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<link title="Weak" rel="Chapter" href="Weak.html"><title>Lazy</title>
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<center><h1>Module <a href="type_Lazy.html">Lazy</a></h1></center>
<br>
<pre><span class="keyword">module</span> Lazy: <code class="type">sig end</code></pre><div class="info">
Deferred computations.<br>
</div>
<hr width="100%">
<br><code><span class="keyword">type</span> <a name="TYPEt"></a><code class="type">'a</code> t = <code class="type">'a lazy_t</code> </code>
<div class="info">
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.
<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>
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.<br>
</div>
<br>
<pre><span class="keyword">exception</span> <a name="EXCEPTIONUndefined"></a>Undefined</pre>
<pre><span class="keyword">val</span> <a name="VALforce"></a>force : <code class="type">'a <a href="Lazy.html#TYPEt">t</a> -> 'a</code></pre><div class="info">
<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.
Raise <code class="code"><span class="constructor">Undefined</span></code> if the forcing of <code class="code">x</code> tries to force <code class="code">x</code> itself
recursively.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALforce_val"></a>force_val : <code class="type">'a <a href="Lazy.html#TYPEt">t</a> -> 'a</code></pre><div class="info">
<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.
Raise <code class="code"><span class="constructor">Undefined</span></code> if the forcing of <code class="code">x</code> tries to force <code class="code">x</code> itself
recursively.
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 <code class="code"><span class="constructor">Undefined</span></code>.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALlazy_from_fun"></a>lazy_from_fun : <code class="type">(unit -> 'a) -> 'a <a href="Lazy.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">lazy_from_fun f</code> is the same as <code class="code"><span class="keyword">lazy</span> (f ())</code> but slightly more
efficient.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALlazy_from_val"></a>lazy_from_val : <code class="type">'a -> 'a <a href="Lazy.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">lazy_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>.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALlazy_is_val"></a>lazy_is_val : <code class="type">'a <a href="Lazy.html#TYPEt">t</a> -> bool</code></pre><div class="info">
<code class="code">lazy_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.<br>
</div>
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