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>Control.Concurrent.MVar</TITLE
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>base-4.1.0.0: Basic libraries</TD
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>Control.Concurrent.MVar</FONT
></TD
><TD ALIGN="right"
><TABLE CLASS="narrow" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="infohead"
>Portability</TD
><TD CLASS="infoval"
>non-portable (concurrency)</TD
></TR
><TR
><TD CLASS="infohead"
>Stability</TD
><TD CLASS="infoval"
>experimental</TD
></TR
><TR
><TD CLASS="infohead"
>Maintainer</TD
><TD CLASS="infoval"
>libraries@haskell.org</TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
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><TD
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="section4"
><B
>Contents</B
></TD
></TR
><TR
><TD
><DL
><DT
><A HREF="#1"
><TT
>MVar</TT
>s
</A
></DT
></DL
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
>Description</TD
></TR
><TR
><TD CLASS="doc"
>Synchronising variables
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
>Synopsis</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
>  <A HREF="#t%3AMVar"
>MVar</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AnewEmptyMVar"
>newEmptyMVar</A
> ::  <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a)</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AnewMVar"
>newMVar</A
> ::  a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a)</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AtakeMVar"
>takeMVar</A
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AputMVar"
>putMVar</A
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AreadMVar"
>readMVar</A
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AswapMVar"
>swapMVar</A
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AtryTakeMVar"
>tryTakeMVar</A
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> a)</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AtryPutMVar"
>tryPutMVar</A
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Bool.html#t%3ABool"
>Bool</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AisEmptyMVar"
>isEmptyMVar</A
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Bool.html#t%3ABool"
>Bool</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AwithMVar"
>withMVar</A
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; (a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> b) -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> b</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AmodifyMVar_"
>modifyMVar_</A
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; (a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> a) -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AmodifyMVar"
>modifyMVar</A
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; (a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> (a, b)) -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> b</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AaddMVarFinalizer"
>addMVarFinalizer</A
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
> -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="1"
><A NAME="1"
><TT
>MVar</TT
>s
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
>  <A NAME="t:MVar"
><A NAME="t%3AMVar"
></A
></A
><B
>MVar</B
> a </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
>An <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> (pronounced &quot;em-var&quot;) is a synchronising variable, used
for communication between concurrent threads.  It can be thought of
as a a box, which may be empty or full.
</TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:MVar')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:MVar" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="Data-Typeable.html#t%3ATypeable1"
>Typeable1</A
> <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Data-Eq.html#t%3AEq"
>Eq</A
> (<A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a)</TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:newEmptyMVar"
><A NAME="v%3AnewEmptyMVar"
></A
></A
><B
>newEmptyMVar</B
> ::  <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a)</TD
></TR
><TR
><TD CLASS="doc"
>Create an <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> which is initially empty.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:newMVar"
><A NAME="v%3AnewMVar"
></A
></A
><B
>newMVar</B
> ::  a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a)</TD
></TR
><TR
><TD CLASS="doc"
>Create an <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> which contains the supplied value.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:takeMVar"
><A NAME="v%3AtakeMVar"
></A
></A
><B
>takeMVar</B
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
><P
>Return the contents of the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>.  If the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> is currently
 empty, <TT
><A HREF="Control-Concurrent-MVar.html#v%3AtakeMVar"
>takeMVar</A
></TT
> will wait until it is full.  After a <TT
><A HREF="Control-Concurrent-MVar.html#v%3AtakeMVar"
>takeMVar</A
></TT
>, 
 the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> is left empty.
</P
><P
>There are two further important properties of <TT
><A HREF="Control-Concurrent-MVar.html#v%3AtakeMVar"
>takeMVar</A
></TT
>:
</P
><UL
><LI
> <TT
><A HREF="Control-Concurrent-MVar.html#v%3AtakeMVar"
>takeMVar</A
></TT
> is single-wakeup.  That is, if there are multiple
     threads blocked in <TT
><A HREF="Control-Concurrent-MVar.html#v%3AtakeMVar"
>takeMVar</A
></TT
>, and the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> becomes full,
     only one thread will be woken up.  The runtime guarantees that
     the woken thread completes its <TT
><A HREF="Control-Concurrent-MVar.html#v%3AtakeMVar"
>takeMVar</A
></TT
> operation.
</LI
><LI
> When multiple threads are blocked on an <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>, they are
     woken up in FIFO order.  This is useful for providing
     fairness properties of abstractions built using <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>s.
</LI
></UL
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:putMVar"
><A NAME="v%3AputMVar"
></A
></A
><B
>putMVar</B
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="doc"
><P
>Put a value into an <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>.  If the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> is currently full,
 <TT
><A HREF="Control-Concurrent-MVar.html#v%3AputMVar"
>putMVar</A
></TT
> will wait until it becomes empty.
</P
><P
>There are two further important properties of <TT
><A HREF="Control-Concurrent-MVar.html#v%3AputMVar"
>putMVar</A
></TT
>:
</P
><UL
><LI
> <TT
><A HREF="Control-Concurrent-MVar.html#v%3AputMVar"
>putMVar</A
></TT
> is single-wakeup.  That is, if there are multiple
     threads blocked in <TT
><A HREF="Control-Concurrent-MVar.html#v%3AputMVar"
>putMVar</A
></TT
>, and the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> becomes empty,
     only one thread will be woken up.  The runtime guarantees that
     the woken thread completes its <TT
><A HREF="Control-Concurrent-MVar.html#v%3AputMVar"
>putMVar</A
></TT
> operation.
</LI
><LI
> When multiple threads are blocked on an <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>, they are
     woken up in FIFO order.  This is useful for providing
     fairness properties of abstractions built using <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>s.
</LI
></UL
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:readMVar"
><A NAME="v%3AreadMVar"
></A
></A
><B
>readMVar</B
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
>This is a combination of <TT
><A HREF="Control-Concurrent-MVar.html#v%3AtakeMVar"
>takeMVar</A
></TT
> and <TT
><A HREF="Control-Concurrent-MVar.html#v%3AputMVar"
>putMVar</A
></TT
>; ie. it takes the value
  from the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>, puts it back, and also returns it.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:swapMVar"
><A NAME="v%3AswapMVar"
></A
></A
><B
>swapMVar</B
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
>Take a value from an <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>, put a new value into the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> and
  return the value taken. Note that there is a race condition whereby
  another process can put something in the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> after the take
  happens but before the put does.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:tryTakeMVar"
><A NAME="v%3AtryTakeMVar"
></A
></A
><B
>tryTakeMVar</B
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> a)</TD
></TR
><TR
><TD CLASS="doc"
>A non-blocking version of <TT
><A HREF="Control-Concurrent-MVar.html#v%3AtakeMVar"
>takeMVar</A
></TT
>.  The <TT
><A HREF="Control-Concurrent-MVar.html#v%3AtryTakeMVar"
>tryTakeMVar</A
></TT
> function
 returns immediately, with <TT
><A HREF="Data-Maybe.html#v%3ANothing"
>Nothing</A
></TT
> if the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> was empty, or
 <TT
><TT
><A HREF="Data-Maybe.html#v%3AJust"
>Just</A
></TT
> a</TT
> if the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> was full with contents <TT
>a</TT
>.  After <TT
><A HREF="Control-Concurrent-MVar.html#v%3AtryTakeMVar"
>tryTakeMVar</A
></TT
>,
 the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> is left empty.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:tryPutMVar"
><A NAME="v%3AtryPutMVar"
></A
></A
><B
>tryPutMVar</B
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Bool.html#t%3ABool"
>Bool</A
></TD
></TR
><TR
><TD CLASS="doc"
>A non-blocking version of <TT
><A HREF="Control-Concurrent-MVar.html#v%3AputMVar"
>putMVar</A
></TT
>.  The <TT
><A HREF="Control-Concurrent-MVar.html#v%3AtryPutMVar"
>tryPutMVar</A
></TT
> function
 attempts to put the value <TT
>a</TT
> into the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>, returning <TT
><A HREF="../ghc-prim/GHC-Bool.html#v%3ATrue"
>True</A
></TT
> if
 it was successful, or <TT
><A HREF="../ghc-prim/GHC-Bool.html#v%3AFalse"
>False</A
></TT
> otherwise.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:isEmptyMVar"
><A NAME="v%3AisEmptyMVar"
></A
></A
><B
>isEmptyMVar</B
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Bool.html#t%3ABool"
>Bool</A
></TD
></TR
><TR
><TD CLASS="doc"
><P
>Check whether a given <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> is empty.
</P
><P
>Notice that the boolean value returned  is just a snapshot of
 the state of the MVar. By the time you get to react on its result,
 the MVar may have been filled (or emptied) - so be extremely
 careful when using this operation.   Use <TT
><A HREF="Control-Concurrent-MVar.html#v%3AtryTakeMVar"
>tryTakeMVar</A
></TT
> instead if possible.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:withMVar"
><A NAME="v%3AwithMVar"
></A
></A
><B
>withMVar</B
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; (a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> b) -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> b</TD
></TR
><TR
><TD CLASS="doc"
><TT
><A HREF="Control-Concurrent-MVar.html#v%3AwithMVar"
>withMVar</A
></TT
> is a safe wrapper for operating on the contents of an
  <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>.  This operation is exception-safe: it will replace the
  original contents of the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> if an exception is raised (see
  <A HREF="Control-Exception.html"
>Control.Exception</A
>).
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:modifyMVar_"
><A NAME="v%3AmodifyMVar_"
></A
></A
><B
>modifyMVar_</B
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; (a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> a) -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="doc"
>A safe wrapper for modifying the contents of an <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>.  Like <TT
><A HREF="Control-Concurrent-MVar.html#v%3AwithMVar"
>withMVar</A
></TT
>, 
  <TT
><A HREF="Control-Concurrent-MVar.html#v%3AmodifyMVar"
>modifyMVar</A
></TT
> will replace the original contents of the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> if an
  exception is raised during the operation.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:modifyMVar"
><A NAME="v%3AmodifyMVar"
></A
></A
><B
>modifyMVar</B
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; (a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> (a, b)) -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> b</TD
></TR
><TR
><TD CLASS="doc"
>A slight variation on <TT
><A HREF="Control-Concurrent-MVar.html#v%3AmodifyMVar_"
>modifyMVar_</A
></TT
> that allows a value to be
  returned (<TT
>b</TT
>) in addition to the modified value of the <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
>.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:addMVarFinalizer"
><A NAME="v%3AaddMVarFinalizer"
></A
></A
><B
>addMVarFinalizer</B
> ::  <A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
> a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
> -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="doc"
>Add a finalizer to an <TT
><A HREF="Control-Concurrent-MVar.html#t%3AMVar"
>MVar</A
></TT
> (GHC only).  See <A HREF="Foreign-ForeignPtr.html"
>Foreign.ForeignPtr</A
> and
 <A HREF="System-Mem-Weak.html"
>System.Mem.Weak</A
> for more about finalizers.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="botbar"
>Produced by <A HREF="http://www.haskell.org/haddock/"
>Haddock</A
> version 2.4.2</TD
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