Safe Haskell | Safe |
---|---|
Language | Haskell98 |
- data Handle :: *
- data HandlePosn :: *
- data IOMode :: *
- data BufferMode :: *
- data SeekMode :: *
- 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
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.
data HandlePosn :: * Source
See openFile
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.
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 |
A mode that determines the effect of hSeek
hdl mode i
.
AbsoluteSeek | the position of |
RelativeSeek | the position of |
SeekFromEnd | the position of |
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; orisPermissionError
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.
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 forhFlush
; - 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
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 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:
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; orisPermissionError
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; orisPermissionError
if another system resource limit would be exceeded.
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; orisPermissionError
if another system resource limit would be exceeded.
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) Source
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 Source
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 Source
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.
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 2010 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 2010, this is an opaque type.
catch :: IO a -> (IOError -> IO a) -> IO a Source
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.
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]])
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]])