1 {-# OPTIONS_GHC -fno-implicit-prelude #-}
2 -----------------------------------------------------------------------------
5 -- Copyright : (c) The University of Glasgow 2001
6 -- License : BSD-style (see the file libraries/base/LICENSE)
8 -- Maintainer : libraries@haskell.org
10 -- Portability : portable
12 -- The standard IO library.
14 -----------------------------------------------------------------------------
19 IO, -- instance MonadFix
20 fixIO, -- :: (a -> IO a) -> IO a
22 -- * Files and handles
24 FilePath, -- :: String
26 Handle, -- abstract, instance of: Eq, Show.
28 -- ** Standard handles
30 -- | Three handles are allocated during program initialisation,
31 -- and are initially open.
33 stdin, stdout, stderr, -- :: Handle
35 -- * Opening and closing files
40 openFile, -- :: FilePath -> IOMode -> IO Handle
41 IOMode(ReadMode,WriteMode,AppendMode,ReadWriteMode),
45 hClose, -- :: Handle -> IO ()
49 -- | These functions are also exported by the "Prelude".
51 readFile, -- :: FilePath -> IO String
52 writeFile, -- :: FilePath -> String -> IO ()
53 appendFile, -- :: FilePath -> String -> IO ()
59 -- * Operations on handles
61 -- ** Determining and changing the size of a file
63 hFileSize, -- :: Handle -> IO Integer
64 #ifdef __GLASGOW_HASKELL__
65 hSetFileSize, -- :: Handle -> Integer -> IO ()
68 -- ** Detecting the end of input
70 hIsEOF, -- :: Handle -> IO Bool
73 -- ** Buffering operations
75 BufferMode(NoBuffering,LineBuffering,BlockBuffering),
76 hSetBuffering, -- :: Handle -> BufferMode -> IO ()
77 hGetBuffering, -- :: Handle -> IO BufferMode
78 hFlush, -- :: Handle -> IO ()
80 -- ** Repositioning handles
82 hGetPosn, -- :: Handle -> IO HandlePosn
83 hSetPosn, -- :: HandlePosn -> IO ()
84 HandlePosn, -- abstract, instance of: Eq, Show.
86 hSeek, -- :: Handle -> SeekMode -> Integer -> IO ()
87 SeekMode(AbsoluteSeek,RelativeSeek,SeekFromEnd),
89 hTell, -- :: Handle -> IO Integer
92 -- ** Handle properties
94 hIsOpen, hIsClosed, -- :: Handle -> IO Bool
95 hIsReadable, hIsWritable, -- :: Handle -> IO Bool
96 hIsSeekable, -- :: Handle -> IO Bool
98 -- ** Terminal operations
100 #if !defined(__NHC__)
101 hIsTerminalDevice, -- :: Handle -> IO Bool
103 hSetEcho, -- :: Handle -> Bool -> IO ()
104 hGetEcho, -- :: Handle -> IO Bool
107 -- ** Showing handle state
109 #ifdef __GLASGOW_HASKELL__
110 hShow, -- :: Handle -> IO String
113 -- * Text input and output
117 hWaitForInput, -- :: Handle -> Int -> IO Bool
118 hReady, -- :: Handle -> IO Bool
119 hGetChar, -- :: Handle -> IO Char
120 hGetLine, -- :: Handle -> IO [Char]
121 hLookAhead, -- :: Handle -> IO Char
122 hGetContents, -- :: Handle -> IO [Char]
126 hPutChar, -- :: Handle -> Char -> IO ()
127 hPutStr, -- :: Handle -> [Char] -> IO ()
128 hPutStrLn, -- :: Handle -> [Char] -> IO ()
129 hPrint, -- :: Show a => Handle -> a -> IO ()
131 -- ** Special cases for standard input and output
133 -- | These functions are also exported by the "Prelude".
135 interact, -- :: (String -> String) -> IO ()
136 putChar, -- :: Char -> IO ()
137 putStr, -- :: String -> IO ()
138 putStrLn, -- :: String -> IO ()
139 print, -- :: Show a => a -> IO ()
140 getChar, -- :: IO Char
141 getLine, -- :: IO String
142 getContents, -- :: IO String
143 readIO, -- :: Read a => String -> IO a
144 readLn, -- :: Read a => IO a
146 -- * Binary input and output
149 openBinaryFile, -- :: FilePath -> IOMode -> IO Handle
150 hSetBinaryMode, -- :: Handle -> Bool -> IO ()
151 #if !defined(__NHC__)
152 hPutBuf, -- :: Handle -> Ptr a -> Int -> IO ()
153 hGetBuf, -- :: Handle -> Ptr a -> Int -> IO Int
155 #if !defined(__NHC__) && !defined(__HUGS__)
156 hPutBufNonBlocking, -- :: Handle -> Ptr a -> Int -> IO Int
157 hGetBufNonBlocking, -- :: Handle -> Ptr a -> Int -> IO Int
162 #ifdef __GLASGOW_HASKELL__
168 #ifdef __GLASGOW_HASKELL__
170 import GHC.IOBase -- Together these four Prelude modules define
171 import GHC.Handle -- all the stuff exported by IO for the GHC version
183 import Hugs.Prelude ( throw, Exception(NonTermination) )
184 import Control.Exception ( bracket )
185 import System.IO.Unsafe ( unsafeInterleaveIO )
192 , IOMode (ReadMode,WriteMode,AppendMode,ReadWriteMode)
193 , BufferMode (NoBuffering,LineBuffering,BlockBuffering)
194 , SeekMode (AbsoluteSeek,RelativeSeek,SeekFromEnd)
195 , stdin, stdout, stderr
196 , openFile -- :: FilePath -> IOMode -> IO Handle
197 , hClose -- :: Handle -> IO ()
198 , hFileSize -- :: Handle -> IO Integer
199 , hIsEOF -- :: Handle -> IO Bool
200 , isEOF -- :: IO Bool
201 , hSetBuffering -- :: Handle -> BufferMode -> IO ()
202 , hGetBuffering -- :: Handle -> IO BufferMode
203 , hFlush -- :: Handle -> IO ()
204 , hGetPosn -- :: Handle -> IO HandlePosn
205 , hSetPosn -- :: HandlePosn -> IO ()
206 , hSeek -- :: Handle -> SeekMode -> Integer -> IO ()
207 , hWaitForInput -- :: Handle -> Int -> IO Bool
208 , hGetChar -- :: Handle -> IO Char
209 , hGetLine -- :: Handle -> IO [Char]
210 , hLookAhead -- :: Handle -> IO Char
211 , hGetContents -- :: Handle -> IO [Char]
212 , hPutChar -- :: Handle -> Char -> IO ()
213 , hPutStr -- :: Handle -> [Char] -> IO ()
214 , hPutStrLn -- :: Handle -> [Char] -> IO ()
215 , hPrint -- :: Handle -> [Char] -> IO ()
216 , hReady -- :: Handle -> [Char] -> IO ()
217 , hIsOpen, hIsClosed -- :: Handle -> IO Bool
218 , hIsReadable, hIsWritable -- :: Handle -> IO Bool
219 , hIsSeekable -- :: Handle -> IO Bool
223 , FilePath -- :: String
225 import NHC.IOExtras (fixIO)
228 -- -----------------------------------------------------------------------------
231 #ifdef __GLASGOW_HASKELL__
232 -- | Write a character to the standard output device
233 -- (same as 'hPutChar' 'stdout').
235 putChar :: Char -> IO ()
236 putChar c = hPutChar stdout c
238 -- | Write a string to the standard output device
239 -- (same as 'hPutStr' 'stdout').
241 putStr :: String -> IO ()
242 putStr s = hPutStr stdout s
244 -- | The same as 'putStr', but adds a newline character.
246 putStrLn :: String -> IO ()
247 putStrLn s = do putStr s
250 -- | The 'print' function outputs a value of any printable type to the
251 -- standard output device.
252 -- Printable types are those that are instances of class 'Show'; 'print'
253 -- converts values to strings for output using the 'show' operation and
256 -- For example, a program to print the first 20 integers and their
257 -- powers of 2 could be written as:
259 -- > main = print ([(n, 2^n) | n <- [0..19]])
261 print :: Show a => a -> IO ()
262 print x = putStrLn (show x)
264 -- | Read a character from the standard input device
265 -- (same as 'hGetChar' 'stdin').
268 getChar = hGetChar stdin
270 -- | Read a line from the standard input device
271 -- (same as 'hGetLine' 'stdin').
274 getLine = hGetLine stdin
276 -- | The 'getContents' operation returns all user input as a single string,
277 -- which is read lazily as it is needed
278 -- (same as 'hGetContents' 'stdin').
280 getContents :: IO String
281 getContents = hGetContents stdin
283 -- | The 'interact' function takes a function of type @String->String@
284 -- as its argument. The entire input from the standard input device is
285 -- passed to this function as its argument, and the resulting string is
286 -- output on the standard output device.
288 interact :: (String -> String) -> IO ()
289 interact f = do s <- getContents
292 -- | The 'readFile' function reads a file and
293 -- returns the contents of the file as a string.
294 -- The file is read lazily, on demand, as with 'getContents'.
296 readFile :: FilePath -> IO String
297 readFile name = openFile name ReadMode >>= hGetContents
299 -- | The computation 'writeFile' @file str@ function writes the string @str@,
300 -- to the file @file@.
301 writeFile :: FilePath -> String -> IO ()
302 writeFile f txt = withFile f WriteMode (\ hdl -> hPutStr hdl txt)
304 -- | The computation 'appendFile' @file str@ function appends the string @str@,
305 -- to the file @file@.
307 -- Note that 'writeFile' and 'appendFile' write a literal string
308 -- to a file. To write a value of any printable type, as with 'print',
309 -- use the 'show' function to convert the value to a string first.
311 -- > main = appendFile "squares" (show [(x,x*x) | x <- [0,0.1..2]])
313 appendFile :: FilePath -> String -> IO ()
314 appendFile f txt = withFile f AppendMode (\ hdl -> hPutStr hdl txt)
316 -- | The 'readLn' function combines 'getLine' and 'readIO'.
318 readLn :: Read a => IO a
319 readLn = do l <- getLine
323 -- | The 'readIO' function is similar to 'read' except that it signals
324 -- parse failure to the 'IO' monad instead of terminating the program.
326 readIO :: Read a => String -> IO a
327 readIO s = case (do { (x,t) <- reads s ;
331 [] -> ioError (userError "Prelude.readIO: no parse")
332 _ -> ioError (userError "Prelude.readIO: ambiguous parse")
333 #endif /* __GLASGOW_HASKELL__ */
336 -- | Computation 'hReady' @hdl@ indicates whether at least one item is
337 -- available for input from handle @hdl@.
339 -- This operation may fail with:
341 -- * 'System.IO.Error.isEOFError' if the end of file has been reached.
343 hReady :: Handle -> IO Bool
344 hReady h = hWaitForInput h 0
346 -- | The same as 'hPutStr', but adds a newline character.
348 hPutStrLn :: Handle -> String -> IO ()
349 hPutStrLn hndl str = do
353 -- | Computation 'hPrint' @hdl t@ writes the string representation of @t@
354 -- given by the 'shows' function to the file or channel managed by @hdl@
355 -- and appends a newline.
357 -- This operation may fail with:
359 -- * 'System.IO.Error.isFullError' if the device is full; or
361 -- * 'System.IO.Error.isPermissionError' if another system resource limit would be exceeded.
363 hPrint :: Show a => Handle -> a -> IO ()
364 hPrint hdl = hPutStrLn hdl . show
365 #endif /* !__NHC__ */
367 -- | @'withFile' name mode act@ opens a file using 'openFile' and passes
368 -- the resulting handle to the computation @act@. The handle will be
369 -- closed on exit from 'withFile', whether by normal termination or by
370 -- raising an exception.
371 withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r
372 withFile name mode = bracket (openFile name mode) hClose
374 -- | @'withBinaryFile' name mode act@ opens a file using 'openBinaryFile'
375 -- and passes the resulting handle to the computation @act@. The handle
376 -- will be closed on exit from 'withBinaryFile', whether by normal
377 -- termination or by raising an exception.
378 withBinaryFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r
379 withBinaryFile name mode = bracket (openBinaryFile name mode) hClose
381 -- ---------------------------------------------------------------------------
384 #if defined(__GLASGOW_HASKELL__) || defined(__HUGS__)
385 fixIO :: (a -> IO a) -> IO a
387 ref <- newIORef (throw NonTermination)
388 ans <- unsafeInterleaveIO (readIORef ref)
390 writeIORef ref result
393 -- NOTE: we do our own explicit black holing here, because GHC's lazy
394 -- blackholing isn't enough. In an infinite loop, GHC may run the IO
395 -- computation a few times before it notices the loop, which is wrong.
399 -- Assume a unix platform, where text and binary I/O are identical.
400 openBinaryFile = openFile
401 hSetBinaryMode _ _ = return ()
405 -- Implementations should enforce as far as possible, at least locally to the
406 -- Haskell process, multiple-reader single-writer locking on files.
407 -- That is, /there may either be many handles on the same file which manage
408 -- input, or just one handle on the file which manages output/. If any
409 -- open or semi-closed handle is managing a file for output, no new
410 -- handle can be allocated for that file. If any open or semi-closed
411 -- handle is managing a file for input, new handles can only be allocated
412 -- if they do not manage output. Whether two files are the same is
413 -- implementation-dependent, but they should normally be the same if they
414 -- have the same absolute path name and neither has been renamed, for
417 -- /Warning/: the 'readFile' operation holds a semi-closed handle on
418 -- the file until the entire contents of the file have been consumed.
419 -- It follows that an attempt to write to a file (using 'writeFile', for
420 -- example) that was earlier opened by 'readFile' will usually result in
421 -- failure with 'System.IO.Error.isAlreadyInUseError'.
423 -- -----------------------------------------------------------------------------
426 #ifdef __GLASGOW_HASKELL__
427 -- Copied here to avoid recursive dependency with Control.Exception
429 :: IO a -- ^ computation to run first (\"acquire resource\")
430 -> (a -> IO b) -- ^ computation to run last (\"release resource\")
431 -> (a -> IO c) -- ^ computation to run in-between
432 -> IO c -- returns the value from the in-between computation
433 bracket before after thing =
438 (\e -> do { after a; throw e })