分開

承接上次的 Haskell 效能測試，我突然想到應該把 IO code 和 pure code 再分清楚一點，讓 State monad 單純傳回 list of results，最後用個 foldr 把這個 list 轉成一串 IO () 再全部接起來執行。因為 Haskell 的 laziness，這樣寫的執行期表現和上次沒什麼差別。

 1: import Control.Monad.State
 2: 
 3: cycleLength 1 k = k
 4: cycleLength n k | even n    = cycleLength (n `div` 2) $! (k + 1)
 5:                 | otherwise = cycleLength (3 * n + 1) $! (k + 1)
 6: 
 7: driver :: State [Int] [Int]
 8: driver = do eof <- gets null
 9:             if eof then return []
10:                    else do this <- commit
11:                            remains <- driver
12:                            return (this : remains)
13: 
14: commit :: State [Int] Int
15: commit = do i <- nextInt
16:             j <- nextInt
17:             return
18:               (maximum . map (flip cycleLength 1) $ [min i j .. max i j])
19:          where nextInt = State $ \(x : xs) -> (x, xs)
20: 
21: main = getContents >>= foldr ((>>) . print) (return ()) .
22:                        evalState driver . map read . words

驚人的是單單這樣改一下就讓執行時間下降到 0.6--0.7 秒，和 Java 不相上下！現在我的偏心得到 justification 了 XD。

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而且現在我也覺得 IO monad 和 State monad 相處得比較融洽了 XD。