Commit 55e738bf authored by sof's avatar sof
Browse files

[project @ 1999-09-11 18:30:49 by sof]

Experimental support for re-routeing I/O on a Handle to that of another
for the duration of an IO action. Used to provide the following three
(exported) actions

   withStdin, withStdout, withStderr :: Handle -> IO a -> IO a

which 'replaces' one of the standard handles with that of the supplied
handle while performing the second action.
parent e982571c
......@@ -46,6 +46,10 @@ module IOExts
, hGetEcho
, hIsTerminalDevice
, hConnectTo
, withHandleFor
, withStdout
, withStdin
, withStderr
#endif
, trace
#ifdef __HUGS__
......@@ -68,6 +72,7 @@ import ST
#else
import PrelBase
import PrelIOBase
import IO
import PrelHandle ( openFileEx, IOModeEx(..),
hSetEcho, hGetEcho, getHandleFd
)
......@@ -209,3 +214,53 @@ foreign import ccall "freeHaskellFunctionPtr"
\end{code}
(Experimental)
Support for redirecting I/O on a handle to another for the
duration of an IO action. To re-route a handle, it is first
flushed, followed by replacing its innards (i.e., FILE_OBJECT)
with that of the other. This happens before and after the
action is executed.
If the action raises an exception, the handle is replaced back
to its old contents, but without flushing it first - as this
may provoke exceptions. Notice that the action may perform
I/O on either Handle, with the result that the I/O is interleaved.
(Why you would want to do this, is a completely different matter.)
ToDo: probably want to restrict what kind of handles can be
replaced with another - i.e., don't want to be able to replace
a writeable handle with a readable one.
\begin{code}
withHandleFor :: Handle
-> Handle
-> IO a
-> IO a
withHandleFor h1 h2 act = do
h1_fo <- getFO h1
plugIn h1_fo
where
plugIn h1_fo = do
hFlush h2
h2_fo <- withHandle h2 $ \ h2_ -> return (h2_{haFO__=h1_fo}, haFO__ h2_)
catch (act >>= \ x -> hFlush h2 >> setFO h2 h2_fo >> return x)
(\ err -> setFO h2 h2_fo >> ioError err)
setFO h fo =
withHandle h $ \ h_ -> return (h_{haFO__=fo}, ())
getFO h =
wantRWHandle "withHandleFor" h $ \ h_ ->
return (haFO__ h_)
\end{code}
Derived @withHandleFor@ combinators and, at the moment, these
are exported from @IOExts@ and not @withHandleFor@ itself.
\begin{code}
withStdin h a = withHandleFor h stdin a
withStdout h a = withHandleFor h stdout a
withStderr h a = withHandleFor h stderr a
\end{code}
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