11  Input/Output

module IO (     Handle, HandlePosn,     IOMode(ReadMode,WriteMode,AppendMode,ReadWriteMode),     BufferMode(NoBuffering,LineBuffering,BlockBuffering),     SeekMode(AbsoluteSeek,RelativeSeek,SeekFromEnd),     stdin, stdout, stderr,      openFile, hClose,      hFileSize, hIsEOF, isEOF,     hSetBuffering, hGetBuffering, hFlush,      hGetPosn, hSetPosn, hSeek,      hWaitForInput, hReady, hGetChar, hGetLine, hLookAhead, hGetContents,      hPutChar, hPutStr, hPutStrLn, hPrint,     hIsOpen, hIsClosed, hIsReadable, hIsWritable, hIsSeekable,     isAlreadyExistsError, isDoesNotExistError, isAlreadyInUseError,      isFullError, isEOFError,     isIllegalOperation, isPermissionError, isUserError,      ioeGetErrorString, ioeGetHandle, ioeGetFileName,     try, bracket, bracket_ ) where import Ix(Ix) data Handle = ... -- implementation-dependent instance Eq Handle where ... instance Show Handle where ..           -- implementation-dependent data HandlePosn = ... -- implementation-dependent instance Eq HandlePosn where ... instance Show HandlePosn where ---      -- implementation-dependent data IOMode      =  ReadMode | WriteMode | AppendMode | ReadWriteMode                     deriving (Eq, Ord, Ix, Bounded, Enum, Read, Show) data BufferMode  =  NoBuffering | LineBuffering                   |  BlockBuffering (Maybe Int)                     deriving (Eq, Ord, Read, Show) data SeekMode    =  AbsoluteSeek | RelativeSeek | SeekFromEnd                     deriving (Eq, Ord, Ix, Bounded, Enum, Read, Show) stdin, stdout, stderr :: Handle openFile              :: FilePath -> IOMode -> IO Handle hClose                :: Handle -> IO ()

hFileSize             :: Handle -> IO Integer hIsEOF                :: Handle -> IO Bool isEOF                 :: IO Bool isEOF                 =  hIsEOF stdin hSetBuffering         :: Handle  -> BufferMode -> IO () hGetBuffering         :: Handle  -> IO BufferMode hFlush                :: Handle -> IO ()  hGetPosn              :: Handle -> IO HandlePosn hSetPosn              :: HandlePosn -> IO ()  hSeek                 :: Handle -> SeekMode -> Integer -> IO ()  hWaitForInput       :: Handle -> Int -> IO Bool hReady                :: Handle -> IO Bool  hReady h       =  hWaitForInput h 0 hGetChar              :: Handle -> IO Char hGetLine              :: Handle -> IO String hLookAhead            :: Handle -> IO Char hGetContents          :: Handle -> IO String hPutChar              :: Handle -> Char -> IO () hPutStr               :: Handle -> String -> IO () hPutStrLn             :: Handle -> String -> IO () hPrint                :: Show a => Handle -> a -> IO () hIsOpen               :: Handle -> IO Bool hIsClosed             :: Handle -> IO Bool hIsReadable           :: Handle -> IO Bool hIsWritable           :: Handle -> IO Bool hIsSeekable           :: Handle -> IO Bool isAlreadyExistsError  :: IOError -> Bool isDoesNotExistError   :: IOError -> Bool isAlreadyInUseError   :: IOError -> Bool isFullError           :: IOError -> Bool isEOFError            :: IOError -> Bool isIllegalOperation    :: IOError -> Bool isPermissionError     :: IOError -> Bool isUserError           :: IOError -> Bool ioeGetErrorString     :: IOError -> String ioeGetHandle          :: IOError -> Maybe Handle ioeGetFileName        :: IOError -> Maybe FilePath try                   :: IO a -> Either IOError a bracket               :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c bracket_              :: IO a -> (a -> IO b) -> IO c -> IO c

The monadic I/O system used in Haskell is described by the Haskell language report. Commonly used I/O functions such as print are part of the standard prelude and need not be explicitly imported. This library contain more advanced I/O features. Some related operations on file systems are contained in the Directory library.

11.1  I/O Errors

• isAlreadyExistsError -- the operation failed because one of its arguments already exists.
• isDoesNotExistError -- the operation failed because one of its arguments does not exist.
• isAlreadyInUseError -- the operation failed because one of its arguments is a single-use resource, which is already being used (for example, opening the same file twice for writing might give this error).
• isFullError -- the operation failed because the device is full.
• isEOFError -- the operation failed because the end of file has been reached.
• isIllegalOperation -- the operation is not possible.
• isPermissionError -- the operation failed because the user does not have sufficient operating system privilege to perform that operation.
• isUserError -- a programmer-defined error value has been raised using fail.

Any computation which returns an IO result may fail with isIllegalOperationError. Additional errors which could be raised by an implementation are listed after the corresponding operation. In some cases, an implementation will not be able to distinguish between the possible error causes. In this case it should return isIllegalOperationError.

Three additional functions are provided to obtain information about an error value. These are ioeGetHandle which returns Just hdl if the error value refers to handle hdl and Nothing otherwise; ioeGetFileName which returns Just name if the error value refers to file name, and Nothing otherwise; and ioeGetErrorString which returns a string. For "user" errors (those which are raised using fail), the string returned by ioeGetErrorString is the argument that was passed to fail; for all other errors, the string is implementation-dependent.

The try function returns an error in a computation explicitly using the Either type.

The bracket function captures a common allocate, compute, deallocate idiom in which the deallocation step must occur even in the case of an error during computation. This is similar to try-catch-finally in Java.

try            :: IO a -> IO (Either IOError a)
try f          =  catch (do r <- f
                            return (Right r))
                        (return . Left)

bracket        :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
bracket before after m = do
        x  <- before
        rs <- try (m x)
        after x
        case rs of
           Right r -> return r
           Left  e -> fail e

-- variant of the above where middle computation doesn't want x
bracket_        :: IO a -> (a -> IO b) -> IO c -> IO c
bracket_ before after m = do
         x  <- before
         rs <- try m
         after x
         case rs of
            Right r -> return r
            Left  e -> fail e

11.2  Files and Handles

File and directory names are values of type String, whose precise meaning is operating system dependent. Files can be opened, yielding a handle which can then be used to operate on the contents of that file.

Haskell defines operations to read and write characters from and to files, represented by values of type Handle. Each value of this type is a handle: a record used by the Haskell run-time system to manage I/O with file system objects. A handle has at least the following properties:

• whether it manages input or output or both;
• whether it is open, closed or semi-closed;
• whether the object is seekable;
• whether buffering is disabled, or enabled on a line or block basis;
• a buffer (whose length may be zero).

Most handles will also have a current I/O position indicating where the next input or output operation will occur. A handle is readable if it manages only input or both input and output; likewise, it is writable if it manages only output or both input and output. A handle is open when first allocated. Once it is closed it can no longer be used for either input or output, though an implementation cannot re-use its storage while references remain to it. Handles are in the Show and Eq classes. The string produced by showing a handle is system dependent; it should include enough information to identify the handle for debugging. A handle is equal according to == only to itself (pointer equality); no attempt is made to compare the internal state of different handles for equality.

11.2.1  Semi-Closed Handles

The operation hGetContents puts a handle hdl into an intermediate state, semi-closed. In this state, hdl is effectively closed, but items are read from hdl on demand and accumulated in a special stream returned by hGetContents hdl. Any operation except for hClose that fails because a handle is closed, also fails if a handle is semi-closed. A semi-closed handle becomes closed:

• if hClose is applied to it;
• if an I/O error occurs when reading an item from the file item from the stream;
• or once the entire contents of the file has been read.

Once a semi-closed handle becomes closed, the contents of the associated stream becomes fixed, and is the list of those items which were successfully read from that handle. Any I/O errors encountered when a handle is semi-closed are simply discarded.

11.2.2  Standard Handles

Three handles are allocated during program initialisation. The first two (stdin and stdout) manage input or output from the Haskell program's standard input or output channel respectively. The third (stderr) manages output to the standard error channel. These handles are initially open.

11.3  Opening and Closing Files

11.3.1  Opening Files

Computation openFile file mode allocates and returns a new, open handle to manage the file file. It manages input if mode is ReadMode, output if mode is WriteMode or AppendMode, and both input and output if mode is ReadWriteMode.

If the file does not exist and it is opened for output, it should be created as a new file. If mode is WriteMode and the file already exists, then it should be truncated to zero length. Some operating systems delete empty files, so there is no guarantee that the file will exist following an openFile with mode WriteMode unless it is subsequently written to successfully. The handle is positioned at the end of the file if mode is AppendMode, and otherwise at the beginning (in which case its internal I/O position is 0). The initial buffer mode is implementation-dependent.

If openFile fails on a file opened for output, the file may still have been created if it did not already exist.

Implementations should enforce as far as possible, locally to the Haskell process, multiple-reader single-writer locking on files. Thus there may either be many handles on the same file which manage input, or just one handle on the file which manages output. If any open or semi-closed handle is managing a file for output, no new handle can be allocated for that file. If any open or semi-closed handle is managing a file for input, new handles can only be allocated if they do not manage output. Whether two files are the same is implementation-dependent, but they should normally be the same if they have the same absolute path name and neither has been renamed, for example.

Error reporting: the openFile computation may fail with isAlreadyInUseError if the file is already open and cannot be reopened; isDoesNotExistError if the file does not exist; or isPermissionError if the user does not have permission to open the file.

11.3.2  Closing Files

Computation hClose hdl makes handle hdl closed. Before the computation finishes, if hdl is writable its buffer is flushed as for hFlush. If the operation fails for any reason, any further operations on the handle will still fail as if hdl had been successfully closed.

11.4  Determining the Size of a File

For a handle hdl which is attached to a physical file, hFileSize hdl returns the size of that file in 8-bit bytes (>= 1).

11.4.1  Detecting the End of Input

For a readable handle hdl, computation hIsEOF hdl returns True if no further input can be taken from hdl or for a physical file, if the current I/O position is equal to the length of the file. Otherwise, it returns False. The computation isEOF is identical, except that it works only on stdin.

11.4.2  Buffering Operations

Three kinds of buffering are supported: line-buffering, block-buffering or no-buffering. These modes have the following effects. For output, items are written out from the internal buffer according to the buffer mode:

• line-buffering: the entire buffer is written out whenever a newline is output, the buffer overflows, a flush is issued, or the handle is closed.
• block-buffering: the entire buffer is written out whenever it overflows, a flush is issued, or the handle is closed.
• no-buffering: output is written immediately, and never stored in the buffer.

Similarly, input occurs according to the buffer mode for handle hdl.

• line-buffering: when the buffer for hdl is not empty, the next item is obtained from the buffer; otherwise, when the buffer is empty, characters up to and including the next newline character are read into the buffer. No characters are available until the newline character is available.
• block-buffering: when the buffer for hdl becomes empty, the next block of data is read into the buffer.
• no-buffering: the next input item is read and returned.

For most implementations, physical files will normally be block-buffered and terminals will normally be line-buffered.

Computation hSetBuffering hdl mode sets the mode of buffering for handle hdl on subsequent reads and writes.

• If mode is LineBuffering, line-buffering is enabled if possible.
• If mode is BlockBuffering size, then block-buffering is enabled if possible. The size of the buffer is n items if size is Just n and is otherwise implementation-dependent.
• If mode is NoBuffering, then buffering is disabled if possible.

If the buffer mode is changed from BlockBuffering or LineBuffering to NoBuffering, then

• if hdl is writable, the buffer is flushed as for hFlush;
• if hdl is not writable, the contents of the buffer is discarded.

Error reporting: the hSetBuffering computation may fail with isPermissionError if the handle has already been used for reading or writing and the implementation does not allow the buffering mode to be changed.

Computation hGetBuffering hdl returns the current buffering mode for hdl.

The default buffering mode when a handle is opened is implementation-dependent and may depend on the file system object which is attached to that handle.

11.4.3  Flushing Buffers

Computation hFlush hdl causes any items buffered for output in handle hdl to be sent immediately to the operating system.

Error reporting: the hFlush computation may fail with: isFullError if the device is full; isPermissionError if a system resource limit would be exceeded.

11.5  Repositioning Handles

11.5.1  Revisiting an I/O Position

Computation hGetPosn hdl returns the current I/O position of hdl as a value of the abstract type HandlePosn. Computation hSetPosn p sets the position of hdl to a previously obtained position p.

Error reporting: the hSetPosn computation may fail with: isPermissionError if a system resource limit would be exceeded.

11.5.2  Seeking to a new Position

Computation hSeek hdl mode i sets the position of handle hdl depending on mode. If mode is:

• AbsoluteSeek: the position of hdl is set to i.
• RelativeSeek: the position of hdl is set to offset i from the current position.
• SeekFromEnd: the position of hdl is set to offset i from the end of the file.

If hdl is block- or line-buffered, then seeking to a position which is not in the current buffer will first cause any items in the output buffer to be written to the device, and then cause the input buffer to be discarded. Some handles may not be seekable (see hIsSeekable), or only support a subset of the possible positioning operations (for instance, it may only be possible to seek to the end of a tape, or to a positive offset from the beginning or current position). It is not possible to set a negative I/O position, or for a physical file, an I/O position beyond the current end-of-file.

Error reporting: the hSeek computation may fail with: isPermissionError if a system resource limit would be exceeded.

11.6  Handle Properties

The functions hIsOpen, hIsClosed, hIsReadable, hIsWritable and hIsSeekable return information about the properties of a handle. Each of these returns True if the handle has the specified property, and False otherwise.

11.7  Text Input and Output

Here we define a standard set of input operations for reading characters and strings from text files, using handles. Many of these functions are generalizations of Prelude functions. I/O in the Prelude generally uses stdin and stdout; here, handles are explicitly specified by the I/O operation.

11.7.1  Checking for Input

Computation hWaitForInput hdl t waits until input is available on handle hdl. It returns True as soon as input is available on hdl, or False if no input is available within t milliseconds.

Computation hReady hdl indicates whether at least one item is available for input from handle hdl.

Computation hGetChar hdl reads a character from the file or channel managed by hdl.

Computation hGetLine hdl reads a line from the file or channel managed by hdl, similar to getLine in the Prelude.

Error reporting: the hWaitForInput, hReady and hGetChar computations may fail with: isEOFError if the end of file has been reached.

11.7.2  Reading Ahead

Computation hLookAhead hdl returns the next character from handle hdl without removing it from the input buffer, blocking until a character is available.

Error reporting: the hLookahead computation may fail with: isEOFError if the end of file has been reached.

Computation hGetContents hdl returns the list of characters corresponding to the unread portion of the channel or file managed by hdl, which is made semi-closed.

Error reporting: the hGetContents computation may fail with: isEOFError if the end of file has been reached.

Computation hPutChar hdl c writes the character c to the file or channel managed by hdl. Characters may be buffered if buffering is enabled for hdl.

Computation hPutStr hdl s writes the string s to the file or channel managed by hdl.

Computation hPrint hdl t writes the string representation of t given by the shows function to the file or channel managed by hdl and appends a newline.

Error reporting: the hPutChar, hPutStr and hPrint computations may fail with: isFull-Error if the device is full; or isPermissionError if another system resource limit would be exceeded.

11.8  Examples

Here are some simple examples to illustrate Haskell I/O.

11.8.1  Summing Two Numbers

This program reads and sums two Integers.

import IO

main = do
         hSetBuffering stdout NoBuffering            
         putStr   "Enter an integer: "        
         x1 <- readNum 
         putStr   "Enter another integer: "          
         x2 <- readNum                          
         putStr  ("Their sum is " ++ show (x1+x2) ++ "\n")
       where readNum :: IO Integer
             readNum =  do { line <- getLine; readIO line }

11.8.2  Copying Files

A simple program to create a copy of a file, with all lower-case characters translated to upper-case. This program will not allow a file to be copied to itself. This version uses character-level I/O. Note that exactly two arguments must be supplied to the program.

import IO
import System

main = do 
         ~[f1,f2] <- getArgs     -- This ~ is required - see below
         h1 <- openFile f1 ReadMode     
         h2 <- openFile f2 WriteMode 
         copyFile h1 h2            
         hClose h1                  
         hClose h2

copyFile h1 h2 = do
                   eof <- hIsEOF h1
                   if eof then return () else
                      do
                        c <- hGetChar h1
                        hPutChar h2 (toUpper c)   
                        copyFile h1 h2

An equivalent but much shorter version, using string I/O is:

import System

main = do
         ~[f1,f2] <- getArgs  -- This ~ is required; see below
         s <- readFile f1
         writeFile f2 (map toUpper s)

The ~ used in the patterns above is a necessary consequence of the do-notation which has been used for the I/O operations. In general, if the pattern on the left of any <- fails to match, the value of the entire do-expression is defined to be the "zero" in the underlying monad. However, since the IO monad has no zero, the ~ is required in order to force the pattern to be irrefutable. Without the ~, a class error would occur because there is no instance of IO for class MonadZero.