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POStripping actions from ANTLR grammar changes its parsing algorithm
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8438755
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8442798
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1
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2011-12-08T22:43:11.733
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2011-12-12T17:44:02.183
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2011-12-12T17:44:02.183
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text
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I have a grammar <code>Foo.xtext</code> (too complex to include it here). Xtext generates <code>InternalFoo.g</code> from it. <a href="http://eclipse.org/Xtext/documentation/indigo/new_and_noteworthy.php" rel="nofollow">After some tweaking</a> it also generates <code>DebugInternalFoo.g</code> which claims to be the same thing without actions. Now, I strip off actions with ANTLR directly <pre><code>java -cp antlr-3.4.jar org.antlr.tool.Strip Internal.g > Stripped.g </code></pre> I'd expect the three grammars to behave the same way when I check them. But here is what I experienced <ul> <li><code>InternalFoo.g</code> - error, rule assignment has non-LL(*) decision</li> <li><code>DebugInternalFoo.g</code> - no problem, parses fine</li> <li><code>Stripped.g</code> - warnings at rule assignment, decision can match using multiple alternatives. It fails to parse properly.</li> </ul> Is it possible that a grammar parses a text differently with or without actions? Or is it a bug in any of the action-remover tools? (The rule in question has syntactic predicates, and without them, it would really have a non-LL(*) decision.) UPDATE: I partly found what caused the problem. The rule in question was like this <pre><code>trickyRule: ({ some complex action}) (expression '=')=>... </code></pre> Stripping with Antlr removed the action, but left an empty group there: <pre><code>// Stripped.g trickyRule: () (expression '=')=>... </code></pre> The generation of the debug grammar removes both the action, and the now empty group around it: <pre><code>// DebugInternalFoo.g trickyRule: (expression '=')=>... </code></pre> So the lesson learned is: an empty group before a syntactic predicate is not the same as nothing at all.
Tags
<antlr><grammar><antlr3><xtext>
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Stripping actions from ANTLR grammar changes its parsing algorithm
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1. PO
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1. USAdam Schmideg
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1. USAdam Schmideg
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