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
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