競速 Reprise

承接兩年前的 C/C++/Java/Ruby 大亂鬥，這次我寫了 Haskell 版本：

 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] (IO ())
 8: driver = do eof <- gets null
 9:             if eof then return (return ())
10:                    else do this <- commit
11:                            remains <- driver
12:                            return (this >> remains)
13: 
14: commit :: State [Int] (IO ())
15: commit = do i <- nextInt
16:             j <- nextInt
17:             return $ print
18:               (maximum . map (flip cycleLength 1) $ [min i j .. max i j])
19:          where nextInt = State $ \(x : xs) -> (x, xs)
20: 
21: main = getContents >>= evalState driver . map read . words

用 ghc 編譯並開 -O 最佳化，執行時間大約是 1.1 秒，是 C++ 版本的 5 倍多。其實還勉強可以接受啦 XD。（偏心？XD）至於程式碼我覺得有一點點彆扭，總覺得 IO monad 和 State monad 沒有搭配得很好。

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現在在讀 Real World Haskell 的線上 beta 版！