Paths

A Path is simply a FilePath with a type-level tag indicating where this path is rooted (relative to the current directory, absolute path, relative to a web domain, whatever). Most operations on Path are just lifted versions of the operations on the underlying FilePath. The tag however allows us to give a lot of operations a more meaningful type. For instance, it does not make sense to append two absolute paths together; instead, we can only append an unrooted path to another path. It also means we avoid bugs where we use one kind of path where we expect another.

Reinterpret the root of a path

This literally just changes the type-level tag; use with caution!

Type-level tag for unrooted paths

Unrooted paths need a root before they can be interpreted.

Reinterpret an unrooted path

This is an alias for castRoot; see comments there.

Forget a path's root

This is an alias for castRoot; see comments there.

Convert a relative/unrooted Path to a FilePath (using POSIX style directory separators).

See also toAbsoluteFilePath

Convert from a relative/unrooted FilePath (using POSIX style directory separators).

A path fragment (like a single directory or filename)

A file system root can be interpreted as an (absolute) FilePath

Abstract over a file system root

see fromFilePath

Wrapper around withFile

Wrapper around openBinaryTempFileWithDefaultPermissions

NOTE: The caller is responsible for cleaning up the temporary file.

Return the immediate children of a directory

Filters out "." and "..".

Recursive traverse a directory structure

Returns a set of paths relative to the directory specified. The list is lazily constructed, so that directories are only read when required. (This is also essential to ensure that this function does not build the entire result in memory before returning, potentially running out of heap.)

See openFile

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.

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.

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

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.

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.

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

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.