This library is the home for miscellaneous IO-related extensions.
fixIO :: (a -> IO a) -> IO a
fixIO
allows recursive IO operations to be defined.
The first argument to fixIO
should be a function
that takes its own output as an argument (sometimes called "tying the
knot").
unsafePerformIO :: IO a -> a
This is the "back door" into the IO
monad, allowing
IO
computation to be performed at any time. For
this to be safe, the IO
computation should be
free of side effects and independent of its environment.
If the I/O computation wrapped in unsafePerformIO
performs side effects, then the relative order in which those side
effects take place (relative to the main I/O trunk, or other calls to
unsafePerformIO
) is indeterminate.
However, it is less well known that
unsafePerformIO
is not type safe. For example:
test :: IORef [a] test = unsafePerformIO $ newIORef [] main = do writeIORef test [42] bang <- readIORef test print (bang :: [Char])
This program will core dump. This problem with polymorphic references is
well known in the ML community, and does not arise with normal monadic use
of references. There is no easy way to make it impossible once you use
unsafePerformIO
. Indeed, it is possible to write
coerce :: a -> b
with the help of unsafePerformIO
.
So be careful!
unsafeInterleaveIO :: IO a -> IO a
unsafeInterleaveIO
allows IO
computation to be deferred lazily. When passed a value of type
IO a
, the IO
will only be
performed when the value of the a
is demanded.
This is used to implement lazy file reading, see
IO.hGetContents
.
data IORef -- instance of: Eq newIORef :: a -> IO (IORef a) readIORef :: IORef a -> IO a writeIORef :: IORef a -> a -> IO () modifyIORef :: IORef a -> (a -> a) -> IO () mkWeakIORef :: IORef a -> IO () -> IO (Weak (IORef a)) -- deprecated, use modifyIORef updateIORef :: IORef a -> (a -> a) -> IO ()
data IOArray -- instance of: Eq newIOArray :: Ix ix => (ix,ix) -> elt -> IO (IOArray ix elt) boundsIOArray :: Ix ix => IOArray ix elt -> (ix, ix) readIOArray :: Ix ix => IOArray ix elt -> ix -> IO elt writeIOArray :: Ix ix => IOArray ix elt -> ix -> elt -> IO () freezeIOArray :: Ix ix => IOArray ix elt -> IO (Array ix elt) thawIOArray :: Ix ix => Array ix elt -> IO (IOArray ix elt) unsafeFreezeIOArray :: Ix ix => IOArray ix elt -> IO (Array ix elt) unsafeThawIOArray :: Ix ix => Array ix elt -> IO (IOArray ix elt)
Note: unsafeFreezeIOArray
and
unsafeThawIOArray
are not provided by Hugs.
data IOModeEx = BinaryMode IOMode | TextMode IOMode deriving (Eq, Read, Show) openFileEx :: FilePath -> IOModeEx -> IO Handle hSetBinaryMode :: Handle -> Bool -> IO Bool
GHC's implementation of the IO library distinguishes between binary- and text-mode files. This unfortunate hack is imposed on us by the need to support Win32 platforms.
On Win32, files opened in text mode are subject to CR-LF translation. When reading a handle in text mode, CR-LF sequences in the physical file are translated into lone LFs in the stream presented to the Haskell program. Writes to a text mode handle are subject to the inverse transformation.
On Unix platforms there is no such translation. What you get is exactly the contents of the file, and vice versa.
Unfortunately this behaviour makes it difficult to correctly implement file-positioning operations in text mode on Win32. If you want to use such operations, you must first place the handle in binary mode. Failure to do so results in IO exceptions being raised. This applies only to Win32, and not to any other platforms. If your programs use seek operations and you want them to be portable between Unix and Win32, you need to ensure the relevant handles are in binary mode.
You can get hold of a binary-mode file handle one of two ways. Either
open the file with openFileEx
, which allows the
mode to be specified. Or, if you already have an open handle,
use hSetBinaryMode
to change its mode.
Also as a result of this, note that on Win32 there are also several
operations which, whist still allowed, may give different results in
text mode than their Unix counterparts. These are: changing buffering
modes of a handle (hSetBuffering
), and writing to a
read-write handle. In both cases, the read-buffer associated with the
handle needs to be flushed, and, due to the Win32 text mode
translation, the resulting physical file position following the flush
may be wrong.
This issue of seeking in the presence of a non-identity transform between file and buffer contents will need to be revisited when the library is re-done to properly support Unicode. The present arrangement is the least-worst kludge we could come up with at present.
hGetBuf :: Handle -> Addr -> Int -> IO Int hPutBuf :: Handle -> Addr -> Int -> IO ()
These functions read and write chunks of data to/from a handle.
They will return only when either the full buffer has been transfered,
or the end of file is reached (in the case of
hGetBuf
.
hGetBufBA :: Handle -> MutableByteArray RealWorld a -> Int -> IO Int hPutBufBA :: Handle -> MutableByteArray RealWorld a -> Int -> IO ()
These functions mirror the previous two functions, but operate
on MutableByteArray
s instead of
Addr
s. This may be more convenient and/or faster,
depending on the circumstances.
hIsTerminalDevice :: Handle -> IO Bool hSetEcho :: Handle -> Bool -> IO () hGetEcho :: Handle -> IO Bool
withHandleFor :: Handle -> Handle -> IO a -> IO a withStdout :: Handle -> IO a -> IO a withStdin :: Handle -> IO a -> IO a withStderr :: Handle -> IO a -> IO a
trace :: String -> a -> a
When called, trace
prints the string in its first
argument to standard error, before returning the second argument as
its result. The trace
function is not
referentially transparent, and should only be used for debugging, or
for monitoring execution. Some implementations of
trace
may decorate the string that's output to
indicate that you're tracing.
trace
is implemented using
unsafePerformIO
.
The IO
module provides several predicates
over the IOError
type, such as
isEOFError
,
isDoesNotExistError
, and so on. Here we define
an extended set of these predicates, taking into account more
types of error:
isHardwareFault :: IOError -> Bool isInappropriateType :: IOError -> Bool isInterrupted :: IOError -> Bool isInvalidArgument :: IOError -> Bool isOtherError :: IOError -> Bool isProtocolError :: IOError -> Bool isResourceVanished :: IOError -> Bool isSystemError :: IOError -> Bool isTimeExpired :: IOError -> Bool isUnsatisfiedConstraints :: IOError -> Bool isUnsupportedOperation :: IOError -> Bool isDynIOError :: IOError -> Bool
unsafePtrEq :: a -> a -> Bool slurpFile :: FilePath -> IO (Addr, Int) hConnectTo :: Handle -> Handle -> IO () performGC :: IO () freeHaskellFunctionPtr :: Addr -> IO () getDynIOError :: IOError -> Maybe Dynamic.Dynamic
performGC
triggers an immediate garbage collection
unsafePtrEq
compares two values for pointer equality without
evaluating them. The results are not referentially transparent and
may vary significantly from one compiler to another or in the face of
semantics-preserving program changes. However, pointer equality is useful
in creating a number of referentially transparent constructs such as this
simplified memoisation function:
> cache :: (a -> b) -> (a -> b) > cache f = \x -> unsafePerformIO (check x) > where > ref = unsafePerformIO (newIORef (error "cache", error "cache")) > check x = readIORef ref >>= \ (x',a) -> > if x `unsafePtrEq` x' then > return a > else > let a = f x in > writeIORef ref (x, a) >> > return a
getDynIOError
takes an IOError
as argument. If it is a dynamic IO error, it returns Just
d
, where d
is the dynamic value. Of
(some) use by library providers to provide their own
IOError
types.