{-
https://projecteuler.net/problem=2
Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89,...

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.
-}
module Main where

main :: IO ()
main = print ans

ans = sum $ filter even $ takeWhile(< 4_000_000) fibs

fibs = 1 : 2 : zipWith (+) fibs (tail fibs)