像在I / O库的其他几个部分中一样,隐式使用标准流的函数在System.IO执行相同工作时具有对等的功能,但是左侧有一个额外的参数,类型为Handle,表示正在处理的流。例如,getLine :: IO String有一个对应项hGetLine :: Handle -> IO String。
import System.IO( Handle, FilePath, IOMode( ReadMode ), openFile, hGetLine, hPutStr, hClose, hIsEOF, stderr ) import Control.Monad( when ) dumpFile :: Handle -> FilePath -> Integer -> IO () dumpFile handle filename lineNumber = do -- show file contents line by line end <- hIsEOF handle when ( not end ) $ do line <- hGetLine handle putStrLn $ filename ++ ":" ++ show lineNumber ++ ": " ++ line dumpFile handle filename $ lineNumber + 1 main :: IO () main = do hPutStr stderr "输入文件名: " filename <- getLine handle <- openFile filename ReadMode dumpFile handle filename 1 hClose handle
文件内容example.txt:
This is an example. Hello, world! This is another example.
输入:
输入文件名: example.txt
输出:
example.txt:1: This is an example. example.txt:2: Hello, world! example.txt:3: This is another example