sumeven.hs

{-# LANGUAGE BangPatterns #-}
import Data.List (foldl')

-- お題：リスト中の偶数を全部足す

-- 最高
sumEven1 :: [Int] -> Int
sumEven1 = foldl' (+) 0 . filter even

-- 書いてしまいがちなクソコード
sumEven2 :: [Int] -> Int
sumEven2 = foldl' (\sm x -> if even x then sm + x else sm) 0 

-- 上を分かり易くしたもの
sumEven3 :: [Int] -> Int
sumEven3 = foldl' addIfEven 0
  where
    addIfEven sm x
        | even x    = sm + x
        | otherwise = sm
 
-- 再帰
sumEven4 :: [Int] -> Int
sumEven4 l = sumEven4' l 0
  where
    sumEven4' [] !sm = sm
    sumEven4' (x:xs) !sm
       | even x    = sumEven4' xs (sm + x)
       | otherwise = sumEven4' xs sm
 
main =
  do
    print $ sumEven1 [1..10]
    print $ sumEven2 [1..10]
    print $ sumEven3 [1..10]
    print $ sumEven4 [1..10]