How to understand (debug) why ANTLR4 makes a certain choice?

[Kostya Y]

Hey all

I have a syntax for a templating language I've defined that works most of the time.

However, sometimes an input causes unexpected matches which are not quite clear to me.

Is there some kind of debug/verbose mode I can use to understand why ANTLR decides something is a specific token A even though there is an error parsing it while token B is a perfect match?

I am familiar with the various IDE plugins/lab.antlr4.org, however none of those explain why ANTLR thinkins something is a token of this or that type.

Thanks in advance.

[Andy Hewitt]

Hi, I would use gun in token mode.

 

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[Kostya]

I'm not familiar with it and wasnt able to find any reference to it in the documentation.

Can you please elaborate?

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[Andy Hewitt]

Apologies its ‘grun’ email system replacement.

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[Ross Patterson]

The "TestRig" tool shipped with ANTLR might help.  It's commonly called "grun", but you won't find any code by that name.  You run it from the command line via "java org.antlr.v4.runtime.misc.TestRig", after first compiling the grammar.  It has several options, but the one that will show you the list of tokens is "-tokens".  If the lexer is correctly determining the tokens, but the parser isn't using the syntax rule you expect, you might find the "-gui" option useful - it will open up a window showing the full parse tree.  That gets you the "what", but not the "why".  ANTLR doesn't appear to have a "tell me why you chose that parse alternative" capability.

Ross

[Wanadoo]

You can now do this online in Antlr lab, no build required

Envoyé de mon iPhone

Le 5 déc. 2022 à 16:52, Ross Patterson <ross.pa...@gmail.com> a écrit :

The "TestRig" tool shipped with ANTLR might help.  It's commonly called "grun", but you won't find any code by that name.  You run it from the command line via "java org.antlr.v4.runtime.misc.TestRig", after first compiling the grammar.  It has several options, but the one that will show you the list of tokens is "-tokens".  If the lexer is correctly determining the tokens, but the parser isn't using the syntax rule you expect, you might find the "-gui" option useful - it will open up a window showing the full parse tree.  That gets you the "what", but not the "why".  ANTLR doesn't appear to have a "tell me why you chose that parse alternative" capability.

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[Terence Parr]

Hi. There is a way in the intellij plugin to learn about the different possibilities for parsing but not lexing.  Sorry. All you know is that the lexer matches the longest token or, if ambiguous, the first defined in grammar.

Ter

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