[Haskell-cafe] is monomorphism restriction necessary?

[Alexey Muranov]

Sorry for the question that probably is already answered somewhere, but i do not get it from the basic documentation i have looked at so far.

Is monomorphism restriction really necessary, or is it just a convenience?

In the example from A History of Haskell

    genericLength :: Num a => [b] -> a

    f xs = (len, len)
        where
            len = genericLength xs

can't the monomorphism restriction be replaced with just adding the signature

    f :: Num a => [b] -> (a, a)

?

If yes, are there other cases where there is no way to get the desired behavior without enabling the monomorphism restriction?

Alexey.