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System.IO

Portability | portable
Stability | stable
Maintainer | libraries@haskell.org

Contents

The IO monad
Files and handles
  Standard handles
Opening and closing files
  Opening files
  Closing files
  Special cases
  File locking
Operations on handles
  Determining and changing the size of a file
  Detecting the end of input
  Buffering operations
  Repositioning handles
  Handle properties
  Terminal operations
  Showing handle state
Text input and output
  Text input
  Text output
  Special cases for standard input and output
Binary input and output
Temporary files

Description
The standard IO library.
Synopsis

data IO a
fixIO :: (a -> IO a) -> IO a
type FilePath = String
data Handle
stdin :: Handle
stdout :: Handle
stderr :: Handle
openFile :: FilePath -> IOMode -> IO Handle

data IOMode

= ReadMode
| WriteMode
| AppendMode
| ReadWriteMode

hClose :: Handle -> IO ()
readFile :: FilePath -> IO String
writeFile :: FilePath -> String -> IO ()
appendFile :: FilePath -> String -> IO ()
hFileSize :: Handle -> IO Integer
hSetFileSize :: Handle -> Integer -> IO ()
hIsEOF :: Handle -> IO Bool
isEOF :: IO Bool

data BufferMode

= NoBuffering
| LineBuffering
| BlockBuffering (Maybe Int)

hSetBuffering :: Handle -> BufferMode -> IO ()
hGetBuffering :: Handle -> IO BufferMode
hFlush :: Handle -> IO ()
hGetPosn :: Handle -> IO HandlePosn
hSetPosn :: HandlePosn -> IO ()
data HandlePosn
hSeek :: Handle -> SeekMode -> Integer -> IO ()

data SeekMode

= AbsoluteSeek
| RelativeSeek
| SeekFromEnd

hTell :: Handle -> IO Integer
hIsOpen :: Handle -> IO Bool
hIsClosed :: Handle -> IO Bool
hIsReadable :: Handle -> IO Bool
hIsWritable :: Handle -> IO Bool
hIsSeekable :: Handle -> IO Bool
hIsTerminalDevice :: Handle -> IO Bool
hSetEcho :: Handle -> Bool -> IO ()
hGetEcho :: Handle -> IO Bool
hShow :: Handle -> IO String
hWaitForInput :: Handle -> Int -> IO Bool
hReady :: Handle -> IO Bool
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 ()
interact :: (String -> String) -> IO ()
putChar :: Char -> IO ()
putStr :: String -> IO ()
putStrLn :: String -> IO ()
print :: Show a => a -> IO ()
getChar :: IO Char
getLine :: IO String
getContents :: IO String
readIO :: Read a => String -> IO a
readLn :: Read a => IO a
openBinaryFile :: FilePath -> IOMode -> IO Handle
hSetBinaryMode :: Handle -> Bool -> IO ()
hPutBuf :: Handle -> Ptr a -> Int -> IO ()
hGetBuf :: Handle -> Ptr a -> Int -> IO Int
hPutBufNonBlocking :: Handle -> Ptr a -> Int -> IO Int
hGetBufNonBlocking :: Handle -> Ptr a -> Int -> IO Int
openTempFile :: FilePath -> String -> IO (FilePath, Handle)
openBinaryTempFile :: FilePath -> String -> IO (FilePath, Handle)

The IO monad
data IO a

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.

Instances

Functor IO
Monad IO
MonadFix IO
Typeable1 IO
MArray IOArray e IO
MArray IOUArray Bool IO
MArray IOUArray Char IO
MArray IOUArray Double IO
MArray IOUArray Float IO
MArray IOUArray Int IO
MArray IOUArray Int16 IO
MArray IOUArray Int32 IO
MArray IOUArray Int64 IO
MArray IOUArray Int8 IO
MArray IOUArray Word IO
MArray IOUArray Word16 IO
MArray IOUArray Word32 IO
MArray IOUArray Word64 IO
MArray IOUArray Word8 IO
Storable e => MArray StorableArray e IO
MArray IOUArray (FunPtr a) IO
MArray IOUArray (Ptr a) IO
MArray IOUArray (StablePtr a) IO
Typeable a => Data (IO a)
HPrintfType (IO a)
PrintfType (IO a)

fixIO :: (a -> IO a) -> IO a
Files and handles
type FilePath = String
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

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.

GHC note: a `Handle` will be automatically closed when the garbage collector detects that it has become unreferenced by the program. However, relying on this behaviour is not generally recommended: the garbage collector is unpredictable. If possible, use explicit an explicit hClose to close `Handle`s when they are no longer required. GHC does not currently attempt to free up file descriptors when they have run out, it is your responsibility to ensure that this doesn't happen.

Instances

Data Handle
Eq Handle
Show Handle
Typeable Handle

Standard handles
Three handles are allocated during program initialisation, and are initially open.
stdin :: Handle
A handle managing input from the Haskell program's standard input channel.
stdout :: Handle
A handle managing output to the Haskell program's standard output channel.
stderr :: Handle
A handle managing output to the Haskell program's standard error channel.
Opening and closing files
Opening files
openFile :: FilePath -> IOMode -> IO Handle

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.

Note: if you will be working with files containing binary data, you'll want to be using `openBinaryFile`.

data IOMode

Constructors

ReadMode |
WriteMode |
AppendMode |
ReadWriteMode |

Instances

Enum IOMode
Eq IOMode
Ix IOMode
Ord IOMode
Read IOMode
Show IOMode

Closing files
hClose :: Handle -> IO ()

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 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 String
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 ()
The computation `writeFile` `file str` function writes the string `str`, to the file `file`.
appendFile :: FilePath -> String -> IO ()

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 Integer
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 ()
`hSetFileSize` `hdl` `size` truncates the physical file with handle `hdl` to `size` bytes.
Detecting the end of input
hIsEOF :: Handle -> IO Bool
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`.
isEOF :: IO Bool
The computation `isEOF` is identical to `hIsEOF`, except that it works only on `stdin`.
Buffering operations
data BufferMode

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.

Constructors

NoBuffering | buffering is disabled if possible.
LineBuffering | 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.

Instances

Eq BufferMode
Ord BufferMode
Read BufferMode
Show BufferMode

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

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 BufferMode
Computation `hGetBuffering` `hdl` returns the current buffering mode for `hdl`.
hFlush :: Handle -> IO ()

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 HandlePosn
Computation `hGetPosn` `hdl` returns the current I/O position of `hdl` as a value of the abstract type `HandlePosn`.
hSetPosn :: HandlePosn -> IO ()

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.

data HandlePosn

Instances

Eq HandlePosn
Show HandlePosn

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

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:

- `isPermissionError` if a system resource limit would be exceeded.

data SeekMode

A mode that determines the effect of `hSeek` `hdl mode i`, as follows:
Constructors

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.

Instances

Enum SeekMode
Eq SeekMode
Ix SeekMode
Ord SeekMode
Read SeekMode
Show SeekMode

hTell :: Handle -> IO Integer
Handle properties
hIsOpen :: Handle -> IO Bool
hIsClosed :: Handle -> IO Bool
hIsReadable :: Handle -> IO Bool
hIsWritable :: Handle -> IO Bool
hIsSeekable :: Handle -> IO Bool
Terminal operations
hIsTerminalDevice :: Handle -> IO Bool
Is the handle connected to a terminal?
hSetEcho :: Handle -> Bool -> IO ()
Set the echoing status of a handle connected to a terminal.
hGetEcho :: Handle -> IO Bool
Get the echoing status of a handle connected to a terminal.
Showing handle state
hShow :: Handle -> IO String
`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 Bool

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.

If `t` is less than zero, then `hWaitForInput` waits indefinitely. NOTE: in the current implementation, this is the only case that works correctly (if `t` is non-zero, then all other concurrent threads are blocked until data is available).

This operation may fail with:

- `isEOFError` if the end of file has been reached.

hReady :: Handle -> IO Bool

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

This operation may fail with:

- `isEOFError` if the end of file has been reached.

hGetChar :: Handle -> IO Char

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:

- `isEOFError` if the end of file has been reached.

hGetLine :: Handle -> IO String

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 Char

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 String

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:

- `isEOFError` if the end of file has been reached.

Text output
hPutChar :: Handle -> Char -> IO ()

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:

- `isFullError` if the device is full; or
- `isPermissionError` if another system resource limit would be exceeded.

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

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

This operation may fail with:

- `isFullError` if the device is full; or
- `isPermissionError` if another system resource limit would be exceeded.

hPutStrLn :: Handle -> String -> IO ()
The same as `hPutStr`, but adds a newline character.
hPrint :: Show a => Handle -> a -> IO ()

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:

- `isFullError` if the device is full; or
- `isPermissionError` if another system resource limit would be exceeded.

Special cases for standard input and output
These functions are also exported by the Prelude.
interact :: (String -> String) -> IO ()
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 ()
Write a character to the standard output device (same as `hPutChar` `stdout`).
putStr :: String -> IO ()
Write a string to the standard output device (same as `hPutStr` `stdout`).
putStrLn :: String -> IO ()
The same as `putStr`, but adds a newline character.
print :: Show a => a -> IO ()

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 Char
Read a character from the standard input device (same as `hGetChar` `stdin`).
getLine :: IO String
Read a line from the standard input device (same as `hGetLine` `stdin`).
getContents :: IO String
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 a
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 a
The `readLn` function combines `getLine` and `readIO`.
Binary input and output
openBinaryFile :: FilePath -> IOMode -> IO Handle

Like `openFile`, but open the file in binary mode. On Windows, reading a file in text mode (which is the default) will translate CRLF to LF, and writing will translate LF to CRLF. This is usually what you want with text files. With binary files this is undesirable; also, as usual under Microsoft operating systems, text mode treats control-Z as EOF. Binary mode turns off all special treatment of end-of-line and end-of-file characters. (See also `hSetBinaryMode`.)

hSetBinaryMode :: Handle -> Bool -> IO ()
Select binary mode (`True`) or text mode (`False`) on a open handle. (See also `openBinaryFile`.)
hPutBuf :: Handle -> Ptr a -> Int -> IO ()

`hPutBuf` `hdl buf count` writes `count` 8-bit bytes from the buffer `buf` to the handle `hdl`. It returns ().

This operation may fail with:

- `ResourceVanished` if the handle is a pipe or socket, and the reading end is closed. (If this is a POSIX system, and the program has not asked to ignore SIGPIPE, then a SIGPIPE may be delivered instead, whose default action is to terminate the program).

hGetBuf :: Handle -> Ptr a -> Int -> IO Int

`hGetBuf` `hdl buf count` reads data from the handle `hdl` into the buffer `buf` until either EOF is reached or `count` 8-bit bytes have been read. It returns the number of bytes actually read. This may be zero if EOF was reached before any data was read (or if `count` is zero).

`hGetBuf` never raises an EOF exception, instead it returns a value smaller than `count`.

If the handle is a pipe or socket, and the writing end is closed, `hGetBuf` will behave as if EOF was reached.

hPutBufNonBlocking :: Handle -> Ptr a -> Int -> IO Int
hGetBufNonBlocking :: Handle -> Ptr a -> Int -> IO Int

`hGetBufNonBlocking` `hdl buf count` reads data from the handle `hdl` into the buffer `buf` until either EOF is reached, or `count` 8-bit bytes have been read, or there is no more data available to read immediately.

`hGetBufNonBlocking` is identical to `hGetBuf`, except that it will never block waiting for data to become available, instead it returns only whatever data is available. To wait for data to arrive before calling `hGetBufNonBlocking`, use `hWaitForInput`.

If the handle is a pipe or socket, and the writing end is closed, `hGetBufNonBlocking` will behave as if EOF was reached.

Temporary files
openTempFile

:: FilePath | Directory in which to create the file
-> String | File name template. If the template is "foo.ext" then the create file will be "fooXXX.ext" where XXX is some random number.
-> IO (FilePath, Handle) |
The function creates a temporary file in ReadWrite mode. The created file isn't deleted automatically, so you need to delete it manually.

openBinaryTempFile :: FilePath -> String -> IO (FilePath, Handle)
Like `openTempFile`, but opens the file in binary mode. See `openBinaryFile` for more comments.
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