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euler / haskell / 2.hs

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-- 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.
--
-- Answer: 4613732

fibs = map fst $ iterate (\(a, b) -> (b, a+b)) (1, 2)

main = print $ sum $ takeWhile (< 4000000) $ filter even fibs