[Haskell-beginners] find an element in a list

[Fabien R]

As a newbie, I'm studying the pdf 'the Haskell road to logic, math and 
programming' and I'm stuck with one exercise.
I want to extract x if x is at the beginning of a list.
I thought to use something like this:
extractIfBegins x [xs] | [xs] == (x:ys) = [ys]	 			| otherwise = [xs]

But ghci complains that ys is not defined.
Without giving the answer, can someone give a hint about the approach to 
follow ?

--
Fabien