|The IO monad
|The jailed IO monad.
|=> Maybe FilePath||A white list of handles that are always accessible.
|-> [Handle]||The jailed IO computation to run.
|-> IO a||Run the computation from within the insecure real world.
|-> IO a|
|Run a jailed IO computation. The IO computation will be able to access all
files that are within the specified jail directory. All file accesses outside
the jail directory will be refused. Only file handles opened from within the
jailed computation and the handles from the white list will be accessible to
the operations requiring a file handle. No smuggling in of foreign handles,
border patrol is very strict. When the jail path is specified as Nothing no
file access will be possible at all, this means the computation can only rely
on the white listed handles.
|Like MonadIO, but for jailed computations.
|Files and handles
|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.
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
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 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.
|Three handles are allocated during program initialisation,
and are initially open.
|A handle managing input from the Haskell program's standard input channel.
|A handle managing output to the Haskell program's standard output channel.
|A handle managing output to the Haskell program's standard error channel.
|Opening and closing files
|Operations on handles
|Determining and changing the size of a file
|Detecting the end of input
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 System.IO.hFlush is issued, or the handle is closed.
- block-buffering: the entire buffer is written out whenever it
overflows, a System.IO.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 System.IO.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.
|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.
|A mode that determines the effect of hSeek hdl mode i, as follows:
|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.
|Terminal operations (not portable: GHC/Hugs only)
|Showing handle state (not portable: GHC only)
|Text input and output
|Special cases for standard input and output
|Binary input and output
|Produced by Haddock version 2.6.0|