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The IO monad

data IO a Source

A value of type IO a is a computation which, when performed, does some I/O before returning a value of type a.

There is really only one way to "perform" an I/O action: bind it to Main.main in your program. When your program is run, the I/O will be performed. It isn't possible to perform I/O from an arbitrary function, unless that function is itself in the IO monad and called at some point, directly or indirectly, from Main.main.

IO is a monad, so IO actions can be combined using either the do-notation or the >> and >>= operations from the Monad class.


fixIO :: (a -> IO a) -> IO aSource

Files and handles

type FilePath = StringSource

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.

data Handle Source

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; no attempt is made to compare the internal state of different handles for equality.

Standard handles

Three handles are allocated during program initialisation, and are initially open.

stdin :: HandleSource

A handle managing input from the Haskell program's standard input channel.

stdout :: HandleSource

A handle managing output to the Haskell program's standard output channel.

stderr :: HandleSource

A handle managing output to the Haskell program's standard error channel.

Opening and closing files

Opening files

withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO rSource

withFile name mode act opens a file using openFile and passes the resulting handle to the computation act. The handle will be closed on exit from withFile, whether by normal termination or by raising an exception. If closing the handle raises an exception, then this exception will be raised by withFile rather than any exception raised by act.

openFile :: FilePath -> IOMode -> IO HandleSource

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 position is 0). The initial buffer mode is implementation-dependent.

This operation 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.

Closing files

hClose :: Handle -> IO ()Source

Computation hClose hdl makes handle hdl closed. Before the computation finishes, if hdl is writable its buffer is flushed as for hFlush. Performing hClose on a handle that has already been closed has no effect; doing so is not an error. All other operations on a closed handle will fail. If hClose fails for any reason, any further operations (apart from hClose) on the handle will still fail as if hdl had been successfully closed.

Special cases

These functions are also exported by the Prelude.

readFile :: FilePath -> IO StringSource

The readFile function reads a file and returns the contents of the file as a string. The file is read lazily, on demand, as with getContents.

writeFile :: FilePath -> String -> IO ()Source

The computation writeFile file str function writes the string str, to the file file.

appendFile :: FilePath -> String -> IO ()Source

The computation appendFile file str function appends the string str, to the file file.

Note that writeFile and appendFile write a literal string to a file. To write a value of any printable type, as with print, use the show function to convert the value to a string first.

 main = appendFile "squares" (show [(x,x*x) | x <- [0,0.1..2]])

File locking

Implementations should enforce as far as possible, at least locally to the Haskell process, multiple-reader single-writer locking on files. That is, 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.

Warning: the readFile operation holds a semi-closed handle on the file until the entire contents of the file have been consumed. It follows that an attempt to write to a file (using writeFile, for example) that was earlier opened by readFile will usually result in failure with isAlreadyInUseError.

Operations on handles

Determining and changing the size of a file

hFileSize :: Handle -> IO IntegerSource

For a handle hdl which attached to a physical file, hFileSize hdl returns the size of that file in 8-bit bytes.

hSetFileSize :: Handle -> Integer -> IO ()Source

hSetFileSize hdl size truncates the physical file with handle hdl to size bytes.

Detecting the end of input

hIsEOF :: Handle -> IO BoolSource

For a readable handle hdl, 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.

NOTE: hIsEOF may block, because it has to attempt to read from the stream to determine whether there is any more data to be read.

isEOF :: IO BoolSource

The computation isEOF is identical to hIsEOF, except that it works only on stdin.

Buffering operations

data BufferMode Source

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, or flushed, from the internal buffer according to the buffer mode:

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

An implementation is free to flush the buffer more frequently, but not less frequently, than specified above. The output buffer is emptied as soon as it has been written out.

Similarly, input occurs according to the buffer mode for the handle:

  • line-buffering: when the buffer for the handle 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 or the buffer is full.
  • block-buffering: when the buffer for the handle becomes empty, the next block of data is read into the buffer.
  • no-buffering: the next input item is read and returned. The hLookAhead operation implies that even a no-buffered handle may require a one-character buffer.

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



buffering is disabled if possible.


line-buffering should be enabled if possible.

BlockBuffering (Maybe Int)

block-buffering should be enabled if possible. The size of the buffer is n items if the argument is Just n and is otherwise implementation-dependent.

hSetBuffering :: Handle -> BufferMode -> IO ()Source

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

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.

This operation 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.

hGetBuffering :: Handle -> IO BufferModeSource

Computation hGetBuffering hdl returns the current buffering mode for hdl.

hFlush :: Handle -> IO ()Source

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

This operation may fail with:

  • isFullError if the device is full;
  • isPermissionError if a system resource limit would be exceeded. It is unspecified whether the characters in the buffer are discarded or retained under these circumstances.

Repositioning handles

hGetPosn :: Handle -> IO HandlePosnSource

Computation hGetPosn hdl returns the current I/O position of hdl as a value of the abstract type HandlePosn.

hSetPosn :: HandlePosn -> IO ()Source

If a call to hGetPosn hdl returns a position p, then computation hSetPosn p sets the position of hdl to the position it held at the time of the call to hGetPosn.

This operation may fail with:

  • isPermissionError if a system resource limit would be exceeded.

hSeek :: Handle -> SeekMode -> Integer -> IO ()Source

Computation hSeek hdl mode i sets the position of handle hdl depending on mode. The offset i is given in terms of 8-bit bytes.

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.

This operation may fail with:

  • isIllegalOperationError if the Handle is not seekable, or does not support the requested seek mode.
  • isPermissionError if a system resource limit would be exceeded.

data SeekMode Source

A mode that determines the effect of hSeek hdl mode i.



the position of hdl is set to i.


the position of hdl is set to offset i from the current position.


the position of hdl is set to offset i from the end of the file.

hTell :: Handle -> IO IntegerSource

Computation hTell hdl returns the current position of the handle hdl, as the number of bytes from the beginning of the file. The value returned may be subsequently passed to hSeek to reposition the handle to the current position.

This operation may fail with:

  • isIllegalOperationError if the Handle is not seekable.

Handle properties

Each of these operations returns True if the handle has the the specified property, or False otherwise.

Terminal operations

hIsTerminalDevice :: Handle -> IO BoolSource

Is the handle connected to a terminal?

hSetEcho :: Handle -> Bool -> IO ()Source

Set the echoing status of a handle connected to a terminal.

hGetEcho :: Handle -> IO BoolSource

Get the echoing status of a handle connected to a terminal.

Showing handle state

hShow :: Handle -> IO StringSource

hShow is in the IO monad, and gives more comprehensive output than the (pure) instance of Show for Handle.

Text input and output

Text input

hWaitForInput :: Handle -> Int -> IO BoolSource

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. Note that hWaitForInput waits until one or more full characters are available, which means that it needs to do decoding, and hence may fail with a decoding error.

If t is less than zero, then hWaitForInput waits indefinitely.

This operation may fail with:

  • isEOFError if the end of file has been reached.
  • a decoding error, if the input begins with an invalid byte sequence in this Handle's encoding.

hReady :: Handle -> IO BoolSource

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

This operation may fail with:

hGetChar :: Handle -> IO CharSource

Computation hGetChar hdl reads a character from the file or channel managed by hdl, blocking until a character is available.

This operation may fail with:

hGetLine :: Handle -> IO StringSource

Computation hGetLine hdl reads a line from the file or channel managed by hdl.

This operation may fail with:

  • isEOFError if the end of file is encountered when reading the first character of the line.

If hGetLine encounters end-of-file at any other point while reading in a line, it is treated as a line terminator and the (partial) line is returned.

hLookAhead :: Handle -> IO CharSource

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

This operation may fail with:

  • isEOFError if the end of file has been reached.

hGetContents :: Handle -> IO StringSource

Computation hGetContents hdl returns the list of characters corresponding to the unread portion of the channel or file managed by hdl, which is put 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 list returned by hGetContents hdl.

Any operation that fails because a handle is closed, also fails if a handle is semi-closed. The only exception is hClose. A semi-closed handle becomes closed:

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

Once a semi-closed handle becomes closed, the contents of the associated list becomes fixed. The contents of this final list is only partially specified: it will contain at least all the items of the stream that were evaluated prior to the handle becoming closed.

Any I/O errors encountered while a handle is semi-closed are simply discarded.

This operation may fail with:

Text output

hPutChar :: Handle -> Char -> IO ()Source

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

This operation may fail with:

hPutStr :: Handle -> String -> IO ()Source

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

This operation may fail with:

hPutStrLn :: Handle -> String -> IO ()Source

The same as hPutStr, but adds a newline character.

hPrint :: Show a => Handle -> a -> IO ()Source

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.

This operation may fail with:

Special cases for standard input and output

These functions are also exported by the Prelude.

interact :: (String -> String) -> IO ()Source

The interact function takes a function of type String->String as its argument. The entire input from the standard input device is passed to this function as its argument, and the resulting string is output on the standard output device.

putChar :: Char -> IO ()Source

Write a character to the standard output device (same as hPutChar stdout).

putStr :: String -> IO ()Source

Write a string to the standard output device (same as hPutStr stdout).

putStrLn :: String -> IO ()Source

The same as putStr, but adds a newline character.

print :: Show a => a -> IO ()Source

The print function outputs a value of any printable type to the standard output device. Printable types are those that are instances of class Show; print converts values to strings for output using the show operation and adds a newline.

For example, a program to print the first 20 integers and their powers of 2 could be written as:

 main = print ([(n, 2^n) | n <- [0..19]])

getChar :: IO CharSource

Read a character from the standard input device (same as hGetChar stdin).

getLine :: IO StringSource

Read a line from the standard input device (same as hGetLine stdin).

getContents :: IO StringSource

The getContents operation returns all user input as a single string, which is read lazily as it is needed (same as hGetContents stdin).

readIO :: Read a => String -> IO aSource

The readIO function is similar to read except that it signals parse failure to the IO monad instead of terminating the program.

readLn :: Read a => IO aSource

The readLn function combines getLine and readIO.