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<div class="document" id="pattern-variables">
<h1 class="title">Pattern Variables</h1>
<p>To take this to the next level, you might want to ask "Who are the sons of
Thomas and Norma?". In this case, you are passing <tt class="docutils literal">Thomas</tt> and <tt class="docutils literal">Norma</tt>
<em>into</em> Pyke, and you'd like Pyke to pass something back <em>out</em> to you as part of
the answer (in addition to the thumbs up).</p>
<p>Pattern variables serve as output parameters. They start with a <tt class="docutils literal">$</tt>:</p>
<pre class="literal-block">
family.son_of($son, Thomas, Norma)
</pre>
<p>You can use whatever name you like after the <tt class="docutils literal">$</tt>. Pyke will answer with a
thumbs up <em>binding</em> <tt class="docutils literal">$son</tt> to <tt class="docutils literal">Bruce</tt>. If you don't like that answer, you
can reject that answer and ask Pyke for another answer ("nope, try again!").
Each time Pyke finds another son for Thomas and Norma, it answers with another
thumbs up and <tt class="docutils literal">$son</tt> <em>bound</em> to a different value.</p>
<p>If you reject the last son of <tt class="docutils literal">Thomas</tt> and <tt class="docutils literal">Norma</tt> (or if <tt class="docutils literal">Thomas</tt> and
<tt class="docutils literal">Norma</tt> have no sons in the first place), Pyke will answer with a thumbs
down.</p>
<p>We say that Pyke <em>binds</em> <tt class="docutils literal">Bruce</tt> to the pattern variable <tt class="docutils literal">$son</tt> when it
comes back with its first thumbs up. When we tell Pyke "nope, try again!",
Pyke must first <em>unbind</em> <tt class="docutils literal">$son</tt> before it can go on and <em>bind</em> it to the
next value. The "nope" part does the <em>unbinding</em>, and the "try again" part
does the <em>binding</em> again to a new value.</p>
<p>So at any point in time, a pattern variable is either <em>bound</em> to a value or
<em>unbound</em>. If we follow a particular pattern variable through time, we might
see that it is alternately bound and unbound many times as Pyke tries to find
a suitable answer to your question. Specifically, when Pyke comes back with
the final thumbs down, <tt class="docutils literal">$son</tt> is unbound.</p>
<div class="section" id="anonymous-pattern-variables">
<h2>Anonymous Pattern Variables</h2>
<p>Suppose we want to know who Norma's sons are? In this case we don't care
about the father. We use <em>anonymous variables</em> as "don't care" placeholders.</p>
<p>An anonymous variable is any pattern variable who's name starts with an
underscore (<tt class="docutils literal">_</tt>). The rest of the name doesn't matter and just serves as
documentation (and so <tt class="docutils literal">$_</tt> is all that's strictly needed).</p>
<p>So "Who are Norma's sons?" looks like:</p>
<pre class="literal-block">
family.son_of($son, $_father, Norma)
</pre>
<p>We're giving Norma as input to Pyke, and asking for the <tt class="docutils literal">$son</tt> as output and
telling Pyke that we don't care about the <tt class="docutils literal">$_father</tt>.</p>
<p>Anonymous variables are never bound to values. (So you could say that they
are always unbound).</p>
</div>
<div class="section" id="pattern-variable-identity">
<h2>Pattern Variable Identity</h2>
<p>Now this is very important, so pay attention! The same pattern variable
<em>name</em> means the same <em>pattern variable</em>. Thus, if you say <tt class="docutils literal">$son</tt> twice,
it's the <em>same</em> pattern variable. And, a pattern variable can only be bound
to one value (at a time), so you mean the <em>same</em> data value must appear in
both places.</p>
<div class="note">
<p class="first admonition-title">Note</p>
<p class="last">This does <em>not</em> apply to <a class="reference internal" href="#anonymous-pattern-variables">anonymous pattern variables</a>. Since they are
never bound to a value, each use of the same anonymous variable can match
different data values.</p>
</div>
<p>So if you wanted to see all of the sons with the same name as their fathers,
you would ask:</p>
<pre class="literal-block">
family.son_of($father, $father, $_mother)
</pre>
<div class="note">
<p class="first admonition-title">Note</p>
<p>The Pyke <a class="reference external" href="../../examples.html#family-relations">family_relations</a> example never uses the same name for both
father and son because that would cause confusion about which of them
was the father to both of their sons and daughters.</p>
<p>In these cases, it modifies the name of the son to make it unique.</p>
<p class="last">Thus, this question would always fail within the family_relations
example...</p>
</div>
<p>And so here is the complete explanation of how pattern variables are matched
to a data value.</p>
<p>First, the pattern variable is checked to see if it is already bound to a
value.</p>
<p>If it is bound to a value, then this bound value has to match the data for
the match to succeed.</p>
<p>If it is unbound, then the pattern variable is bound to the data value as a
by-product of doing the match, and the match always succeeds.</p>
<p>And so pattern variables only "match any value" when they are unbound. And in
matching that value, they become bound to it as a by-product of doing the
match. Once bound to a value, a pattern variable will only match that value
(much like a literal pattern).</p>
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<h3>More:</h3>
<div class="right-item"><a href="literal_patterns.html">Literal Patterns</a><p>Explanation of <em>literal patterns</em>.</p>
</div>
<div class="right-item"><a href="pattern_variables.html">Pattern Variables</a><p>Explanation of <em>pattern variables</em>.</p>
</div>
<div class="right-item"><a href="tuple_patterns.html">Tuple Patterns</a><p>Explanation of <em>tuple patterns</em>.</p>
</div>
<div class="right-item"><a href="matching_patterns.html">Matching Two Patterns</a><p>Explanation of matching two patterns together, vs matching a pattern
to data.</p>
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Copyright © 2007-2009 Bruce Frederiksen
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