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<h1>Module <a href="type_Stdlib.Lazy.html">Stdlib.Lazy</a></h1>
<pre><span id="MODULELazy"><span class="keyword">module</span> Lazy</span>: <code class="type"><a href="Lazy.html">Lazy</a></code></pre><hr width="100%">
<pre><span id="TYPEt"><span class="keyword">type</span> <code class="type">'a</code> t</span> = <code class="type">'a <a href="CamlinternalLazy.html#TYPEt">CamlinternalLazy.t</a></code> </pre>
<div class="info ">
<div class="info-desc">
<p>A value of type <code class="code"><span class="keywordsign">'</span>a <span class="constructor">Lazy</span>.t</code> is a deferred computation, called
a suspension, that has a result of type <code class="code"><span class="keywordsign">'</span>a</code>. The special
expression syntax <code class="code"><span class="keyword">lazy</span> (expr)</code> makes a suspension of the
computation of <code class="code">expr</code>, without computing <code class="code">expr</code> itself yet.
"Forcing" the suspension will then compute <code class="code">expr</code> and return its
result. Matching a suspension with the special pattern syntax
<code class="code"><span class="keyword">lazy</span>(pattern)</code> also computes the underlying expression and
tries to bind it to <code class="code">pattern</code>:</p>
<pre class="codepre"><code class="code"> <span class="keyword">let</span> lazy_option_map f x =
<span class="keyword">match</span> x <span class="keyword">with</span>
<span class="keywordsign">|</span> <span class="keyword">lazy</span> (<span class="constructor">Some</span> x) <span class="keywordsign">-></span> <span class="constructor">Some</span> (<span class="constructor">Lazy</span>.force f x)
<span class="keywordsign">|</span> _ <span class="keywordsign">-></span> <span class="constructor">None</span>
</code></pre>
<p>Note: If lazy patterns appear in multiple cases in a pattern-matching,
lazy expressions may be forced even outside of the case ultimately selected
by the pattern matching. In the example above, the suspension <code class="code">x</code> is always
computed.</p>
<p>Note: <code class="code">lazy_t</code> is the built-in type constructor used by the compiler
for the <code class="code"><span class="keyword">lazy</span></code> keyword. You should not use it directly. Always use
<code class="code"><span class="constructor">Lazy</span>.t</code> instead.</p>
<p>Note: <code class="code"><span class="constructor">Lazy</span>.force</code> is not thread-safe. If you use this module in
a multi-threaded program, you will need to add some locks.</p>
<p>Note: if the program is compiled with the <code class="code">-rectypes</code> option,
ill-founded recursive definitions of the form <code class="code"><span class="keyword">let</span> <span class="keyword">rec</span> x = <span class="keyword">lazy</span> x</code>
or <code class="code"><span class="keyword">let</span> <span class="keyword">rec</span> x = <span class="keyword">lazy</span>(<span class="keyword">lazy</span>(...(<span class="keyword">lazy</span> x)))</code> are accepted by the type-checker
and lead, when forced, to ill-formed values that trigger infinite
loops in the garbage collector and other parts of the run-time system.
Without the <code class="code">-rectypes</code> option, such ill-founded recursive definitions
are rejected by the type-checker.</p>
</div>
</div>
<pre><span id="EXCEPTIONUndefined"><span class="keyword">exception</span> Undefined</span></pre>
<pre><span id="VALforce"><span class="keyword">val</span> force</span> : <code class="type">'a <a href="Lazy.html#TYPEt">t</a> -> 'a</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">force x</code> forces the suspension <code class="code">x</code> and returns its result.
If <code class="code">x</code> has already been forced, <code class="code"><span class="constructor">Lazy</span>.force x</code> returns the
same value again without recomputing it. If it raised an exception,
the same exception is raised again.</p>
</div>
<ul class="info-attributes">
<li><b>Raises</b> <code>Undefined</code> if the forcing of <code class="code">x</code> tries to force <code class="code">x</code> itself
recursively.</li>
</ul>
</div>
<pre><span id="VALforce_val"><span class="keyword">val</span> force_val</span> : <code class="type">'a <a href="Lazy.html#TYPEt">t</a> -> 'a</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">force_val x</code> forces the suspension <code class="code">x</code> and returns its
result. If <code class="code">x</code> has already been forced, <code class="code">force_val x</code>
returns the same value again without recomputing it.</p>
<p>If the computation of <code class="code">x</code> raises an exception, it is unspecified
whether <code class="code">force_val x</code> raises the same exception or <a href="Lazy.html#EXCEPTIONUndefined"><code class="code"><span class="constructor">Lazy</span>.<span class="constructor">Undefined</span></code></a>.</p>
</div>
<ul class="info-attributes">
<li><b>Raises</b> <code>Undefined</code> if the forcing of <code class="code">x</code> tries to force <code class="code">x</code> itself
recursively.</li>
</ul>
</div>
<pre><span id="VALfrom_fun"><span class="keyword">val</span> from_fun</span> : <code class="type">(unit -> 'a) -> 'a <a href="Lazy.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">from_fun f</code> is the same as <code class="code"><span class="keyword">lazy</span> (f ())</code> but slightly more efficient.</p>
<p><code class="code">from_fun</code> should only be used if the function <code class="code">f</code> is already defined.
In particular it is always less efficient to write
<code class="code">from_fun (<span class="keyword">fun</span> () <span class="keywordsign">-></span> expr)</code> than <code class="code"><span class="keyword">lazy</span> expr</code>.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.00.0</li>
</ul>
</div>
<pre><span id="VALfrom_val"><span class="keyword">val</span> from_val</span> : <code class="type">'a -> 'a <a href="Lazy.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">from_val v</code> returns an already-forced suspension of <code class="code">v</code>.
This is for special purposes only and should not be confused with
<code class="code"><span class="keyword">lazy</span> (v)</code>.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.00.0</li>
</ul>
</div>
<pre><span id="VALis_val"><span class="keyword">val</span> is_val</span> : <code class="type">'a <a href="Lazy.html#TYPEt">t</a> -> bool</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">is_val x</code> returns <code class="code"><span class="keyword">true</span></code> if <code class="code">x</code> has already been forced and
did not raise an exception.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.00.0</li>
</ul>
</div>
<pre><span id="VALlazy_from_fun"><span class="keyword">val</span> lazy_from_fun</span> : <code class="type">(unit -> 'a) -> 'a <a href="Lazy.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-deprecated">
<span class="warning">Deprecated.</span>synonym for <code class="code">from_fun</code>.</div>
</div>
<pre><span id="VALlazy_from_val"><span class="keyword">val</span> lazy_from_val</span> : <code class="type">'a -> 'a <a href="Lazy.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-deprecated">
<span class="warning">Deprecated.</span>synonym for <code class="code">from_val</code>.</div>
</div>
<pre><span id="VALlazy_is_val"><span class="keyword">val</span> lazy_is_val</span> : <code class="type">'a <a href="Lazy.html#TYPEt">t</a> -> bool</code></pre><div class="info ">
<div class="info-deprecated">
<span class="warning">Deprecated.</span>synonym for <code class="code">is_val</code>.</div>
</div>
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