself to fall into any of the usual ways I walk blindly into these sorts of 
problems this time.