haskell - 在 haskell 中从 stdin 读取的正确方法

Question

我有一个程序，根据给定的参数以不同的方式工作:

• 如果有 2 个参数 - 它采用第二个参数作为文件名，从中读取，然后简单地将其打印出来。
• 如果有 1 个参数 - 它从 stdin 读取并打印出来。

代码如下:

main :: IO ()
main = do

-- Read given arguments
args <- getArgs

-- If file containing gramma was given
if length args == 2 then do
    hfile <- openFile (last args) ReadMode
    content <- hGetContents hfile
    let inGramma = getGramma content
    doJob (head args) inGramma
    hClose hfile
    return ()

-- If no file was given - reads from stdin
else if length args == 1 then do
    content <- getContents
    let inGramma = getGramma content
    doJob (head args) inGramma
    return ()

else do putStrLn "Invalid count of arguments!"

问题是，当它从标准输入读取时，在每一个新行(按下输入键)之后，它都会打印该行，然后读取下一个。我需要它等待整个输入然后将其打印出来(在 Ctrl+D 之后)。

以下是该代码中使用的函数:

-- | Structure of gramma
data GrammaStruct = Gramma
    { nonTerminals :: NonTerminals
    , terminals :: Terminals
    , start :: Start
    , rules :: Rules
    } deriving (Eq)

-- | Representation of gramma
instance Show GrammaStruct where
    show (Gramma n t s r) = 
                init (showSplit n) ++ 
        "\n" ++ init (showSplit t) ++ 
        "\n" ++ showStart s ++ 
        "\n" ++ init (showRules r)

-- | Print gramma
showGramma :: GrammaStruct -> IO ()
showGramma gr = do
    putStrLn $ show gr

-- | Transforms string given from file of stdin into gramma representation in app
getGramma :: String -> GrammaStruct
getGramma hIn = procLns (lines hIn)

-- | Depending on option given, provides required functionality
doJob :: String -> GrammaStruct -> IO ()
doJob op gramma
        | op == "-i" = showGramma gramma

谢谢。

Answer 1

这里的问题是 getContents 使用惰性 IO，使得输入流被逐行处理。如果您想在开始执行作业之前强制它读取整个输入，您可以使用以下技巧:

...
if length args == 1 then do
    content <- getContents
    length content `seq` return ()   -- force the input to be fully read now
    let inGramma = getGramma content
    doJob (head args) inGramma
    return ()

或者，使用evaluate，或者在Hackage中查找提供严格getContents的严格IO模块。例如，我刚刚发现 strict-io 包提供 System.IO.Strict.getContents 。使用它你应该能够编写(未经测试)

import qualified System.IO.Strict as S
...
if length args == 1 then do
    content <- run S.getContents
    ...