jail- Jailed IO monad.




The IO monad

data IO a Source

The jailed IO monad.



:: Maybe FilePath

The jail directory or Nothing for not allowing file access.

-> [Handle]

A white list of handles that are always accessible.

-> IO a

The jailed IO computation to run.

-> IO a

Run the computation from within the insecure real world.

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.

class Monad m => JailIO m whereSource

Like MonadIO, but for jailed computations.


jailIO :: IO a -> m aSource


JailIO IO 
JailIO m => JailIO (ListT m) 
JailIO m => JailIO (IdentityT m) 
(Error e, JailIO m) => JailIO (ErrorT e m) 
JailIO m => JailIO (ContT r m) 
JailIO m => JailIO (ReaderT r m) 
JailIO m => JailIO (StateT r m) 
(Monoid r, JailIO m) => JailIO (WriterT r m) 
(Monoid w, JailIO m) => JailIO (RWST r w s m) 

Files and handles

type FilePath = String

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

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.

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.

stdin :: Handle

A handle managing input from the Haskell program's standard input channel.

stdout :: Handle

A handle managing output to the Haskell program's standard output channel.

stderr :: Handle

A handle managing output to the Haskell program's standard error channel.

Opening and closing files

Opening files

withFile :: FilePath -> IOMode -> (Handle -> IO a) -> IO aSource

Closing files

Special cases

File locking

Operations on handles

Determining and changing the size of a file

Detecting the end of input

Buffering operations

data BufferMode

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.



buffering is disabled if possible.


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.

Repositioning handles

data SeekMode

A mode that determines the effect of hSeek hdl mode i, as follows:



the position of hdl is set to i.


the position of hdl is set to offset i from the current position.


the position of hdl is set to offset i from the end of the file.

Handle properties

Terminal operations (not portable: GHC/Hugs only)

Showing handle state (not portable: GHC only)

Text input and output

Text input

Text output

hPrint :: Show a => Handle -> a -> IO ()Source

Special cases for standard input and output

print :: Show a => a -> IO ()Source

readIO :: Read a => String -> IO aSource

readLn :: Read a => IO aSource

Binary input and output

Temporary files