IO

haskell98-2.0.0.2: Compatibility with Haskell 98

Safe Haskell | Safe

IO

Synopsis

- data Handle
- data HandlePosn
- data IOMode
  - = ReadMode
  - | WriteMode
  - | AppendMode
  - | ReadWriteMode
- data BufferMode
  - = NoBuffering
  - | LineBuffering
  - | BlockBuffering (Maybe Int)
- data SeekMode
  - = AbsoluteSeek
  - | RelativeSeek
  - | SeekFromEnd
- stdin :: Handle
- stdout :: Handle
- stderr :: Handle
- openFile :: FilePath -> IOMode -> IO Handle
- hClose :: Handle -> IO ()
- hFileSize :: Handle -> IO Integer
- hIsEOF :: Handle -> IO Bool
- isEOF :: IO Bool
- 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
- 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 -> IO (Either IOError a)
- bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
- bracket_ :: IO a -> (a -> IO b) -> IO c -> IO c
- data IO a
- type FilePath = String
- type IOError = IOException
- ioError :: IOError -> IO a
- userError :: String -> IOError
- catch :: IO a -> (IOError -> IO a) -> IO a
- 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
- readFile :: FilePath -> IO String
- writeFile :: FilePath -> String -> IO ()
- appendFile :: FilePath -> String -> IO ()
- readIO :: Read a => String -> IO a
- readLn :: Read a => IO a

# Documentation

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.

Instances

Eq Handle |
Show Handle |
Typeable Handle |

data HandlePosn Source

Instances

Eq HandlePosn |
Show HandlePosn |

data IOMode Source

See `openFile`

Constructors

ReadMode |
WriteMode |
AppendMode |
ReadWriteMode |

Instances

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

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.

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 |

data SeekMode Source

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

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 |
Ord SeekMode |
Read SeekMode |
Show SeekMode |
Ix SeekMode |

stdin :: Handle Source

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

stdout :: Handle Source

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

stderr :: Handle Source

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

openFile :: FilePath -> IOMode -> IO Handle Source

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`.

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.

hFileSize :: Handle -> IO Integer Source

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

hIsEOF :: Handle -> IO Bool Source

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 Bool Source

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

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 BufferMode Source

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.

hGetPosn :: Handle -> IO HandlePosn Source

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.

hWaitForInput :: Handle -> Int -> IO Bool Source

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.

NOTE for GHC users: unless you use the `-threaded` flag, `hWaitForInput t` where `t >= 0` will block all other Haskell threads for the duration of the call. It behaves like a `safe` foreign call in this respect.

hReady :: Handle -> IO Bool Source

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 Source

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 Source

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 Source

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 Source

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.

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:

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

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:

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

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:

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

hIsOpen :: Handle -> IO Bool Source

hIsClosed :: Handle -> IO Bool Source

hIsReadable :: Handle -> IO Bool Source

hIsWritable :: Handle -> IO Bool Source

hIsSeekable :: Handle -> IO Bool Source

isAlreadyExistsError :: IOError -> Bool Source

An error indicating that an `IO` operation failed because one of its arguments already exists.

isDoesNotExistError :: IOError -> Bool Source

An error indicating that an `IO` operation failed because one of its arguments does not exist.

isAlreadyInUseError :: IOError -> Bool Source

An error indicating that an `IO` 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 :: IOError -> Bool Source

An error indicating that an `IO` operation failed because the device is full.

isEOFError :: IOError -> Bool Source

An error indicating that an `IO` operation failed because the end of file has been reached.

isIllegalOperation :: IOError -> Bool Source

An error indicating that an `IO` operation failed because the operation was not possible. Any computation which returns an `IO` result may fail with `isIllegalOperation`. In some cases, an implementation will not be able to distinguish between the possible error causes. In this case it should fail with `isIllegalOperation`.

isPermissionError :: IOError -> Bool Source

An error indicating that an `IO` operation failed because the user does not have sufficient operating system privilege to perform that operation.

isUserError :: IOError -> Bool Source

A programmer-defined error value constructed using `userError`.

ioeGetErrorString :: IOError -> String Source

ioeGetHandle :: IOError -> Maybe Handle Source

ioeGetFileName :: IOError -> Maybe FilePath Source

try :: IO a -> IO (Either IOError a)

The construct `try` `comp` exposes IO errors which occur within a computation, and which are not fully handled.

Non-I/O exceptions are not caught by this variant; to catch all exceptions, use `try` from Control.Exception.

bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c

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.

This version handles only IO errors, as defined by Haskell 98. The version of `bracket` in Control.Exception handles all exceptions, and should be used instead.

bracket_ :: IO a -> (a -> IO b) -> IO c -> IO c

A variant of `bracket` where the middle computation doesn't want `x`.

This version handles only IO errors, as defined by Haskell 98. The version of `bracket_` in Control.Exception handles all exceptions, and should be used instead.

data IO a

Instances

Monad IO |
Functor IO |

type FilePath = String Source

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.

type IOError = IOException Source

The Haskell 98 type for exceptions in the `IO` monad. Any I/O operation may raise an `IOError` instead of returning a result. For a more general type of exception, including also those that arise in pure code, see Control.Exception.Exception.

In Haskell 98, this is an opaque type.

ioError :: IOError -> IO aSource

Raise an `IOError` in the `IO` monad.

userError :: String -> IOError Source

Construct an `IOError` value with a string describing the error. The `fail` method of the `IO` instance of the `Monad` class raises a `userError`, thus:

     instance Monad IO where
       ...
       fail s = ioError (userError s)

catch :: IO a -> (IOError -> IO a) -> IO a

The `catch` function establishes a handler that receives any `IOError` raised in the action protected by `catch`. An `IOError` is caught by the most recent handler established by one of the exception handling functions. These handlers are not selective: all `IOError`s are caught. Exception propagation must be explicitly provided in a handler by re-raising any unwanted exceptions. For example, in

     f = catch g (\e -> if IO.isEOFError e then return [] else ioError e)

the function `f` returns `[]` when an end-of-file exception (cf. `isEOFError`) occurs in `g`; otherwise, the exception is propagated to the next outer handler.

When an exception propagates outside the main program, the Haskell system prints the associated `IOError` value and exits the program.

Non-I/O exceptions are not caught by this variant; to catch all exceptions, use `catch` from Control.Exception.

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 Char Source

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

getLine :: IO String Source

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

getContents :: IO String Source

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

readFile :: FilePath -> IO String Source

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]])

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`.