This is exercise 3.1 of Real World Haskell. I have my length function like this:

myLength [] = 0
myLength (_:xs) = 1 + (myLength xs)

And I assumed the type signature is like this:
mylength :: [a] -> Num

But when I wrote this into the file and reloaded it into ghci, there is an error.

The type signature for `mylength' lacks an accompanying binding
Failed, modules loaded: none.

And the type signature given by ghci is

myLength :: (Num t1) => [t] -> t1

So how can I modify the function to have a type signature like the first one?