| <html><head><META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>2.1.6. Pattern Variable</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. Pattern Definition"><link rel="prev" href="def_PatternComposition.html" title="2.1.5. Pattern Composition"><link rel="next" href="def_PatternVariableAssignment.html" title="2.1.7. Pattern Variable Assignment"></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">2.1.6. Pattern Variable</th></tr><tr><td align="left" width="20%"><a accesskey="p" href="def_PatternComposition.html">Prev</a> </td><th align="center" width="60%">2.1. Pattern Definition</th><td align="right" width="20%"> <a accesskey="n" href="def_PatternVariableAssignment.html">Next</a></td></tr></table><hr></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="def_PatternVariable"></a>2.1.6. Pattern Variable</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_PatternVariable_Description"></a>Description</th></tr><tr><td valign="top" align="left"><p> | |
| <span class="strong"><strong>Pattern variable</strong></span> | |
| <a name="N10706" class="indexterm"></a> | |
| <a name="N1070B" class="indexterm"></a> | |
| is a variable that appears in a graph pattern, i.e. (i) in the formal parameter list of a | |
| <a href="def_GraphPatternDef.html" title="2.1.1. Graph Pattern Definition">graph pattern definition</a>, | |
| (ii) in the definition of a (local) model element in a | |
| <a href="def_PatternBody.html" title="2.1.2. Pattern Body">pattern body</a>, (iii) in a | |
| <a href="def_PatternVariableAssignment.html" title="2.1.7. Pattern Variable Assignment">pattern variable assignment</a> | |
| or (iv) in the | |
| <a href="def_ActualParameter.html" title="5.8.3. Actual Parameters">actual parameters</a> | |
| of (positive or negative) | |
| <a href="def_PatternComposition.html" title="2.1.5. Pattern Composition">pattern composition</a>. | |
| </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_PatternVariable_Syntax"></a>Syntax</th></tr><tr><td valign="top" align="left"><pre class="programlisting"> | |
| PatternVariableDefAST ::= <a href="def_UpperCaseLiteral.html" title="6.2.3. UpperCase Literal">UpperCaseLiteral</a> | |
| PatternVariableRefAST ::= <a href="def_UpperCaseLiteral.html" title="6.2.3. UpperCase Literal">UpperCaseLiteral</a> | |
| PatternVariableDefRefAST ::= <a href="def_UpperCaseLiteral.html" title="6.2.3. UpperCase Literal">UpperCaseLiteral</a> | |
| </pre><p> | |
| Syntactically, a <span class="strong"><strong>pattern variable</strong></span> | |
| <a name="N1073F" class="indexterm"></a> | |
| is (an ASM) | |
| <span class="strong"><strong>Variable</strong></span> | |
| <a name="N10748" class="indexterm"></a>, i.e. | |
| begins with an uppercase letter that can be followed by an arbitrary number | |
| of alphanumerical characters. A pattern variable can be a | |
| <span class="strong"><strong>pattern variable definition</strong></span> | |
| <a name="N10751" class="indexterm"></a> or | |
| <span class="strong"><strong>pattern variable reference</strong></span> | |
| <a name="N1075C" class="indexterm"></a> | |
| (or something both at the same time). | |
| </p></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_PatternVariable_Semantics"></a>Semantics</th></tr><tr><td valign="top" align="left"><p> | |
| The binding of a | |
| <span class="strong"><strong>pattern variable</strong></span> | |
| is more restrictive compared to that of a ASM | |
| <a href="def_Variable.html" title="5.7.2. Variable">Variable</a>. | |
| </p><p> | |
| Unless being an input parameter of the pattern, a | |
| <span class="strong"><strong>pattern variable</strong></span> | |
| should always be bound to a model element. Furthermore, if a graph pattern is flattened along | |
| <a href="def_PatternComposition.html" title="2.1.5. Pattern Composition">pattern composition</a> | |
| (i.e. by composing the subpatterns into a complex flat pattern), then all output | |
| <span class="strong"><strong>pattern variable definitions</strong></span> | |
| are required to appear in the definition of a model element, i.e. as the | |
| first parameter of an | |
| <a href="def_Entity.html" title="2.2.1. Entity">entity</a> or a | |
| <a href="def_Relation.html" title="2.2.2. Relation">relation</a>. | |
| </p><p> | |
| Moreover, if each <span class="strong"><strong>pattern variable reference</strong></span> should be | |
| resolved to <span class="strong"><strong>pattern variable definitions</strong></span> within the same | |
| pattern body | |
| </p><p> | |
| Note, that input parameters of a pattern may still be bound to arbitrary ASM terms. However, this binding is | |
| already carried out before pattern matching is initiated. | |
| </p></td></tr></table></div><div class="warning" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Warning: Exceptions"><tr><td valign="top" align="center" rowspan="2" width="25"><img alt="[Warning]" src="images/warning.png"></td><th align="left"><a name="def_PatternVariable_Exception"></a>Exceptions</th></tr><tr><td valign="top" align="left"><p> | |
| When binding an output a pattern variable to anything else but model element during pattern matching, a run-time | |
| exception is thrown: | |
| <span class="emphasis"><em>"Invalid pattern variable binding: Pattern variable X should be bound | |
| to a model element and not to term Y."</em></span> | |
| </p></td></tr></table></div><div class="highlights"><a name="def_PatternVariable_UsedBy"></a><p><b>Used By: </b> | |
| <a href="def_PatternComposition.html" title="2.1.5. Pattern Composition">pattern composition</a> | |
| </p></div><div class="highlights"><a name="def_PatternVariable_SeeAlso"></a><p><b>See Also: </b> | |
| <a href="def_Variable.html" title="5.7.2. Variable">Variable</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_PatternComposition.html">Prev</a> </td><td align="center" width="20%"><a accesskey="u" href="sec_PatternDefinition.html">Up</a></td><td align="right" width="40%"> <a accesskey="n" href="def_PatternVariableAssignment.html">Next</a></td></tr><tr><td valign="top" align="left" width="40%">2.1.5. Pattern Composition </td><td align="center" width="20%"><a accesskey="h" href="index.html">Home</a></td><td valign="top" align="right" width="40%"> 2.1.7. Pattern Variable Assignment</td></tr></table></div></body></html> |