[Haskell-beginners] Why the length function I wrote has such a type signature?

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?