Can someone help me with how to resolve this ambiguity?

[Peng Xiao]

The question is also pasted here:

https://stackoverflow.com/questions/74749948/how-do-i-implement-a-antlr4-parser-that-resolve-ambiguity-when-both-rules-use

I need to parse something like below:

Client->iFrame.Initialise() { 

  res = Server.loadStaticResource() 

  if (res.status == 404 || res.status == 503) {   <1> 

    throw Error()

  } 

} 

== Initialisation done! ==                                      <2> 

Client->iFrame.load(data) { 

  moreStatements() 

}

To add more context, this is to be used to render a sequence diagram like below:

It is a Java-like syntax, except that we support a Divider (shown as == Initialisation done! == in the example). Note that:

1. both <1> and <2> uses ==.
2. It is treated as a Divider only if it appears at the beginning of the line (after spaces removed).
3. Any character can be used between the starting == and ending == except for changelines.
4. There could be more = in the Divider component, such as === 3 equals ===.

How should I implement that?

What I have tried?

I was looking for a way to use lookahead in the Lexer, but could not find any good examples. I am currently limiting the content between starting == and ending == to ONE word only.

My implementation is here:

1. The parser: https://github.com/ZenUml/core/blob/b84c9ca0a6d023c5e520bf83a225e4931db134fc/src/g4/sequenceParser.g4#L88-L89
2. The lexer: https://github.com/ZenUml/core/blob/main/src/g4/sequenceLexer.g4

[Eric Vergnaud]

If it’s treated as a divider only when at beginning of line, then one way would be to have a DIVIDER token including the LF such as the following: ‘\n==[^\n]*\n’

Since it’s longer then ‘\n' it will win during recognition. 

Le 10 déc. 2022 à 04:23, Peng Xiao <eagle...@gmail.com> a écrit :

The question is also pasted here:

https://stackoverflow.com/questions/74749948/how-do-i-implement-a-antlr4-parser-that-resolve-ambiguity-when-both-rules-use

I need to parse something like below:

Client->iFrame.Initialise() { 

  res = Server.loadStaticResource() 

  if (res.status == 404 || res.status == 503) {   <1> 

    throw Error()

  } 

} 

== Initialisation done! ==                                      <2> 

Client->iFrame.load(data) { 

  moreStatements() 

}

To add more context, this is to be used to render a sequence diagram like below:

<question-antlr.png>

It is a Java-like syntax, except that we support a Divider (shown as == Initialisation done! == in the example). Note that:

1. both <1> and <2> uses ==.
2. It is treated as a Divider only if it appears at the beginning of the line (after spaces removed).
3. Any character can be used between the starting == and ending == except for changelines.
4. There could be more = in the Divider component, such as === 3 equals ===.

How should I implement that?

What I have tried?

I was looking for a way to use lookahead in the Lexer, but could not find any good examples. I am currently limiting the content between starting == and ending == to ONE word only.

My implementation is here:

1. The parser: https://github.com/ZenUml/core/blob/b84c9ca0a6d023c5e520bf83a225e4931db134fc/src/g4/sequenceParser.g4#L88-L89
2. The lexer: https://github.com/ZenUml/core/blob/main/src/g4/sequenceLexer.g4

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