問題2改良型

prob2_improved.hs

{-
 - 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.

 - -}
{-
makeFibonatchList :: [Int] -> [Int]
makeFibonatchList a 
        | (last a) > 4000000 = init a
        | otherwise = makeFibonatchList $ reverse $ ((sum $ ( last a ):( last (init a) ):[] ):reverse a)
 - -}
makeFibonatchList fibs = 1 : 1 : (map (uncurry (+)) $ zip fibs (tail fibs))

main = do print $ sum $ filter even $ makeFibonatchList [1,2]