eclipse - xtext 处理左递归语法

Question

我正在使用xtext 2.4并且想要同时支持map和set，我的语法如下

<term> ::- <collection>
<collection> ::- <map> | <set>
<map> ::- '{' (<term> ':' <term> (',' <term> ':' <term> )*)? '}'
<set> ::- '{' (<term> (',' <term>)* )+ '}'

因此可以简化为

<term> ::- '{' (<term> ':' <term> (',' <term> ':' <term> )*)? '}' |
           '{' (<term> (',' <term>)* )+ '}'

antlr 提示这个语法开始左递归，我不知道为什么，因为它的右侧有“{”。

即使我想分解相同的部分，我也不知道如何在xtext中做到这一点，因为EObject现在被分成两个生产规则，并且解析的数据不能存储在同一个对象中。

有什么帮助吗？

================================================== =============================我写的对应的xtext语法是

grammar org.xtext.problem.Term with org.eclipse.xtext.common.Terminals

generate term "http://www.xtext.org/problem/Term"

Term:
    term = Collection
;

Collection:
    MyMap | MySet
; 

MyMap:
    {MyMap} '{'( keys+= Term ':' values += Term ( ',' keys+=Term ':' values +=Term)*  )?'}'
;

MySet:
    {MySet} '{'( values += Term ( ',' values +=Term)*  )?'}'
;

错误消息(以 error(211) 开头):

0    [main] INFO  lipse.emf.mwe.utils.StandaloneSetup  - Registering platform uri '/Users/jcwu/opensource/xtext-problemistic'
116  [main] INFO  lipse.emf.mwe.utils.StandaloneSetup  - Adding generated EPackage 'org.eclipse.xtext.xbase.XbasePackage'
454  [main] INFO  clipse.emf.mwe.utils.GenModelHelper  - Registered GenModel 'http://www.eclipse.org/Xtext/Xbase/XAnnotations' from 'platform:/resource/org.eclipse.xtext.xbase/model/Xbase.genmodel'
459  [main] INFO  clipse.emf.mwe.utils.GenModelHelper  - Registered GenModel 'http://www.eclipse.org/xtext/xbase/Xtype' from 'platform:/resource/org.eclipse.xtext.xbase/model/Xbase.genmodel'
479  [main] INFO  clipse.emf.mwe.utils.GenModelHelper  - Registered GenModel 'http://www.eclipse.org/xtext/xbase/Xbase' from 'platform:/resource/org.eclipse.xtext.xbase/model/Xbase.genmodel'
479  [main] INFO  clipse.emf.mwe.utils.GenModelHelper  - Registered GenModel 'http://www.eclipse.org/xtext/common/JavaVMTypes' from 'platform:/resource/org.eclipse.xtext.common.types/model/JavaVMTypes.genmodel'
1660 [main] INFO  ipse.emf.mwe.utils.DirectoryCleaner  - Cleaning /Users/jcwu/opensource/xtext-problemistic/org.xtext.problem.term/../org.xtext.problem.term/src-gen
1665 [main] INFO  ipse.emf.mwe.utils.DirectoryCleaner  - Cleaning /Users/jcwu/opensource/xtext-problemistic/org.xtext.problem.term/../org.xtext.problem.term.ui/src-gen
1666 [main] INFO  ipse.emf.mwe.utils.DirectoryCleaner  - Cleaning /Users/jcwu/opensource/xtext-problemistic/org.xtext.problem.term/../org.xtext.problem.term.tests/src-gen
2033 [main] INFO  ipse.xtext.generator.LanguageConfig  - generating infrastructure for org.xtext.problem.Term with fragments : ImplicitRuntimeFragment, ImplicitUiFragment, GrammarAccessFragment, EcoreGeneratorFragment, SerializerFragment, ResourceFactoryFragment, XtextAntlrGeneratorFragment, ValidatorFragment, ImportNamespacesScopingFragment, QualifiedNamesFragment, BuilderIntegrationFragment, GeneratorFragment, FormatterFragment, LabelProviderFragment, OutlineTreeProviderFragment, QuickOutlineFragment, QuickfixProviderFragment, ContentAssistFragment, XtextAntlrUiGeneratorFragment, Junit4Fragment, RefactorElementNameFragment, TypesGeneratorFragment, XbaseGeneratorFragment, CodetemplatesGeneratorFragment, CompareFragment
4115 [main] INFO  clipse.emf.mwe.utils.GenModelHelper  - Registered GenModel 'http://www.xtext.org/problem/Term' from 'platform:/resource/org.xtext.problem.term/src-gen/org/xtext/problem/Term.genmodel'
error(211): ../org.xtext.problem.term/src-gen/org/xtext/problem/parser/antlr/internal/InternalTerm.g:119:1: [fatal] rule ruleCollection has non-LL(*) decision due to recursive rule invocations reachable from alts 1,2.  Resolve by left-factoring or using syntactic predicates or using backtrack=true option.
5918 [main] ERROR enerator.CompositeGeneratorFragment  - java.io.FileNotFoundException: ../org.xtext.problem.term/src-gen/org/xtext/problem/parser/antlr/internal/InternalTermLexer.java (No such file or directory)
org.eclipse.emf.common.util.WrappedException: java.io.FileNotFoundException: ../org.xtext.problem.term/src-gen/org/xtext/problem/parser/antlr/internal/InternalTermLexer.java (No such file or directory)
    at org.eclipse.xtext.util.Files.readFileIntoString(Files.java:129)
    at org.eclipse.xtext.generator.parser.antlr.AbstractAntlrGeneratorFragment.suppressWarningsImpl(AbstractAntlrGeneratorFragment.java:132)
    at org.eclipse.xtext.generator.parser.antlr.AbstractAntlrGeneratorFragment.suppressWarnings(AbstractAntlrGeneratorFragment.java:142)
    at org.eclipse.xtext.generator.parser.antlr.AbstractAntlrGeneratorFragment.suppressWarnings(AbstractAntlrGeneratorFragment.java:138)
    at org.eclipse.xtext.generator.parser.antlr.XtextAntlrGeneratorFragment.generate(XtextAntlrGeneratorFragment.java:49)
    at org.eclipse.xtext.generator.CompositeGeneratorFragment.generate(CompositeGeneratorFragment.java:92)
    at org.eclipse.xtext.generator.LanguageConfig.generate(LanguageConfig.java:113)
    at org.eclipse.xtext.generator.Generator.generate(Generator.java:361)
    at org.eclipse.xtext.generator.Generator.invokeInternal(Generator.java:128)
    at org.eclipse.emf.mwe.core.lib.AbstractWorkflowComponent.invoke(AbstractWorkflowComponent.java:126)
    at org.eclipse.emf.mwe.core.lib.Mwe2Bridge.invoke(Mwe2Bridge.java:34)
    at org.eclipse.emf.mwe.core.lib.AbstractWorkflowComponent.invoke(AbstractWorkflowComponent.java:201)
    at org.eclipse.emf.mwe2.runtime.workflow.AbstractCompositeWorkflowComponent.invoke(AbstractCompositeWorkflowComponent.java:35)
    at org.eclipse.emf.mwe2.runtime.workflow.Workflow.run(Workflow.java:19)
    at org.eclipse.emf.mwe2.launch.runtime.Mwe2Runner.run(Mwe2Runner.java:102)
    at org.eclipse.emf.mwe2.launch.runtime.Mwe2Runner.run(Mwe2Runner.java:62)
    at org.eclipse.emf.mwe2.launch.runtime.Mwe2Runner.run(Mwe2Runner.java:52)
    at org.eclipse.emf.mwe2.launch.runtime.Mwe2Launcher.run(Mwe2Launcher.java:74)
    at org.eclipse.emf.mwe2.launch.runtime.Mwe2Launcher.main(Mwe2Launcher.java:35)
Caused by: java.io.FileNotFoundException: ../org.xtext.problem.term/src-gen/org/xtext/problem/parser/antlr/internal/InternalTermLexer.java (No such file or directory)
    at java.io.FileInputStream.open(Native Method)
    at java.io.FileInputStream.<init>(FileInputStream.java:138)
    at java.io.FileInputStream.<init>(FileInputStream.java:97)
    at org.eclipse.xtext.util.Files.readFileIntoString(Files.java:126)
    ... 18 more
error(211): ../org.xtext.problem.term.ui/src-gen/org/xtext/problem/ui/contentassist/antlr/internal/InternalTerm.g:176:1: [fatal] rule rule__Collection__Alternatives has non-LL(*) decision due to recursive rule invocations reachable from alts 1,2.  Resolve by left-factoring or using syntactic predicates or using backtrack=true option.
6617 [main] ERROR enerator.CompositeGeneratorFragment  - java.io.FileNotFoundException: ../org.xtext.problem.term.ui/src-gen/org/xtext/problem/ui/contentassist/antlr/internal/InternalTermLexer.java (No such file or directory)
org.eclipse.emf.common.util.WrappedException: java.io.FileNotFoundException: ../org.xtext.problem.term.ui/src-gen/org/xtext/problem/ui/contentassist/antlr/internal/InternalTermLexer.java (No such file or directory)
    at org.eclipse.xtext.util.Files.readFileIntoString(Files.java:129)
    at org.eclipse.xtext.generator.parser.antlr.AbstractAntlrGeneratorFragment.suppressWarningsImpl(AbstractAntlrGeneratorFragment.java:132)
    at org.eclipse.xtext.generator.parser.antlr.AbstractAntlrGeneratorFragment.suppressWarnings(AbstractAntlrGeneratorFragment.java:142)
    at org.eclipse.xtext.generator.parser.antlr.AbstractAntlrGeneratorFragment.suppressWarnings(AbstractAntlrGeneratorFragment.java:138)
    at org.eclipse.xtext.generator.parser.antlr.XtextAntlrUiGeneratorFragment.generate(XtextAntlrUiGeneratorFragment.java:53)
    at org.eclipse.xtext.generator.CompositeGeneratorFragment.generate(CompositeGeneratorFragment.java:92)
    at org.eclipse.xtext.generator.LanguageConfig.generate(LanguageConfig.java:113)
    at org.eclipse.xtext.generator.Generator.generate(Generator.java:361)
    at org.eclipse.xtext.generator.Generator.invokeInternal(Generator.java:128)
    at org.eclipse.emf.mwe.core.lib.AbstractWorkflowComponent.invoke(AbstractWorkflowComponent.java:126)
    at org.eclipse.emf.mwe.core.lib.Mwe2Bridge.invoke(Mwe2Bridge.java:34)
    at org.eclipse.emf.mwe.core.lib.AbstractWorkflowComponent.invoke(AbstractWorkflowComponent.java:201)
    at org.eclipse.emf.mwe2.runtime.workflow.AbstractCompositeWorkflowComponent.invoke(AbstractCompositeWorkflowComponent.java:35)
    at org.eclipse.emf.mwe2.runtime.workflow.Workflow.run(Workflow.java:19)
    at org.eclipse.emf.mwe2.launch.runtime.Mwe2Runner.run(Mwe2Runner.java:102)
    at org.eclipse.emf.mwe2.launch.runtime.Mwe2Runner.run(Mwe2Runner.java:62)
    at org.eclipse.emf.mwe2.launch.runtime.Mwe2Runner.run(Mwe2Runner.java:52)
    at org.eclipse.emf.mwe2.launch.runtime.Mwe2Launcher.run(Mwe2Launcher.java:74)
    at org.eclipse.emf.mwe2.launch.runtime.Mwe2Launcher.main(Mwe2Launcher.java:35)
Caused by: java.io.FileNotFoundException: ../org.xtext.problem.term.ui/src-gen/org/xtext/problem/ui/contentassist/antlr/internal/InternalTermLexer.java (No such file or directory)
    at java.io.FileInputStream.open(Native Method)
    at java.io.FileInputStream.<init>(FileInputStream.java:138)
    at java.io.FileInputStream.<init>(FileInputStream.java:97)
    at org.eclipse.xtext.util.Files.readFileIntoString(Files.java:126)
    ... 18 more
6650 [main] INFO  text.generator.junit.Junit4Fragment  - generating Junit4 Test support classes
6682 [main] INFO  text.generator.junit.Junit4Fragment  - generating Compare Framework infrastructure
7118 [main] INFO  .emf.mwe2.runtime.workflow.Workflow  - Done.

Answer 1

解决您的问题的有效 Xtext 语法是这样的:

Model: term += Term*;

Term: c=Collection;

Collection: ( => Map | Set );

Map: '{' {Map} ( entries += MapEntry ( ',' entries += MapEntry )* )? '}';
MapEntry: key=Term ':' value=Term;

Set: '{' ( values += Term ( ',' values += Term )* )+ '}';

注意事项:

• Collection 规则中的 => 语法谓词。这将 Xtext/ANTLR 引向正确的方向。 (Docu here)

• 即使内容为空，{Map}“简单操作”(Docu here)也会创建一个Map。

• 还需要额外的 MapEntry 规则，否则您将无法保存键/值对。

• 您的语法允许 Set 中同时存在 {}{}{} 和 {}、{}、{}。这可能是也可能不是您想要的。

所以在我看来，关于 LL 递归的 Xtext/ANTLR 错误消息在这种情况下并不是最合适的。这与 LL 递归无关，而是与语法中的歧义有关，有时可以通过应用 => 来解决。有关更多详细信息，请参阅链接的文档。

仅供引用:语法可以解析像这样的简单和嵌套的东西:

// Maps
{}
{ {} : {} }
{ {} : {}, {} : {}}

// Sets
{ {} }
{ {} {} {} {} }
{ {}, {}, {}, {} }

// nested / mixed
{ { { {}:{} } } : {}, {} : { {}:{} }}
{ { { {}:{} } } : {}, {} : { {}:{ {}{}{} } }}