Euler_2.hs

fibonacci :: (Ord a, Num a) => a -> [a]
fibonacci limit = _fibonacci limit [1,2]

_fibonacci :: (Ord a, Num a) => a -> [a] -> [a]
_fibonacci limit x
        | sum < limit = _fibonacci limit (x++[sum])
        | otherwise = x
        where   x1x2 = drop ((length x)-2) x
                sum = ((head x1x2)+(last x1x2))


foldl (+) 0 (filter (\n -> n `mod` 2 == 0) (fibonacci 4000000))