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<link title="Weak" rel="Chapter" href="Weak.html"><title>Stdlib.Seq</title>
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<h1>Module <a href="type_Stdlib.Seq.html">Stdlib.Seq</a></h1>
<pre><span id="MODULESeq"><span class="keyword">module</span> Seq</span>: <code class="type"><a href="Seq.html">Seq</a></code></pre><hr width="100%">
<p>The type <code class="code"><span class="keywordsign">'</span>a t</code> is a <b>delayed list</b>, i.e. a list where some evaluation
is needed to access the next element. This makes it possible to build
infinite sequences, to build sequences as we traverse them, and to transform
them in a lazy fashion rather than upfront.</p>
<pre><span id="TYPEt"><span class="keyword">type</span> <code class="type">'a</code> t</span> = <code class="type">unit -> 'a <a href="Seq.html#TYPEnode">node</a></code> </pre>
<div class="info ">
<div class="info-desc">
<p>The type of delayed lists containing elements of type <code class="code"><span class="keywordsign">'</span>a</code>.
Note that the concrete list node <code class="code"><span class="keywordsign">'</span>a node</code> is delayed under a closure,
not a <code class="code"><span class="keyword">lazy</span></code> block, which means it might be recomputed every time
we access it.</p>
</div>
</div>
<pre><code><span id="TYPEnode"><span class="keyword">type</span> <code class="type">'a</code> node</span> = </code></pre><table class="typetable">
<tr>
<td align="left" valign="top" >
<code><span class="keyword">|</span></code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTnode.Nil"><span class="constructor">Nil</span></span></code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code><span class="keyword">|</span></code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTnode.Cons"><span class="constructor">Cons</span></span> <span class="keyword">of</span> <code class="type">'a * 'a <a href="Seq.html#TYPEt">t</a></code></code></td>
</tr></table>
<div class="info ">
<div class="info-desc">
<p>A fully-evaluated list node, either empty or containing an element
and a delayed tail.</p>
</div>
</div>
<pre><span id="VALempty"><span class="keyword">val</span> empty</span> : <code class="type">'a <a href="Seq.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-desc">
<p>The empty sequence, containing no elements.</p>
</div>
</div>
<pre><span id="VALreturn"><span class="keyword">val</span> return</span> : <code class="type">'a -> 'a <a href="Seq.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-desc">
<p>The singleton sequence containing only the given element.</p>
</div>
</div>
<pre><span id="VALcons"><span class="keyword">val</span> cons</span> : <code class="type">'a -> 'a <a href="Seq.html#TYPEt">t</a> -> 'a <a href="Seq.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">cons x xs</code> is the sequence containing the element <code class="code">x</code> followed by
the sequence <code class="code">xs</code></p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.11</li>
</ul>
</div>
<pre><span id="VALappend"><span class="keyword">val</span> append</span> : <code class="type">'a <a href="Seq.html#TYPEt">t</a> -> 'a <a href="Seq.html#TYPEt">t</a> -> 'a <a href="Seq.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">append xs ys</code> is the sequence <code class="code">xs</code> followed by the sequence <code class="code">ys</code></p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.11</li>
</ul>
</div>
<pre><span id="VALmap"><span class="keyword">val</span> map</span> : <code class="type">('a -> 'b) -> 'a <a href="Seq.html#TYPEt">t</a> -> 'b <a href="Seq.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">map f seq</code> returns a new sequence whose elements are the elements of
<code class="code">seq</code>, transformed by <code class="code">f</code>.
This transformation is lazy, it only applies when the result is traversed.</p>
<p>If <code class="code">seq = [1;2;3]</code>, then <code class="code">map f seq = [f 1; f 2; f 3]</code>.</p>
</div>
</div>
<pre><span id="VALfilter"><span class="keyword">val</span> filter</span> : <code class="type">('a -> bool) -> 'a <a href="Seq.html#TYPEt">t</a> -> 'a <a href="Seq.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-desc">
<p>Remove from the sequence the elements that do not satisfy the
given predicate.
This transformation is lazy, it only applies when the result is
traversed.</p>
</div>
</div>
<pre><span id="VALfilter_map"><span class="keyword">val</span> filter_map</span> : <code class="type">('a -> 'b option) -> 'a <a href="Seq.html#TYPEt">t</a> -> 'b <a href="Seq.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-desc">
<p>Apply the function to every element; if <code class="code">f x = <span class="constructor">None</span></code> then <code class="code">x</code> is dropped;
if <code class="code">f x = <span class="constructor">Some</span> y</code> then <code class="code">y</code> is returned.
This transformation is lazy, it only applies when the result is
traversed.</p>
</div>
</div>
<pre><span id="VALflat_map"><span class="keyword">val</span> flat_map</span> : <code class="type">('a -> 'b <a href="Seq.html#TYPEt">t</a>) -> 'a <a href="Seq.html#TYPEt">t</a> -> 'b <a href="Seq.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-desc">
<p>Map each element to a subsequence, then return each element of this
sub-sequence in turn.
This transformation is lazy, it only applies when the result is
traversed.</p>
</div>
</div>
<pre><span id="VALfold_left"><span class="keyword">val</span> fold_left</span> : <code class="type">('a -> 'b -> 'a) -> 'a -> 'b <a href="Seq.html#TYPEt">t</a> -> 'a</code></pre><div class="info ">
<div class="info-desc">
<p>Traverse the sequence from left to right, combining each element with the
accumulator using the given function.
The traversal happens immediately and will not terminate on infinite
sequences.</p>
<p>Also see <a href="List.html#VALfold_left"><code class="code"><span class="constructor">List</span>.fold_left</code></a></p>
</div>
</div>
<pre><span id="VALiter"><span class="keyword">val</span> iter</span> : <code class="type">('a -> unit) -> 'a <a href="Seq.html#TYPEt">t</a> -> unit</code></pre><div class="info ">
<div class="info-desc">
<p>Iterate on the sequence, calling the (imperative) function on every element.
The traversal happens immediately and will not terminate on infinite
sequences.</p>
</div>
</div>
<pre><span id="VALunfold"><span class="keyword">val</span> unfold</span> : <code class="type">('b -> ('a * 'b) option) -> 'b -> 'a <a href="Seq.html#TYPEt">t</a></code></pre><div class="info ">
<div class="info-desc">
<p>Build a sequence from a step function and an initial value.
<code class="code">unfold f u</code> returns <code class="code">empty</code> if <code class="code">f u</code> returns <code class="code"><span class="constructor">None</span></code>,
or <code class="code"><span class="keyword">fun</span> () <span class="keywordsign">-></span> <span class="constructor">Cons</span> (x, unfold f y)</code> if <code class="code">f u</code> returns <code class="code"><span class="constructor">Some</span> (x, y)</code>.</p>
<p>For example, <code class="code">unfold (<span class="keyword">function</span> [] <span class="keywordsign">-></span> <span class="constructor">None</span> <span class="keywordsign">|</span> h::t <span class="keywordsign">-></span> <span class="constructor">Some</span> (h,t)) l</code>
is equivalent to <code class="code"><span class="constructor">List</span>.to_seq l</code>.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.11</li>
</ul>
</div>
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