-- 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.

fib :: Integer -> Integer
fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib(n-2)

sumEvenFib :: Integer
sumEvenFib = sum $ filter even $ takeWhile (<=4000000) (map fib [0..])

main =  do
    let result = sumEvenFib
    putStrLn $ "Project Euler, Problem 2\n"
        ++ "Answer: " ++ show result