antrl4 why can not parse Compaire lexer but can parse compaire parser

Question

grammar test;

parse:(expr|type)EOF ;

type: ID('<'ID'>')?;
expr:ID Compaire ID;

Compaire:'>' | '<';

ID:[a-z];

I am learning antlr4 , why can not parse Compaire lexer but can parse compaire parser

Answer 1

As already mentioned in the comments: when using tokens like '<' or '>' in parser rules, what really happens is this:

type : ID ('<' ID '>')?;
expr : ID Compaire ID;

T__1     : '<';
T__2     : '>';
Compaire : '>' | '<';

ID:[a-z];

In other words, for all of the '...' tokens ANTLR encounters inside parser rules (that cannot be directly mapped to lexer rules!), ANTLR will create a lexer rule behind the scenes. And in your case, this causes that the token Compaire will never be created because T__1 and T__2 already match these chars.

To fix it, you could do:

// the parser now uses LT and GT, and will not create T__1 and T__2
type : ID ('<' ID '>')?; 
expr : ID compaire ID;

compaire : GT | LT;

GT : '>';
LT : '<';

The above will work, but I recommend to not use literal tokens inside your parser rules and always do:

type : ID (LT ID GT)?; 
expr : ID compaire ID;

compaire : GT | LT;

GT : '>';
LT : '<';