foo.hs

fibs :: (Integral a) => [a]
fibs = 1 : 1 : zipWith (+) fibs (tail fibs)

--https://projecteuler.net/problem=2
problem2 :: (Integral a, Integral b) => a -> b
problem2 n
  | n <= 1 = 0
  | otherwise = sum (filter even (takeWhile (<= (fromIntegral n)) fibs))
 
answer = problem2 (4*10^6)

main = print $ answer