[Haskell-beginners] find an element in a list

[Max Voit]

Hi Fabien,

On Sat, 26 Dec 2015 15:19:17 +0100
Fabien R <theedge456 at free.fr> wrote:

> extractIfBegins x [xs] | [xs] == (x:ys) = [ys]
> 			| otherwise = [xs]
> 
> But ghci complains that ys is not defined.

That is because you cannot pattern match whilst equality testing. The
statement 
  xs == (x:ys)
is problematic therefore. You expect the compiler to see "oh, I don't
know ys, but xs is a list, so I'm just checking x and put the rest into
ys while I'm at it".

> Without giving the answer, can someone give a hint about the approach
> to follow ?

Try pattern matching on the list xs instead. Also take care that it's
xs, not [xs] (the latter notation implies a list with one element xs).

Best,
Max