[project @ 1996-06-27 16:55:06 by partain]

[ghc-hetmet.git] / ghc / lib / prelude / SampleVar.lhs

diff --git a/ghc/lib/prelude/SampleVar.lhs b/ghc/lib/prelude/SampleVar.lhs
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--- a/ghc/lib/prelude/SampleVar.lhs
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-%
-% (c) The GRASP/AQUA Project, Glasgow University, 1995
-%
-\section[SampleVar]{Sample variables}
-
-Sample variables are slightly different from a normal @MVar@:
-
-\begin{itemize}
-\item Reading an empty @SampleVar@ causes the reader to block.
-    (same as @takeMVar@ on empty @MVar@)
-\item Reading a filled @SampleVar@ empties it and returns value.
-    (same as @takeMVar@)
-\item Writing to an empty @SampleVar@ fills it with a value, and
-potentially, wakes up a blocked reader  (same as for @putMVar@ on empty @MVar@).
-\item Writing to a filled @SampleVar@ overwrites the current value.
- (different from @putMVar@ on full @MVar@.)
-\end{itemize}
-
-\begin{code}
-module SampleVar
-       (
-         SampleVar(..),    --:: type _ =
- 
-         newSampleVar,     --:: IO (SampleVar a)
-        emptySampleVar,   --:: SampleVar a -> IO ()
-        readSample,       --:: SampleVar a -> IO a
-        writeSample       --:: SampleVar a -> a -> IO ()
-
-       ) where
-
-import PreludeGlaST
-import PreludePrimIO   ( newEmptyMVar, newMVar, putMVar,
-                         readMVar, swapMVar, takeMVar, _MVar
-                       )
-\end{code}
-
-\begin{code}
-
-type SampleVar a
- = _MVar (Int,         -- 1  == full
-                       -- 0  == empty
-                       -- <0 no of readers blocked
-          _MVar a)
-
-\end{code}
-
-Initally, a @SampleVar@ is empty/unfilled.
-
-\begin{code}
-
-newSampleVar :: IO (SampleVar a)
-newSampleVar
- = newEmptyMVar          >>= \ val ->
-   newMVar (0,val)
-
-emptySampleVar :: SampleVar a -> IO ()
-emptySampleVar v
- = takeMVar v         >>= \ (readers,var) ->
-   if readers >= 0 then
-     putMVar v (0,var)
-   else
-     putMVar v (readers,var)
-
-\end{code}
-
-
-
-\begin{code}
-
---
--- filled => make empty and grab sample
--- not filled => try to grab value, empty when read val.
---
-readSample :: SampleVar a -> IO a
-readSample svar
- = takeMVar svar                >>= \ (readers,val) ->
-   putMVar svar (readers-1,val) >>
-   takeMVar val
-
---
--- filled => overwrite
--- not filled => fill, write val
---
-writeSample :: SampleVar a -> a -> IO ()
-writeSample svar v
- = takeMVar svar  >>= \ (readers, val) ->
-   case readers of
-     1 -> 
-       swapMVar val v      >> 
-       putMVar svar (1,val)
-     _ -> 
-       putMVar val v >> 
-       putMVar svar (min 1 (readers+1), val)
-
-\end{code}