chevrotain: Playground error message - question

Hi,

I have an issue that appeared with the playground. It may be just my misakes, but the error message displayed there is not insightful and I can’t progress further after quiet the time, so I am posting here. I looked for a Stack-overflow tag but I could not find one, Please let me know if there is a better place to ask these things, as I see from the current issues that there arent really such help-me question there.

Alright so here, we go. The code in the playground is as follows:

(function jsonGrammarOnlyExample() {
  // ----------------- Lexer -----------------
  const createToken = chevrotain.createToken;
  const Lexer = chevrotain.Lexer;
  const CstParser = chevrotain.CstParser;

  const LSquare = createToken({name: "LSquare", pattern: /\[/});
  const RSquare = createToken({name: "RSquare", pattern: /]/});
  const Slash = createToken({name: "Slash", pattern: /\//});
  const Comma = createToken({name: "Comma", pattern: /,/});
  const MetaChr = createToken({name: "MetaChr", pattern: /[,!@#$%^&*()"':{}=+-;]+/});
  const StringLiteral = createToken({
    name: "StringLiteral", pattern: /(:?[^\\"\n\r]+|\\(:?[bfnrtv"\\/]|u[0-9a-fA-F]{4}))*/
  });
  const NumberLiteral = createToken({
    name: "NumberLiteral", pattern: /-?(0|[1-9]\d*)(\.\d+)?([eE][+-]?\d+)?/
  });
  const WhiteSpace = createToken({
    name: "WhiteSpace",
    pattern: /\s+/,
    group: Lexer.SKIPPED
  });

  const tokens = [WhiteSpace, NumberLiteral, StringLiteral, LSquare, RSquare, Comma, Slash, MetaChr];

  const yedLexer = new Lexer(tokens, {
    // Less position info tracked, reduces verbosity of the playground output.
    positionTracking: "onlyStart"
  });

  // Labels only affect error messages and Diagrams.
  LSquare.LABEL = "'['";
  RSquare.LABEL = "']'";
  Comma.LABEL = "','";
  Slash.LABEL = "/";

  // ----------------- parser -----------------
  // eventClause
  // : StringLiteral+
  // guardClause
  // : StringLiteral*
  // actionsClause
  // : StringLiteral*
  // edgeClause:
  // [eventClause] [\[ guardClause \]] [/ actionsClause] |

  class Parser extends CstParser {
    constructor() {
      super(tokens, {
        recoveryEnabled: false
      })

      const $ = this;

      $.RULE("yed", () => {
        $.OPTION1(() => {
          $.SUBRULE($.eventClause);
        })
        $.OPTION2(() => {
          $.CONSUME(LSquare);
          $.SUBRULE($.guardClause);
          $.CONSUME(RSquare);
        })
        $.OPTION3(() => {
          $.CONSUME(Slash);
          $.SUBRULE($.actionsClause);
        })
      });

      $.RULE("eventClause", () => {
        $.MANY(() => {
          $.CONSUME(StringLiteral)
        })
      });

      $.RULE("guardClause", () => {
        $.MANY(() => {
          $.CONSUME(StringLiteral)
        })
      });

      $.RULE("actionsClause", () => {
        $.MANY(() => {
          $.CONSUME(StringLiteral)
        })
      });

      // very important to call this after all the rules have been setup.
      // otherwise the parser may not work correctly as it will lack information
      // derived from the self analysis.
      this.performSelfAnalysis();

    }
  }

  // wrapping it all together
  // reuse the same parser instance.
  const parser = new Parser([]);

  // ----------------- Interpreter -----------------
  const BaseCstVisitor = parser.getBaseCstVisitorConstructor()

  class yedInterpreter extends BaseCstVisitor {

    constructor() {
      super(tokens, {recoveryEnabled: false})
      // This helper will detect any missing or redundant methods on this visitor
      this.validateVisitor()
    }

    yed (ctx) {
      return {event: this.visit(ctx.eventClause), guard: this.visit(ctx.guardClause), action: this.visit(ctx.actionsClause)}
    }

    eventClause (ctx){ return ctx.StringLiteral ? ctx.StringLiteral : ""}
    guardClause (ctx){ return ctx.StringLiteral ? ctx.StringLiteral : ""}
    actionsClause (ctx){ return ctx.StringLiteral ? ctx.StringLiteral : ""}
  }

  // for the playground to work the returned object must contain these fields
  return {
    lexer: yedLexer,
    parser: Parser,
    visitor: yedInterpreter,
    defaultRule: "yed"
  };
}())

The input I pass is m m m m / a.

The error message I get is:

Errors during evaluation of the implementation: 
Cannot read property 'PATTERN' of undefined

If that matters, running on Windows in the Brave browser (Chromium 81).

The language I want is fairly simple: a [b] / c for instance shoud be recognized and a, b, c can all be empty strings too. For the STRING_LITERAL, I reused the JSON_LITERAL and removed the starting and end quotes ". When doing that, I get the aforementioned error. If you put the quotes back, the error is no longer but then the string literals are not parsed (because they do not start with a quote).

Any ideas what I can be doing wrong? I don’t see which PATTERN Chevrotain is trying and failing at validating.