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<head><title>optP -- making a parser optional</title>
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<div><a href="index.html" title="a framework for building parsers">Parsing</a> > <a href="___Parser.html" title="the class of all parsers">Parser</a> > <a href="_opt__P.html" title="making a parser optional">optP</a></div>
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<div><h1>optP -- making a parser optional</h1>
<div class="single"><h2>Synopsis</h2>
<ul><li><div class="list"><dl class="element"><dt class="heading">Usage: </dt><dd class="value"><div><tt>optP p</tt></div>
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<li><div class="single">Inputs:<ul><li><span><tt>p</tt>, <span>a <a href="___Parser.html">parser</a></span></span></li>
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<li><div class="single">Outputs:<ul><li><span><span>a <a href="___Parser.html">parser</a></span>, a parser that accepts tokens accepted by p and returns the value returned by p, or accepts the empty sequence of tokens and returns the symbol <tt>nil</tt>. It prefers to do the former.</span></li>
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<div class="single"><h2>Description</h2>
<div><p>After the first token, the parser is no slower than <tt>p</tt> would have been.</p>
<table class="examples"><tr><td><pre>i1 : (optP constParser "abc" : charAnalyzer) "abc"
o1 = abc</pre>
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<tr><td><pre>i2 : (optP constParser "abc" : charAnalyzer) ""
o2 = nil
o2 : Symbol</pre>
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<div class="single"><h2>See also</h2>
<ul><li><span><a href="_const__Parser.html" title="produce a parser that accepts a fixed string, one character at a time">constParser</a> -- produce a parser that accepts a fixed string, one character at a time</span></li>
<li><span><a href="_char__Analyzer.html" title="a lexical analyzer that provides characters from a string one at a time">charAnalyzer</a> -- a lexical analyzer that provides characters from a string one at a time</span></li>
<li><span><a href="___Parser_sp_co_sp__Analyzer.html" title="combine a parser with a lexical analyzer to make a complete system">Parser : Analyzer</a> -- combine a parser with a lexical analyzer to make a complete system</span></li>
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