| <html><head><META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>4.3.8. Iterate Rule</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_ASMCompoundRules.html" title="4.3. Compound ASM Rules"><link rel="prev" href="def_ForallRule.html" title="4.3.7. Forall Rule"><link rel="next" href="def_IfRule.html" title="4.3.9. If Rule"></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">4.3.8. Iterate Rule</th></tr><tr><td align="left" width="20%"><a accesskey="p" href="def_ForallRule.html">Prev</a> </td><th align="center" width="60%">4.3. Compound ASM Rules</th><td align="right" width="20%"> <a accesskey="n" href="def_IfRule.html">Next</a></td></tr></table><hr></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="def_IterateRule"></a>4.3.8. Iterate Rule</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_IterateRule_Description"></a>Description</th></tr><tr><td valign="top" align="left"><p> | |
| Using an <span class="strong"><strong>iterate rule</strong></span><a name="N119A6" class="indexterm"></a> is a | |
| possible way to manage loop execution in a transformation. | |
| </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_IterateRule_Syntax"></a>Syntax</th></tr><tr><td valign="top" align="left"><pre class="programlisting"> | |
| IterateRuleAST ::= <span class="token">iterate</span> <a href="def_AsmRuleInvoc.html" title="4.1.2. ASM Rule Invocation">AsmRuleAST</a> | |
| </pre><p> | |
| The <span class="strong"><strong>iterate rule</strong></span> contains a single body ASM rule. | |
| </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_IterateRule_Semantics"></a>Semantics</th></tr><tr><td valign="top" align="left"><p> | |
| The <span class="strong"><strong>iterate rule</strong></span> applies its body rule as long as possible, i.e. | |
| until its body fails. | |
| </p></td></tr></table></div><div class="highlights"><a name="def_IterateRule_Remark"></a><p><b>Remark. </b> | |
| A rule can fail in the following situations: | |
| <div class="itemizedlist"><ul type="disc"><li><p>it is the <a href="def_FailRule.html" title="4.2.4. Fail Rule">fail rule</a> itself, | |
| </p></li><li><p>it is the <a href="def_ChooseRule.html" title="4.3.6. Choose Rule">choose rule</a> and no matching is found, | |
| </p></li><li><p>it is a <a href="def_CompoundRule.html" title="4.3.1. Compound Rule">compound rule</a> and an executed contained rule fails. | |
| </p></li></ul></div> | |
| Since the execution of an <a href="def_AsmRuleInvoc.html" title="4.1.2. ASM Rule Invocation">ASM rule</a> does not necessarily fails it can | |
| happen that the <span class="strong"><strong>iterate rule</strong></span> does <span class="emphasis"><em>not terminate</em></span>. It can | |
| be avoided only with careful transformation design. | |
| The <span class="strong"><strong>iterate rule</strong></span> construct combined with a single | |
| <a href="def_ChooseRule.html" title="4.3.6. Choose Rule">choose rule</a> | |
| execution of a GT rule (as its body) applies the GT rule as long as possible, | |
| i.e. as long as a matching of the GT rule can be found by the choose | |
| construct. In other terms, first we apply the GT rule on a single | |
| (non-deterministically selected) matching and only after rule application do we | |
| recompute the next available matching. | |
| As a consequence, the incorrect precondition of the GT rule may cause | |
| <span class="emphasis"><em>non-termination</em></span> when combined with the iterate construct. For | |
| instance, the most typical problem is when a transformation designer does not | |
| prevent to apply the GT rule twice on the same matching (by using an | |
| appropriate negative condition). | |
| </p></div><div class="highlights"><a name="def_IterateRule_DefinedIn"></a><p><b>Defined In: </b> | |
| <a href="def_CompoundRule.html" title="4.3.1. Compound Rule">compound rule</a> | |
| </p></div><div class="highlights"><a name="def_IterateRule_SeeAlso"></a><p><b>See Also: </b> | |
| <a href="def_ForallRule.html" title="4.3.7. Forall Rule">forall rule</a>, <a href="def_ChooseRule.html" title="4.3.6. Choose Rule">choose rule</a>, <a href="def_FailRule.html" title="4.2.4. Fail Rule">fail rule</a>, <a href="def_TryRule.html" title="4.3.10. Try Rule">try rule</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_ForallRule.html">Prev</a> </td><td align="center" width="20%"><a accesskey="u" href="sec_ASMCompoundRules.html">Up</a></td><td align="right" width="40%"> <a accesskey="n" href="def_IfRule.html">Next</a></td></tr><tr><td valign="top" align="left" width="40%">4.3.7. Forall Rule </td><td align="center" width="20%"><a accesskey="h" href="index.html">Home</a></td><td valign="top" align="right" width="40%"> 4.3.9. If Rule</td></tr></table></div></body></html> |