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%
% (c) The GRASP/AQUA Project, Glasgow University, 1995
%
\section[ChannelVar]{Channel variables}
Channel variables, are one-element channels described in the Concurrent
Haskell paper (available from @ftp://ftp.dcs.gla.ac.uk/pub/glasgow-fp/drafts@)
\begin{code}
module ChannelVar
(
{- abstract -}
CVar,
newCVar, -- :: IO (CVar a)
writeCVar, -- :: CVar a -> a -> IO ()
readCVar, -- :: CVar a -> IO a
MVar
) where
import Prelude
import PrelConc
\end{code}
@MVars@ provide the basic mechanisms for synchronising access to a shared
resource. @CVars@, or channel variables, provide an abstraction that guarantee
that the producer is not allowed to run riot, but enforces the interleaved
access to the channel variable,i.e., a producer is forced to wait up for
a consumer to remove the previous value before it can deposit a new one in the @CVar@.
\begin{code}
data CVar a
= CVar (MVar a) -- prod -> cons
(MVar ()) -- cons -> prod
newCVar :: IO (CVar a)
writeCVar :: CVar a -> a -> IO ()
readCVar :: CVar a -> IO a
newCVar
= newEmptyMVar >>= \ datum ->
newMVar () >>= \ ack ->
return (CVar datum ack)
writeCVar (CVar datum ack) val
= takeMVar ack >>
putMVar datum val >>
return ()
readCVar (CVar datum ack)
= takeMVar datum >>= \ val ->
putMVar ack () >>
return val
\end{code}
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