Haskell non exhaustive patterns in function

Question

the following code compiles well but when i try to input helper 10 primes [] [] it gives me :Non-exhaustive patterns in function helper

primes = [2, 3, 5, 7,         


        

Answer 1

What you're trying to evaluate:

helper 10 primes   []  []

Your equations for helper:

helper n  []       (v) _      = ...
helper n  (x:y:xs) (v) (c:cs) = ...

Let's try to match the first equation:

• n = 10 matches
• [] = primes does not match

OK, let's try to match the second equation instead:

• n = 10 matches
• (x:y:xs) = primes matches, with
  • x = 2
  • y = 3
  • xs = [5, 7, 11, 13, ...]
• v = [] matches
• (c:cs) = [] does not match

So neither pattern matches. We have a pattern match failure.