This document discusses non-imperative functional programming and demand-driven I/O. It proposes modeling I/O as a type of (a :-> b) where operations return results on demand. This avoids issues with mixing lazy and eager contexts by making all I/O lazy. The document suggests implementing this using a runI function to run a (a :-> b) in the IO monad.

1. Non-Imperative Functional Programming
序章： 要求駆動I/O
2012-04-22
山下伸夫
nobsun@sampou.org

2. IO は命令書だ
main :: IO ()
main = do input <- openFile "i" ReadMode
contents <- hGetContents input
hClose input
output <- openFile "o" WriteMode
hPutStr output contents
hClose output
return ()

3. すぐやらなくてもいいLazyな命令
書いちゃだめでしょ!
main :: IO ()
main = do input <- openFile "i" ReadMode
contents <- hGetContents input
hClose input
output <- openFile "o" WriteMode
hPutStr output contents
hClose output
return ()

4. 命令書と実行結果
main :: IO ()
main = do line1 <- getLine
line2 <- getLine
putStrLn line2
putStrLn line1
ghci> :main
world!↓
Hello,↓
Hello,
world!

5. Lazyな命令があると？
main :: IO ()
main = do line1 <- lazy getLine
line2 <- lazy getLine
putStrLn line2
putStrLn line1
return ()
ghci> :main
？

6. Eagerな文脈にLazyな要素
main :: IO ()
main = do line1 <- lazy getLine
line2 <- lazy getLine
putStrLn line2
putStrLn line1
return ()
ghci> :main
world!↓
world!
Hello,↓
Hello,

7. Eagerな文脈にLazyな要素
！
main :: IO ()
main = do line1 <- lazy getLine 険
line2 <- lazy getLine
putStrLn line2 危
な
putStrLn line1
る
return ()
ghci> :main
world!↓
ぜ
world!
Hello,↓
Hello,
ま

8. 全部Lazyにしてしまえ!
main :: IO ()
main = do line1 <- lazy getLine
line2 <- lazy getLine
lazy (putStrLn line2)
lazy (putStrLn line1)
return ()
ghci> :main
？

9. Lazy ＝ 要求駆動
main :: IO ()
main = do line1 <- lazy getLine
line2 <- lazy getLine
lazy (putStrLn line2)
ね
lazy (putStrLn line1)
return () だ
ghci> :main 然
ghci>
当
。
ま

10. 要求駆動I/O
main :: IO ()
main = do
line1 <- lazy getLine
line2 <- lazy getLine
prn2 <- lazy (putStrLn line2)
prn1 <- lazy (putStrLn line1)
return $! sequencing [force line1
,force line2
,prn2
,prn1]

11. 要求駆動I/O
ス
ー ！
ォ え
force :: a -> ()
force !x = () フ 使
を
sequencing :: [()] -> ()
sequencing sq = case dropWhile (()==) sq of
[] -> ()

12. IO は命令書だ
main :: IO ()
main = do
input <- lazy $ openFile "i" ReadMode
icloseR <- lazy $ hClose input
output <- lazy $ openFile "o" WriteMode
ocloseR <- lazy $ hClose output
contents <- hGetContents input
printR <- lazy $ hPutStr output contents
return $! sequencing
$ [printR,icloseR,ocloseR]

13. ああ IO ()
main :: IO ()
main = do
input <- lazy $ openFile "i" ReadMode
icloseR <- lazy $ hClose input
output <- lazy $ openFile "o" WriteMode
ocloseR <- lazy $ hClose output
contents <- hGetContents input
printR <- lazy $ hPutStr output contents
return $! sequencing
$ [printR,icloseR,ocloseR]

14. ああ IO ()
型はプログラムの
仕様じゃなかった

15. 対話:: a :-> b としてのI/O
type a :-> b = OI a -> b
mainR :: a :-> ()
runI :: (a :-> b) -> IO b
main = runI mainR

16. oi package
実装についてはオイオイ
to be continued