<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Control.Monad.ST.Lazy</title><link href="ocean.css" rel="stylesheet" type="text/css" title="Ocean" /><script src="haddock-util.js" type="text/javascript"></script><script type="text/javascript">//<![CDATA[
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</script></head><body><div id="package-header"><ul class="links" id="page-menu"><li><a href="index.html">Contents</a></li><li><a href="doc-index.html">Index</a></li></ul><p class="caption">base-4.5.1.0: Basic libraries</p></div><div id="content"><div id="module-header"><table class="info"><tr><th>Portability</th><td>non-portable (requires universal quantification for runST)</td></tr><tr><th>Stability</th><td>provisional</td></tr><tr><th>Maintainer</th><td>libraries@haskell.org</td></tr><tr><th>Safe Haskell</th><td>Unsafe</td></tr></table><p class="caption">Control.Monad.ST.Lazy</p></div><div id="table-of-contents"><p class="caption">Contents</p><ul><li><a href="#g:1">The <code><a href="Control-Monad-ST-Lazy.html#t:ST">ST</a></code> monad
</a></li><li><a href="#g:2">Converting between strict and lazy <code><a href="Control-Monad-ST-Lazy.html#t:ST">ST</a></code>
</a></li><li><a href="#g:3">Converting <code><a href="Control-Monad-ST-Lazy.html#t:ST">ST</a></code> To <code><a href="System-IO.html#t:IO">IO</a></code>
</a></li><li><a href="#g:4">Unsafe Functions
</a></li></ul></div><div id="description"><p class="caption">Description</p><div class="doc"><p>This module presents an identical interface to <a href="Control-Monad-ST.html">Control.Monad.ST</a>,
except that the monad delays evaluation of state operations until
a value depending on them is required.
</p></div></div><div id="synopsis"><p id="control.syn" class="caption expander" onclick="toggleSection('syn')">Synopsis</p><ul id="section.syn" class="hide" onclick="toggleSection('syn')"><li class="src short"><span class="keyword">data</span> <a href="#t:ST">ST</a> s a</li><li class="src short"><a href="#v:runST">runST</a> :: (<span class="keyword">forall</span> s. <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a) -> a</li><li class="src short"><a href="#v:fixST">fixST</a> :: (a -> <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a) -> <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a</li><li class="src short"><a href="#v:strictToLazyST">strictToLazyST</a> :: <a href="Control-Monad-ST-Safe.html#t:ST">ST</a> s a -> <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a</li><li class="src short"><a href="#v:lazyToStrictST">lazyToStrictST</a> :: <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a -> <a href="Control-Monad-ST-Safe.html#t:ST">ST</a> s a</li><li class="src short"><span class="keyword">data</span> <a href="#t:RealWorld">RealWorld</a> </li><li class="src short"><a href="#v:stToIO">stToIO</a> :: <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> <a href="Control-Monad-ST-Lazy.html#t:RealWorld">RealWorld</a> a -> <a href="System-IO.html#t:IO">IO</a> a</li><li class="src short"><a href="#v:unsafeInterleaveST">unsafeInterleaveST</a> :: <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a -> <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a</li><li class="src short"><a href="#v:unsafeIOToST">unsafeIOToST</a> :: <a href="System-IO.html#t:IO">IO</a> a -> <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a</li></ul></div><div id="interface"><h1 id="g:1">The <code><a href="Control-Monad-ST-Lazy.html#t:ST">ST</a></code> monad
</h1><div class="top"><p class="src"><span class="keyword">data</span> <a name="t:ST" class="def">ST</a> s a </p><div class="doc"><p>The lazy state-transformer monad.
A computation of type <code><code><a href="Control-Monad-ST-Lazy.html#t:ST">ST</a></code> s a</code> transforms an internal state indexed
by <code>s</code>, and returns a value of type <code>a</code>.
The <code>s</code> parameter is either
</p><ul><li> an unstantiated type variable (inside invocations of <code><a href="Control-Monad-ST-Lazy.html#v:runST">runST</a></code>), or
</li><li> <code><a href="Control-Monad-ST-Lazy.html#t:RealWorld">RealWorld</a></code> (inside invocations of <code><a href="Control-Monad-ST-Lazy.html#v:stToIO">stToIO</a></code>).
</li></ul><p>It serves to keep the internal states of different invocations of
<code><a href="Control-Monad-ST-Lazy.html#v:runST">runST</a></code> separate from each other and from invocations of <code><a href="Control-Monad-ST-Lazy.html#v:stToIO">stToIO</a></code>.
</p><p>The <code><a href="Control-Monad.html#v:-62--62--61-">>>=</a></code> and <code><a href="Control-Monad.html#v:-62--62-">>></a></code> operations are not strict in the state. For example,
</p><pre><code><a href="Control-Monad-ST-Lazy.html#v:runST">runST</a></code> (writeSTRef _|_ v >>= readSTRef _|_ >> return 2) = 2</pre></div><div class="subs instances"><p id="control.i:ST" class="caption collapser" onclick="toggleSection('i:ST')">Instances</p><div id="section.i:ST" class="show"><table><tr><td class="src"><a href="Control-Monad.html#t:Monad">Monad</a> (<a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s)</td><td class="doc empty"> </td></tr><tr><td class="src"><a href="Control-Monad.html#t:Functor">Functor</a> (<a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s)</td><td class="doc empty"> </td></tr><tr><td class="src"><a href="Control-Monad-Fix.html#t:MonadFix">MonadFix</a> (<a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s)</td><td class="doc empty"> </td></tr><tr><td class="src"><a href="Control-Applicative.html#t:Applicative">Applicative</a> (<a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s)</td><td class="doc empty"> </td></tr></table></div></div></div><div class="top"><p class="src"><a name="v:runST" class="def">runST</a> :: (<span class="keyword">forall</span> s. <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a) -> a</p><div class="doc"><p>Return the value computed by a state transformer computation.
The <code>forall</code> ensures that the internal state used by the <code><a href="Control-Monad-ST-Lazy.html#t:ST">ST</a></code>
computation is inaccessible to the rest of the program.
</p></div></div><div class="top"><p class="src"><a name="v:fixST" class="def">fixST</a> :: (a -> <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a) -> <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a</p><div class="doc"><p>Allow the result of a state transformer computation to be used (lazily)
inside the computation.
Note that if <code>f</code> is strict, <code><code><a href="Control-Monad-ST-Lazy.html#v:fixST">fixST</a></code> f = _|_</code>.
</p></div></div><h1 id="g:2">Converting between strict and lazy <code><a href="Control-Monad-ST-Lazy.html#t:ST">ST</a></code>
</h1><div class="top"><p class="src"><a name="v:strictToLazyST" class="def">strictToLazyST</a> :: <a href="Control-Monad-ST-Safe.html#t:ST">ST</a> s a -> <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a</p><div class="doc"><p>Convert a strict <code><a href="Control-Monad-ST-Lazy.html#t:ST">ST</a></code> computation into a lazy one. The strict state
thread passed to <code><a href="Control-Monad-ST-Lazy.html#v:strictToLazyST">strictToLazyST</a></code> is not performed until the result of
the lazy state thread it returns is demanded.
</p></div></div><div class="top"><p class="src"><a name="v:lazyToStrictST" class="def">lazyToStrictST</a> :: <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a -> <a href="Control-Monad-ST-Safe.html#t:ST">ST</a> s a</p><div class="doc"><p>Convert a lazy <code><a href="Control-Monad-ST-Lazy.html#t:ST">ST</a></code> computation into a strict one.
</p></div></div><h1 id="g:3">Converting <code><a href="Control-Monad-ST-Lazy.html#t:ST">ST</a></code> To <code><a href="System-IO.html#t:IO">IO</a></code>
</h1><div class="top"><p class="src"><span class="keyword">data</span> <a name="t:RealWorld" class="def">RealWorld</a> <a href="../ghc-prim-0.2.0.0/src/GHC-Prim.html#RealWorld" class="link">Source</a></p><div class="doc"><p><code>RealWorld</code> is deeply magical. It is <em>primitive</em>, but it is not
<em>unlifted</em> (hence <code>ptrArg</code>). We never manipulate values of type
<code>RealWorld</code>; it's only used in the type system, to parameterise <code>State#</code>.
</p></div><div class="subs instances"><p id="control.i:RealWorld" class="caption collapser" onclick="toggleSection('i:RealWorld')">Instances</p><div id="section.i:RealWorld" class="show"><table><tr><td class="src"><a href="Data-Typeable-Internal.html#t:Typeable">Typeable</a> <a href="Control-Monad-ST-Lazy.html#t:RealWorld">RealWorld</a></td><td class="doc empty"> </td></tr></table></div></div></div><div class="top"><p class="src"><a name="v:stToIO" class="def">stToIO</a> :: <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> <a href="Control-Monad-ST-Lazy.html#t:RealWorld">RealWorld</a> a -> <a href="System-IO.html#t:IO">IO</a> a</p><div class="doc"><p>A monad transformer embedding lazy state transformers in the <code><a href="System-IO.html#t:IO">IO</a></code>
monad. The <code><a href="Control-Monad-ST-Lazy.html#t:RealWorld">RealWorld</a></code> parameter indicates that the internal state
used by the <code><a href="Control-Monad-ST-Lazy.html#t:ST">ST</a></code> computation is a special one supplied by the <code><a href="System-IO.html#t:IO">IO</a></code>
monad, and thus distinct from those used by invocations of <code><a href="Control-Monad-ST-Lazy.html#v:runST">runST</a></code>.
</p></div></div><h1 id="g:4">Unsafe Functions
</h1><div class="top"><p class="src"><a name="v:unsafeInterleaveST" class="def">unsafeInterleaveST</a> :: <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a -> <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a</p><div class="doc"><div class="warning"><p>Deprecated: Please import from Control.Monad.ST.Lazy.Unsafe instead; This will be removed in the next release</p></div></div></div><div class="top"><p class="src"><a name="v:unsafeIOToST" class="def">unsafeIOToST</a> :: <a href="System-IO.html#t:IO">IO</a> a -> <a href="Control-Monad-ST-Lazy.html#t:ST">ST</a> s a</p><div class="doc"><div class="warning"><p>Deprecated: Please import from Control.Monad.ST.Lazy.Unsafe instead; This will be removed in the next release</p></div></div></div></div></div><div id="footer"><p>Produced by <a href="http://www.haskell.org/haddock/">Haddock</a> version 2.11.0</p></div></body></html>
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