How to split an ANTLR grammar file into multiple ones

Question

I have a large grammar file, and plan to split it into multiple ones, so that I can reuse some of those smaller files in another grammar file. I have tried doing it but failed. Can you please tell if such a feature is available, and if so, please direct me towards an example.

Gott

Answer 1

If you want to split lexer and parser.

Lexer:

lexer grammar HelloLexer;
Hello : 'hello' ;
ID : [a-z]+ ;             // match lower-case identifiers
WS : [ \t\r\n]+ -> skip ; // skip spaces, tabs, newlines

Parser:

parser grammar HelloParser;
options { tokenVocab=HelloLexer; }
r  : Hello ID ;      

Remember to name the files HelloLexer.g4 and HelloParser.g4

if you want to import a whole grammar, then you should use the import keyword

grammar Hello;

import OtherGrammar;

Hello : 'hello' ;
ID : [a-z]+ ;             // match lower-case identifiers
WS : [ \t\r\n]+ -> skip ; // skip spaces, tabs, newlines    
r  : Hello ID ;

Homebrew

shouldn't it be import HelloParser;? – Husted

Answer 2

You did not mention ANTLR version, so I am going to assume you are using the current one - 4.x. In ANTLR4 grammars can be imported with import keyword. Something like this:

File: CommonLexerRules.g4

lexer grammar CommonLexerRules;

ID  :   [a-zA-Z]+ ;
...

File: MyParser.g4

grammar MyParser;      
import CommonLexerRules; //includes all rules from lexer CommonLexerRules.g4
...

Rules in the “main grammar” override rules from imported grammars to implement inheritance. See more details here: https://theantlrguy.atlassian.net/wiki/display/ANTLR4/Grammar+Structure#GrammarStructure-GrammarImports

Illyria

yes, the version I am using is antlr4.5-opt. When I try to reference a rule in an imported grammar in the main-grammar, I get this error-message: reference to undefined rule: temp_rule (com.tunnelvisionlabs:antlr4-maven-plugin:4.5:antlr4:default:generate-sources) – Gott

try to reduce your imported and main grammar files to the absolute minimum, just to demonstrate the error, and add files content in your original question – Illyria

To me seems only one import per grammar? Only way I could make it work was main.g4 imports a.g4 which imports b.g4. Not nice. But main.g4 importing a.g4 and b.g4 does indeed cause reference to undefined rules. – Rajah

This might be irrelevant now, but for other viewers, to remove the error undefined rule, write options { tokenVocab = YourLexerHere; } after grammar YourGrammarName; instead of importing your Lexer Grammar. – Cerement