base-4.19.2.0: Core data structures and operations
Copyright(c) The University of Glasgow 2001
LicenseBSD-style (see the file libraries/base/LICENSE)
Maintainerlibraries@haskell.org
Stabilitystable
Portabilityportable
Safe HaskellTrustworthy
LanguageHaskell2010

System.IO

Description

The standard IO library.

Synopsis

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.

Instances

Instances details
MonadFail IO Source #

Since: base-4.9.0.0

Instance details

Defined in Control.Monad.Fail

Methods

fail :: String -> IO a Source #

MonadFix IO Source #

Since: base-2.1

Instance details

Defined in Control.Monad.Fix

Methods

mfix :: (a -> IO a) -> IO a Source #

MonadIO IO Source #

Since: base-4.9.0.0

Instance details

Defined in Control.Monad.IO.Class

Methods

liftIO :: IO a -> IO a Source #

Alternative IO Source #

Takes the first non-throwing IO action's result. empty throws an exception.

Since: base-4.9.0.0

Instance details

Defined in GHC.Base

Methods

empty :: IO a Source #

(<|>) :: IO a -> IO a -> IO a Source #

some :: IO a -> IO [a] Source #

many :: IO a -> IO [a] Source #

Applicative IO Source #

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

pure :: a -> IO a Source #

(<*>) :: IO (a -> b) -> IO a -> IO b Source #

liftA2 :: (a -> b -> c) -> IO a -> IO b -> IO c Source #

(*>) :: IO a -> IO b -> IO b Source #

(<*) :: IO a -> IO b -> IO a Source #

Functor IO Source #

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

fmap :: (a -> b) -> IO a -> IO b Source #

(<$) :: a -> IO b -> IO a Source #

Monad IO Source #

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

(>>=) :: IO a -> (a -> IO b) -> IO b Source #

(>>) :: IO a -> IO b -> IO b Source #

return :: a -> IO a Source #

MonadPlus IO Source #

Takes the first non-throwing IO action's result. mzero throws an exception.

Since: base-4.9.0.0

Instance details

Defined in GHC.Base

Methods

mzero :: IO a Source #

mplus :: IO a -> IO a -> IO a Source #

GHCiSandboxIO IO Source #

Since: base-4.4.0.0

Instance details

Defined in GHC.GHCi

Methods

ghciStepIO :: IO a -> IO a Source #

Monoid a => Monoid (IO a) Source #

Since: base-4.9.0.0

Instance details

Defined in GHC.Base

Methods

mempty :: IO a Source #

mappend :: IO a -> IO a -> IO a Source #

mconcat :: [IO a] -> IO a Source #

Semigroup a => Semigroup (IO a) Source #

Since: base-4.10.0.0

Instance details

Defined in GHC.Base

Methods

(<>) :: IO a -> IO a -> IO a Source #

sconcat :: NonEmpty (IO a) -> IO a Source #

stimes :: Integral b => b -> IO a -> IO a Source #

a ~ () => HPrintfType (IO a) Source #

Since: base-4.7.0.0

Instance details

Defined in Text.Printf

Methods

hspr :: Handle -> String -> [UPrintf] -> IO a

a ~ () => PrintfType (IO a) Source #

Since: base-4.7.0.0

Instance details

Defined in Text.Printf

Methods

spr :: String -> [UPrintf] -> IO a

fixIO :: (a -> IO a) -> IO a Source #

The implementation of mfix for IO. If the function passed to fixIO inspects its argument, the resulting action will throw FixIOException.

Files and handles

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.

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

Instances details
Show Handle Source #

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Handle.Types

Eq Handle Source #

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Handle.Types

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 an explicit hClose to close Handles 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.

Standard handles

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

Opening and closing files

Opening files

withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r Source #

data IOMode Source #

Instances

Instances details
Enum IOMode Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.IOMode

Ix IOMode Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.IOMode

Read IOMode Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.IOMode

Show IOMode Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.IOMode

Eq IOMode Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.IOMode

Ord IOMode Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.IOMode

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.

hClose is an interruptible operation in the sense described in Control.Exception. If hClose is interrupted by an asynchronous exception in the process of flushing its buffers, then the I/O device (e.g., file) will be closed anyway.

Special cases

These functions are also exported by the Prelude.

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.

readFile' :: FilePath -> IO String Source #

The readFile' function reads a file and returns the contents of the file as a string. The file is fully read before being returned, as with getContents'.

Since: base-4.15.0.0

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 Integer Source #

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 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.

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.

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.

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 are 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.

Repositioning handles

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:

data HandlePosn Source #

Instances

Instances details
Show HandlePosn Source #

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Handle

Eq HandlePosn Source #

Since: base-4.1.0.0

Instance details

Defined in GHC.IO.Handle

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:

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

Instances details
Enum SeekMode Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.Device

Ix SeekMode Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.Device

Read SeekMode Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.Device

Show SeekMode Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.Device

Eq SeekMode Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.Device

Ord SeekMode Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.Device

hTell :: Handle -> IO Integer Source #

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:

Handle properties

Terminal operations (not portable: GHC only)

hIsTerminalDevice :: Handle -> IO Bool Source #

Is the handle connected to a terminal?

On Windows the result of hIsTerminalDevide might be misleading, because non-native terminals, such as MinTTY used in MSYS and Cygwin environments, are implemented via redirection. Use System.Win32.Types.withHandleToHANDLE System.Win32.MinTTY.isMinTTYHandle to recognise it. Also consider ansi-terminal package for crossplatform terminal support.

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

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

hGetEcho :: Handle -> IO Bool Source #

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

Showing handle state (not portable: GHC only)

hShow :: Handle -> IO String Source #

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 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 hdl 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:

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:

hGetLine :: Handle -> IO String Source #

Computation hGetLine hdl reads a line from the file or channel managed by hdl. hGetLine does not return the newline as part of the result.

A line is separated by the newline set with hSetNewlineMode or nativeNewline by default. The read newline character(s) are not returned as part of the result.

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

This operation may fail with:

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

Examples

Expand
>>> withFile "/home/user/foo" ReadMode hGetLine >>= putStrLn
this is the first line of the file :O
>>> withFile "/home/user/bar" ReadMode (replicateM 3 . hGetLine)
["this is the first line","this is the second line","this is the third line"]

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:

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:

hGetContents' :: Handle -> IO String Source #

The hGetContents' operation reads all input on the given handle before returning it as a String and closing the handle.

Since: base-4.15.0.0

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 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).

getContents' :: IO String Source #

The getContents' operation returns all user input as a single string, which is fully read before being returned (same as hGetContents' stdin).

Since: base-4.15.0.0

readIO :: Read a => String -> IO a Source #

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 Source #

The readLn function combines getLine and readIO.

Binary input and output

hSetBinaryMode :: Handle -> Bool -> IO () Source #

Select binary mode (True) or text mode (False) on a open handle. (See also openBinaryFile.)

This has the same effect as calling hSetEncoding with char8, together with hSetNewlineMode with noNewlineTranslation.

hPutBuf :: Handle -> Ptr a -> Int -> IO () Source #

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

hPutBuf ignores any text encoding that applies to the Handle, writing the bytes directly to the underlying file or device.

hPutBuf ignores the prevailing TextEncoding and NewlineMode on the Handle, and writes bytes directly.

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 Source #

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.

hGetBuf ignores the prevailing TextEncoding and NewlineMode on the Handle, and reads bytes directly.

hGetBufSome :: Handle -> Ptr a -> Int -> IO Int Source #

hGetBufSome hdl buf count reads data from the handle hdl into the buffer buf. If there is any data available to read, then hGetBufSome returns it immediately; it only blocks if there is no data to be 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).

hGetBufSome 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, hGetBufSome will behave as if EOF was reached.

hGetBufSome ignores the prevailing TextEncoding and NewlineMode on the Handle, and reads bytes directly.

hGetBufNonBlocking :: Handle -> Ptr a -> Int -> IO Int Source #

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.

hGetBufNonBlocking ignores the prevailing TextEncoding and NewlineMode on the Handle, and reads bytes directly.

NOTE: on Windows, this function does not work correctly; it behaves identically to hGetBuf.

Temporary files

openTempFile Source #

Arguments

:: FilePath

Directory in which to create the file

-> String

File name template. If the template is "foo.ext" then the created file will be "fooXXX.ext" where XXX is some random number. Note that this should not contain any path separator characters. On Windows, the template prefix may be truncated to 3 chars, e.g. "foobar.ext" will be "fooXXX.ext".

-> 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.

The file is created with permissions such that only the current user can read/write it.

With some exceptions (see below), the file will be created securely in the sense that an attacker should not be able to cause openTempFile to overwrite another file on the filesystem using your credentials, by putting symbolic links (on Unix) in the place where the temporary file is to be created. On Unix the O_CREAT and O_EXCL flags are used to prevent this attack, but note that O_EXCL is sometimes not supported on NFS filesystems, so if you rely on this behaviour it is best to use local filesystems only.

openBinaryTempFile :: FilePath -> String -> IO (FilePath, Handle) Source #

Like openTempFile, but opens the file in binary mode. See openBinaryFile for more comments.

openTempFileWithDefaultPermissions :: FilePath -> String -> IO (FilePath, Handle) Source #

Like openTempFile, but uses the default file permissions

Unicode encoding/decoding

A text-mode Handle has an associated TextEncoding, which is used to decode bytes into Unicode characters when reading, and encode Unicode characters into bytes when writing.

The default TextEncoding is the same as the default encoding on your system, which is also available as localeEncoding. (GHC note: on Windows, we currently do not support double-byte encodings; if the console's code page is unsupported, then localeEncoding will be latin1.)

Encoding and decoding errors are always detected and reported, except during lazy I/O (hGetContents, getContents, and readFile), where a decoding error merely results in termination of the character stream, as with other I/O errors.

hSetEncoding :: Handle -> TextEncoding -> IO () Source #

The action hSetEncoding hdl encoding changes the text encoding for the handle hdl to encoding. The default encoding when a Handle is created is localeEncoding, namely the default encoding for the current locale.

To create a Handle with no encoding at all, use openBinaryFile. To stop further encoding or decoding on an existing Handle, use hSetBinaryMode.

hSetEncoding may need to flush buffered data in order to change the encoding.

hGetEncoding :: Handle -> IO (Maybe TextEncoding) Source #

Return the current TextEncoding for the specified Handle, or Nothing if the Handle is in binary mode.

Note that the TextEncoding remembers nothing about the state of the encoder/decoder in use on this Handle. For example, if the encoding in use is UTF-16, then using hGetEncoding and hSetEncoding to save and restore the encoding may result in an extra byte-order-mark being written to the file.

Unicode encodings

data TextEncoding Source #

A TextEncoding is a specification of a conversion scheme between sequences of bytes and sequences of Unicode characters.

For example, UTF-8 is an encoding of Unicode characters into a sequence of bytes. The TextEncoding for UTF-8 is utf8.

Instances

Instances details
Show TextEncoding Source #

Since: base-4.3.0.0

Instance details

Defined in GHC.IO.Encoding.Types

latin1 :: TextEncoding Source #

The Latin1 (ISO8859-1) encoding. This encoding maps bytes directly to the first 256 Unicode code points, and is thus not a complete Unicode encoding. An attempt to write a character greater than '\255' to a Handle using the latin1 encoding will result in an error.

utf8 :: TextEncoding Source #

The UTF-8 Unicode encoding

utf8_bom :: TextEncoding Source #

The UTF-8 Unicode encoding, with a byte-order-mark (BOM; the byte sequence 0xEF 0xBB 0xBF). This encoding behaves like utf8, except that on input, the BOM sequence is ignored at the beginning of the stream, and on output, the BOM sequence is prepended.

The byte-order-mark is strictly unnecessary in UTF-8, but is sometimes used to identify the encoding of a file.

utf16 :: TextEncoding Source #

The UTF-16 Unicode encoding (a byte-order-mark should be used to indicate endianness).

utf16le :: TextEncoding Source #

The UTF-16 Unicode encoding (little-endian)

utf16be :: TextEncoding Source #

The UTF-16 Unicode encoding (big-endian)

utf32 :: TextEncoding Source #

The UTF-32 Unicode encoding (a byte-order-mark should be used to indicate endianness).

utf32le :: TextEncoding Source #

The UTF-32 Unicode encoding (little-endian)

utf32be :: TextEncoding Source #

The UTF-32 Unicode encoding (big-endian)

localeEncoding :: TextEncoding Source #

The Unicode encoding of the current locale

This is the initial locale encoding: if it has been subsequently changed by setLocaleEncoding this value will not reflect that change.

char8 :: TextEncoding Source #

An encoding in which Unicode code points are translated to bytes by taking the code point modulo 256. When decoding, bytes are translated directly into the equivalent code point.

This encoding never fails in either direction. However, encoding discards information, so encode followed by decode is not the identity.

Since: base-4.4.0.0

mkTextEncoding :: String -> IO TextEncoding Source #

Look up the named Unicode encoding. May fail with

The set of known encodings is system-dependent, but includes at least:

  • UTF-8
  • UTF-16, UTF-16BE, UTF-16LE
  • UTF-32, UTF-32BE, UTF-32LE

There is additional notation (borrowed from GNU iconv) for specifying how illegal characters are handled:

  • a suffix of //IGNORE, e.g. UTF-8//IGNORE, will cause all illegal sequences on input to be ignored, and on output will drop all code points that have no representation in the target encoding.
  • a suffix of //TRANSLIT will choose a replacement character for illegal sequences or code points.
  • a suffix of //ROUNDTRIP will use a PEP383-style escape mechanism to represent any invalid bytes in the input as Unicode codepoints (specifically, as lone surrogates, which are normally invalid in UTF-32). Upon output, these special codepoints are detected and turned back into the corresponding original byte.

In theory, this mechanism allows arbitrary data to be roundtripped via a String with no loss of data. In practice, there are two limitations to be aware of:

  1. This only stands a chance of working for an encoding which is an ASCII superset, as for security reasons we refuse to escape any bytes smaller than 128. Many encodings of interest are ASCII supersets (in particular, you can assume that the locale encoding is an ASCII superset) but many (such as UTF-16) are not.
  2. If the underlying encoding is not itself roundtrippable, this mechanism can fail. Roundtrippable encodings are those which have an injective mapping into Unicode. Almost all encodings meet this criterion, but some do not. Notably, Shift-JIS (CP932) and Big5 contain several different encodings of the same Unicode codepoint.

On Windows, you can access supported code pages with the prefix CP; for example, "CP1250".

Newline conversion

In Haskell, a newline is always represented by the character '\n'. However, in files and external character streams, a newline may be represented by another character sequence, such as '\r\n'.

A text-mode Handle has an associated NewlineMode that specifies how to translate newline characters. The NewlineMode specifies the input and output translation separately, so that for instance you can translate '\r\n' to '\n' on input, but leave newlines as '\n' on output.

The default NewlineMode for a Handle is nativeNewlineMode, which does no translation on Unix systems, but translates '\r\n' to '\n' and back on Windows.

Binary-mode Handles do no newline translation at all.

hSetNewlineMode :: Handle -> NewlineMode -> IO () Source #

Set the NewlineMode on the specified Handle. All buffered data is flushed first.

data Newline Source #

The representation of a newline in the external file or stream.

Constructors

LF
'\n'
CRLF
'\r\n'

Instances

Instances details
Read Newline Source #

Since: base-4.3.0.0

Instance details

Defined in GHC.IO.Handle.Types

Show Newline Source #

Since: base-4.3.0.0

Instance details

Defined in GHC.IO.Handle.Types

Eq Newline Source #

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.Handle.Types

Ord Newline Source #

Since: base-4.3.0.0

Instance details

Defined in GHC.IO.Handle.Types

nativeNewline :: Newline Source #

The native newline representation for the current platform: LF on Unix systems, CRLF on Windows.

data NewlineMode Source #

Specifies the translation, if any, of newline characters between internal Strings and the external file or stream. Haskell Strings are assumed to represent newlines with the '\n' character; the newline mode specifies how to translate '\n' on output, and what to translate into '\n' on input.

Constructors

NewlineMode 

Fields

noNewlineTranslation :: NewlineMode Source #

Do no newline translation at all.

noNewlineTranslation  = NewlineMode { inputNL  = LF, outputNL = LF }

universalNewlineMode :: NewlineMode Source #

Map '\r\n' into '\n' on input, and '\n' to the native newline representation on output. This mode can be used on any platform, and works with text files using any newline convention. The downside is that readFile >>= writeFile might yield a different file.

universalNewlineMode  = NewlineMode { inputNL  = CRLF,
                                      outputNL = nativeNewline }

nativeNewlineMode :: NewlineMode Source #

Use the native newline representation on both input and output

nativeNewlineMode  = NewlineMode { inputNL  = nativeNewline
                                   outputNL = nativeNewline }