Syntax of semantic predicates in Antlr4

问题:

In What is a 'semantic predicate' in ANTLR3? Bart Kiers gives a very well overview about the different semantic predicates in Antlr3.

Too bad the syntax/semantics were seemingly changed in Antlr4, so this does not compile:

end_of_statement
    :   ';'
    |   EOF
    |   {input.LT(1).getType() == RBRACE}? =>
    ;

RBRACE
    : '}'
    ;

Could someone please tell me how to do the third case of end_of_statement: Accept if the next token is a '}' but do not consume it.

回答1:

There is now just a single type of semantic predicates, which looks like this:

{ <<boolean-epxression>> }?

And the input attribute from the abstract class Parser (which your generated parser extends from) now has an underscore in front of it.

So, in your case, the following ANTLR v3 syntax:

{input.LT(1).getType() == RBRACE}? =>

would look like this in ANTLR v4:

{_input.LT(1).getType() == RBRACE}?