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>Data.IORef</TITLE
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>base-4.1.0.0: Basic libraries</TD
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>Data.IORef</FONT
></TD
><TD ALIGN="right"
><TABLE CLASS="narrow" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="infohead"
>Portability</TD
><TD CLASS="infoval"
>portable</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
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><TD CLASS="section4"
><B
>Contents</B
></TD
></TR
><TR
><TD
><DL
><DT
><A HREF="#1"
>IORefs
</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"
>Mutable references in the IO monad.
</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%3AIORef"
>IORef</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AnewIORef"
>newIORef</A
> ::  a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
> a)</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AreadIORef"
>readIORef</A
> ::  <A HREF="Data-IORef.html#t%3AIORef"
>IORef</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%3AwriteIORef"
>writeIORef</A
> ::  <A HREF="Data-IORef.html#t%3AIORef"
>IORef</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%3AmodifyIORef"
>modifyIORef</A
> ::  <A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
> a -&gt; (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%3AatomicModifyIORef"
>atomicModifyIORef</A
> ::  <A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
> a -&gt; (a -&gt; (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%3AmkWeakIORef"
>mkWeakIORef</A
> ::  <A HREF="Data-IORef.html#t%3AIORef"
>IORef</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="System-Mem-Weak.html#t%3AWeak"
>Weak</A
> (<A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
> 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"
>IORefs
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
>  <A NAME="t:IORef"
><A NAME="t%3AIORef"
></A
></A
><B
>IORef</B
> a </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
>A mutable variable in the <TT
><A HREF="System-IO.html#t%3AIO"
>IO</A
></TT
> monad
</TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:IORef')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:IORef" 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="Data-IORef.html#t%3AIORef"
>IORef</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Data-Eq.html#t%3AEq"
>Eq</A
> (<A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
> a)</TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:newIORef"
><A NAME="v%3AnewIORef"
></A
></A
><B
>newIORef</B
> ::  a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> (<A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
> a)</TD
></TR
><TR
><TD CLASS="doc"
>Build a new <TT
><A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
></TT
>
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:readIORef"
><A NAME="v%3AreadIORef"
></A
></A
><B
>readIORef</B
> ::  <A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
> a -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> a</TD
></TR
><TR
><TD CLASS="doc"
>Read the value of an <TT
><A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
></TT
>
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:writeIORef"
><A NAME="v%3AwriteIORef"
></A
></A
><B
>writeIORef</B
> ::  <A HREF="Data-IORef.html#t%3AIORef"
>IORef</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"
>Write a new value into an <TT
><A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
></TT
>
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:modifyIORef"
><A NAME="v%3AmodifyIORef"
></A
></A
><B
>modifyIORef</B
> ::  <A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
> a -&gt; (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"
>Mutate the contents of an <TT
><A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
></TT
>
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:atomicModifyIORef"
><A NAME="v%3AatomicModifyIORef"
></A
></A
><B
>atomicModifyIORef</B
> ::  <A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
> a -&gt; (a -&gt; (a, b)) -&gt; <A HREF="System-IO.html#t%3AIO"
>IO</A
> b</TD
></TR
><TR
><TD CLASS="doc"
><P
>Atomically modifies the contents of an <TT
><A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
></TT
>.
</P
><P
>This function is useful for using <TT
><A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
></TT
> in a safe way in a multithreaded
 program.  If you only have one <TT
><A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
></TT
>, then using <TT
><A HREF="Data-IORef.html#v%3AatomicModifyIORef"
>atomicModifyIORef</A
></TT
> to
 access and modify it will prevent race conditions.
</P
><P
>Extending the atomicity to multiple <TT
><A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
></TT
>s is problematic, so it
 is recommended that if you need to do anything more complicated
 then using Control.Concurrent.MVar.MVar instead is a good idea.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:mkWeakIORef"
><A NAME="v%3AmkWeakIORef"
></A
></A
><B
>mkWeakIORef</B
> ::  <A HREF="Data-IORef.html#t%3AIORef"
>IORef</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="System-Mem-Weak.html#t%3AWeak"
>Weak</A
> (<A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
> a))</TD
></TR
><TR
><TD CLASS="doc"
>Make a <TT
><A HREF="System-Mem-Weak.html#t%3AWeak"
>Weak</A
></TT
> pointer to an <TT
><A HREF="Data-IORef.html#t%3AIORef"
>IORef</A
></TT
>
</TD
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><TR
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></TD
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