Documentation

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.

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.

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

>>> 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"]

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.

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.
• isPermissionError if another system resource limit would be exceeded.

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

Note that hPutStr is not concurrency safe unless the BufferMode of hdl is set to LineBuffering or BlockBuffering:

>>> let f = forkIO . hPutStr stdout
>>> in do hSetBuffering stdout NoBuffering; f "This is a longer string"; f ":D"; f "Hello Haskell"; pure ()
This: HDiesl lao  lHoansgkeerl lstring

>>> let f = forkIO . hPutStr stdout
>>> in do hSetBuffering stdout LineBuffering; f "This is a longer string"; f ":D"; f "Hello Haskell"; pure ()
This is a longer string:DHello Haskell

This operation may fail with:

• isFullError if the device is full.
• isPermissionError if another system resource limit would be exceeded.

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 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 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.

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).

The same as hPutStr, but adds a newline character.

This operation may fail with the same errors, and has the same issues with concurrency, as hPutStr!

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

This is a strict version of hGetContents

Since: base-4.15.0.0