Stuck on an issue?

Lightrun Answers was designed to reduce the constant googling that comes with debugging 3rd party libraries. It collects links to all the places you might be looking at while hunting down a tough bug.

And, if you’re still stuck at the end, we’re happy to hop on a call to see how we can help out.

How does "OPTION" work?

See original GitHub issue

I’m currently working on this set of tokens:

[ Let {
    image: 'let',
    startOffset: 0,
    startLine: 1,
    startColumn: 1,
    endLine: 1,
    endColumn: 3,
    isInsertedInRecovery: false },
  Identifier {
    image: 'a',
    startOffset: 4,
    startLine: 1,
    startColumn: 5,
    endLine: 1,
    endColumn: 5,
    isInsertedInRecovery: false },
  Equals {
    image: '=',
    startOffset: 6,
    startLine: 1,
    startColumn: 7,
    endLine: 1,
    endColumn: 7,
    isInsertedInRecovery: false },
  NumberLiteral {
    image: '1',
    startOffset: 8,
    startLine: 1,
    startColumn: 9,
    endLine: 1,
    endColumn: 9,
    isInsertedInRecovery: false } ]

I’m trying to process it like so:

		this.RULE("LetStatement", () => {
			this.CONSUME(Let);
			this.CONSUME(Identifier);
			this.CONSUME(Equals);
			this.CONSUME(NumberLiteral);
			this.OPTION(() => {
				this.CONSUME(Semicolon);
			});
		});

This will fail with:

/srv/http/lang/node_modules/chevrotain/lib/src/parse/parser_public.js:1153
                    throw e;
                    ^

TypeError: Cannot read property 'accept' of undefined
    at getLookaheadPathsForOptionalProd (/srv/http/lang/node_modules/chevrotain/lib/src/parse/grammar/lookahead.js:369:16)
    at buildLookaheadFuncForOptionalProd (/srv/http/lang/node_modules/chevrotain/lib/src/parse/grammar/lookahead.js:38:26)
    at buildLookaheadForOption (/srv/http/lang/node_modules/chevrotain/lib/src/parse/grammar/lookahead.js:43:12)
    at ChiParser.Parser.getLookaheadFuncFor (/srv/http/lang/node_modules/chevrotain/lib/src/parse/parser_public.js:1601:36)
    at ChiParser.Parser.getLookaheadFuncForOption (/srv/http/lang/node_modules/chevrotain/lib/src/parse/parser_public.js:1561:21)
    at ChiParser.Parser.optionInternal (/srv/http/lang/node_modules/chevrotain/lib/src/parse/parser_public.js:1391:34)
    at ChiParser.Parser.OPTION1 (/srv/http/lang/node_modules/chevrotain/lib/src/parse/parser_public.js:524:21)
    at ChiParser.Parser.OPTION (/srv/http/lang/node_modules/chevrotain/lib/src/parse/parser_public.js:500:29)
    at ChiParser.RULE (/srv/http/lang/src/Parser.js:58:9)
    at ChiParser.invokeRuleWithTry (/srv/http/lang/node_modules/chevrotain/lib/src/parse/parser_public.js:1108:29)

Why? Am I misunderstanding how OPTION works?