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<title>GHC/STRef.lhs</title>
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\begin{code}
<pre><a name="line-1"></a><span class='hs-comment'>{-# OPTIONS_GHC -XNoImplicitPrelude #-}</span>
<a name="line-2"></a><span class='hs-comment'>{-# OPTIONS_HADDOCK hide #-}</span>
<a name="line-3"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-4"></a><span class='hs-comment'>-- |</span>
<a name="line-5"></a><span class='hs-comment'>-- Module : GHC.STRef</span>
<a name="line-6"></a><span class='hs-comment'>-- Copyright : (c) The University of Glasgow, 1994-2002</span>
<a name="line-7"></a><span class='hs-comment'>-- License : see libraries/base/LICENSE</span>
<a name="line-8"></a><span class='hs-comment'>-- </span>
<a name="line-9"></a><span class='hs-comment'>-- Maintainer : cvs-ghc@haskell.org</span>
<a name="line-10"></a><span class='hs-comment'>-- Stability : internal</span>
<a name="line-11"></a><span class='hs-comment'>-- Portability : non-portable (GHC Extensions)</span>
<a name="line-12"></a><span class='hs-comment'>--</span>
<a name="line-13"></a><span class='hs-comment'>-- References in the 'ST' monad.</span>
<a name="line-14"></a><span class='hs-comment'>--</span>
<a name="line-15"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-16"></a>
<a name="line-17"></a><span class='hs-comment'>-- #hide</span>
<a name="line-18"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>GHC</span><span class='hs-varop'>.</span><span class='hs-conid'>STRef</span> <span class='hs-keyword'>where</span>
<a name="line-19"></a>
<a name="line-20"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>GHC</span><span class='hs-varop'>.</span><span class='hs-conid'>ST</span>
<a name="line-21"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>GHC</span><span class='hs-varop'>.</span><span class='hs-conid'>Base</span>
<a name="line-22"></a>
<a name="line-23"></a><a name="STRef"></a><span class='hs-keyword'>data</span> <span class='hs-conid'>STRef</span> <span class='hs-varid'>s</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>STRef</span> <span class='hs-layout'>(</span><span class='hs-conid'>MutVar</span><span class='hs-cpp'>#</span> <span class='hs-varid'>s</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span>
<a name="line-24"></a><span class='hs-comment'>-- ^ a value of type @STRef s a@ is a mutable variable in state thread @s@,</span>
<a name="line-25"></a><span class='hs-comment'>-- containing a value of type @a@</span>
<a name="line-26"></a>
<a name="line-27"></a><a name="newSTRef"></a><span class='hs-comment'>-- |Build a new 'STRef' in the current state thread</span>
<a name="line-28"></a><span class='hs-definition'>newSTRef</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ST</span> <span class='hs-varid'>s</span> <span class='hs-layout'>(</span><span class='hs-conid'>STRef</span> <span class='hs-varid'>s</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span>
<a name="line-29"></a><span class='hs-definition'>newSTRef</span> <span class='hs-varid'>init</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ST</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>s1</span><span class='hs-cpp'>#</span> <span class='hs-keyglyph'>-></span>
<a name="line-30"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>newMutVar</span><span class='hs-cpp'>#</span> <span class='hs-varid'>init</span> <span class='hs-varid'>s1</span><span class='hs-cpp'>#</span> <span class='hs-keyword'>of</span> <span class='hs-layout'>{</span> <span class='hs-layout'>(</span><span class='hs-cpp'>#</span> <span class='hs-varid'>s2</span><span class='hs-cpp'>#</span><span class='hs-layout'>,</span> <span class='hs-varid'>var</span><span class='hs-cpp'>#</span> <span class='hs-cpp'>#</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span>
<a name="line-31"></a> <span class='hs-layout'>(</span><span class='hs-cpp'>#</span> <span class='hs-varid'>s2</span><span class='hs-cpp'>#</span><span class='hs-layout'>,</span> <span class='hs-conid'>STRef</span> <span class='hs-varid'>var</span><span class='hs-cpp'>#</span> <span class='hs-cpp'>#</span><span class='hs-layout'>)</span> <span class='hs-layout'>}</span>
<a name="line-32"></a>
<a name="line-33"></a><a name="readSTRef"></a><span class='hs-comment'>-- |Read the value of an 'STRef'</span>
<a name="line-34"></a><span class='hs-definition'>readSTRef</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>STRef</span> <span class='hs-varid'>s</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ST</span> <span class='hs-varid'>s</span> <span class='hs-varid'>a</span>
<a name="line-35"></a><span class='hs-definition'>readSTRef</span> <span class='hs-layout'>(</span><span class='hs-conid'>STRef</span> <span class='hs-varid'>var</span><span class='hs-cpp'>#</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ST</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>s1</span><span class='hs-cpp'>#</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>readMutVar</span><span class='hs-cpp'>#</span> <span class='hs-varid'>var</span><span class='hs-cpp'>#</span> <span class='hs-varid'>s1</span><span class='hs-cpp'>#</span>
<a name="line-36"></a>
<a name="line-37"></a><a name="writeSTRef"></a><span class='hs-comment'>-- |Write a new value into an 'STRef'</span>
<a name="line-38"></a><span class='hs-definition'>writeSTRef</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>STRef</span> <span class='hs-varid'>s</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ST</span> <span class='hs-varid'>s</span> <span class='hs-conid'>()</span>
<a name="line-39"></a><span class='hs-definition'>writeSTRef</span> <span class='hs-layout'>(</span><span class='hs-conid'>STRef</span> <span class='hs-varid'>var</span><span class='hs-cpp'>#</span><span class='hs-layout'>)</span> <span class='hs-varid'>val</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ST</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>s1</span><span class='hs-cpp'>#</span> <span class='hs-keyglyph'>-></span>
<a name="line-40"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>writeMutVar</span><span class='hs-cpp'>#</span> <span class='hs-varid'>var</span><span class='hs-cpp'>#</span> <span class='hs-varid'>val</span> <span class='hs-varid'>s1</span><span class='hs-cpp'>#</span> <span class='hs-keyword'>of</span> <span class='hs-layout'>{</span> <span class='hs-varid'>s2</span><span class='hs-cpp'>#</span> <span class='hs-keyglyph'>-></span>
<a name="line-41"></a> <span class='hs-layout'>(</span><span class='hs-cpp'>#</span> <span class='hs-varid'>s2</span><span class='hs-cpp'>#</span><span class='hs-layout'>,</span> <span class='hs-conid'>()</span> <span class='hs-cpp'>#</span><span class='hs-layout'>)</span> <span class='hs-layout'>}</span>
<a name="line-42"></a>
<a name="line-43"></a><span class='hs-comment'>-- Just pointer equality on mutable references:</span>
<a name="line-44"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>Eq</span> <span class='hs-layout'>(</span><span class='hs-conid'>STRef</span> <span class='hs-varid'>s</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-45"></a> <span class='hs-conid'>STRef</span> <span class='hs-varid'>v1</span><span class='hs-cpp'>#</span> <span class='hs-varop'>==</span> <span class='hs-conid'>STRef</span> <span class='hs-varid'>v2</span><span class='hs-cpp'>#</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>sameMutVar</span><span class='hs-cpp'>#</span> <span class='hs-varid'>v1</span><span class='hs-cpp'>#</span> <span class='hs-varid'>v2</span><span class='hs-cpp'>#</span>
</pre>\end{code}
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