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<head><title>orP -- parsing alternatives</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="_or__P.html" title="parsing alternatives">orP</a></div>
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<div><h1>orP -- parsing alternatives</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>r = orP(p,q,...)</tt></div>
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<li><div class="single">Inputs:<ul><li><span><tt>(p,q,...)</tt>, a sequence of parsers (of type <a href="___Parser.html" title="the class of all parsers">Parser</a>)</span></li>
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<li><div class="single">Outputs:<ul><li><span><tt>r</tt>, <span>a <a href="___Parser.html">parser</a></span>, a parser that accepts any sequence of tokens acceptable to one of the parsers <tt>p,q,...</tt></span></li>
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<div class="single"><h2>Description</h2>
<div><p>An abbreviation for <tt>orP(p,q)</tt> is <tt>p|q</tt>.</p>
<p>In case of ambiguity, the value returned by the left-most accepting parser is provided.</p>
<p>If one of the arguments is <span>a <a href="../../Macaulay2Doc/html/___String.html">string</a></span> then <a href="_const__Parser.html" title="produce a parser that accepts a fixed string, one character at a time">constParser</a> is used to convert it into a parser.</p>
<p>In an efficient grammar, the first token presented to <tt>r</tt> will be acceptable to at most one of the input parsers, and then the parser returned by <tt>r</tt> will be the parser returned by the single accepting input parser.</p>
<table class="examples"><tr><td><pre>i1 : (constParser "abc" | constParser "def" : charAnalyzer) "abc"
o1 = abc</pre>
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<tr><td><pre>i2 : (constParser "abc" | constParser "def" : charAnalyzer) "def"
o2 = def</pre>
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<tr><td><pre>i3 : (constParser "abc" | "def" : charAnalyzer) "def"
o3 = def</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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<div class="waystouse"><div class="single"><h2>Code</h2>
<pre>../Parsing.m2:81:26-86:46: --source code:
Parser | Parser := (p,q) -> new Parser from ( c -> (
p' := p c;
q' := q c;
if p' === null then q'
else if q' === null then p'
else if c === null then p' else p'|q'))</pre>
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