<html><head><META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>2.1.7.&nbsp;Pattern Variable Assignment</title><link href="style.css" rel="stylesheet" type="text/css"><meta content="DocBook XSL Stylesheets V1.72.0" name="generator"><link rel="start" href="index.html" title="The VIATRA2 Model Transformation Framework"><link rel="up" href="sec_PatternDefinition.html" title="2.1.&nbsp;Pattern Definition"><link rel="prev" href="def_PatternVariable.html" title="2.1.6.&nbsp;Pattern Variable"><link rel="next" href="def_CheckCondition.html" title="2.1.8.&nbsp;Check Condition"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table summary="Navigation header" width="100%"><tr><th align="center" colspan="3">2.1.7.&nbsp;Pattern Variable Assignment</th></tr><tr><td align="left" width="20%"><a accesskey="p" href="def_PatternVariable.html">Prev</a>&nbsp;</td><th align="center" width="60%">2.1.&nbsp;Pattern Definition</th><td align="right" width="20%">&nbsp;<a accesskey="n" href="def_CheckCondition.html">Next</a></td></tr></table><hr></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="def_PatternVariableAssignment"></a>2.1.7.&nbsp;Pattern Variable Assignment</h4></div></div></div><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Tip: Description"><tr><td valign="top" align="center" rowspan="2" width="25"><img alt="[Tip]" src="images/tip.png"></td><th align="left"><a name="def_PatternVariableAssignment_Description"></a>Description</th></tr><tr><td valign="top" align="left"><p>
			A
			<span class="strong"><strong>pattern variable assignment</strong></span>
			<a name="N107C1" class="indexterm"></a>
			is part of a
			<a href="def_PatternBody.html" title="2.1.2.&nbsp;Pattern Body">pattern body</a>. 
			It defines the equality of two locally visible variable.
		</p></td></tr></table></div><div class="important" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Important: Syntax"><tr><td valign="top" align="center" rowspan="2" width="25"><img alt="[Important]" src="images/important.png"></td><th align="left"><a name="def_PatternVariableAssignment_Syntax"></a>Syntax</th></tr><tr><td valign="top" align="left"><pre class="programlisting">
PatternVariableAssignmentAST ::= <a href="def_PatternVariable.html" title="2.1.6.&nbsp;Pattern Variable">PatternVariableRefAST</a> <span class="token">=</span> <a href="def_PatternVariable.html" title="2.1.6.&nbsp;Pattern Variable">PatternVariableRefAST</a>
</pre></td></tr></table></div><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Note: Semantics"><tr><td valign="top" align="center" rowspan="2" width="25"><img alt="[Note]" src="images/note.png"></td><th align="left"><a name="def_PatternVariableAssignment_Semantics"></a>Semantics</th></tr><tr><td valign="top" align="left"><p>
			By default, two pattern variables are bound to different model elements according to the injective pattern
			matching principle. By
			<span class="strong"><strong>pattern variable assignment</strong></span>, 
			two pattern variables are declared to be equal, i.e. to store (share) the same
			model element.
		</p><p>
			The effect of a
			<span class="strong"><strong>pattern variable assignment</strong></span>
			<a name="N107EC" class="indexterm"></a>
			$X = Y $depends on the bindings of pattern variables as follows:
		</p><div class="itemizedlist"><ul type="disc"><li><p>
					If both	<code class="computeroutput">X</code>
					and	<code class="computeroutput">Y</code>
					are bound (input) pattern variables, or both of them are free variables, then the pattern matcher
					treats <code class="computeroutput">X = Y</code>
					as a constraint. Furthermore, it obviously removes injectivity checks for the pair
					<code class="computeroutput">X</code>
					and	<code class="computeroutput">Y</code>.
				</p></li><li><p>
					If exactly one of pattern variables
					<code class="computeroutput">X</code>
					and	<code class="computeroutput">Y</code>
					are free (e.g. let it be <code class="computeroutput">X</code>), 
					then the value of the bound variable
					<code class="computeroutput">Y</code>
					is assigned to the free variable
					<code class="computeroutput">X</code>.
				</p></li></ul></div></td></tr></table></div><div class="caution" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Caution: Constraints"><tr><td valign="top" align="center" rowspan="2" width="25"><img alt="[Caution]" src="images/caution.png"></td><th align="left"><a name="def_PatternVariableAssignment_Constraints"></a>Constraints</th></tr><tr><td valign="top" align="left"><p>
			At least one of the variables in the assignment have to appear in the pattern body either as
			<a href="def_PatternBody.html" title="2.1.2.&nbsp;Pattern Body">locally defined body element</a>
			or in a (positive)
			<a href="def_PatternComposition.html" title="2.1.5.&nbsp;Pattern Composition">pattern composition</a>, 
			otherwise a compile-time error is generated.
		</p></td></tr></table></div><div class="highlights"><a name="def_PatternVariableAssignment_DefinedIn"></a><p><b>Defined In:&nbsp;</b>
				<a href="def_PatternBody.html" title="2.1.2.&nbsp;Pattern Body">pattern body</a>
			</p></div></div><div class="navfooter"><hr><table summary="Navigation footer" width="100%"><tr><td align="left" width="40%"><a accesskey="p" href="def_PatternVariable.html">Prev</a>&nbsp;</td><td align="center" width="20%"><a accesskey="u" href="sec_PatternDefinition.html">Up</a></td><td align="right" width="40%">&nbsp;<a accesskey="n" href="def_CheckCondition.html">Next</a></td></tr><tr><td valign="top" align="left" width="40%">2.1.6.&nbsp;Pattern Variable&nbsp;</td><td align="center" width="20%"><a accesskey="h" href="index.html">Home</a></td><td valign="top" align="right" width="40%">&nbsp;2.1.8.&nbsp;Check Condition</td></tr></table></div></body></html>