versioning.js

/*
 * Example Of using **Grammar Inheritance** to support multiple versions of the same grammar.
 */
import { createToken, Lexer, CstParser } from "chevrotain";

// ----------------- lexer -----------------
const Select = createToken({ name: "Select", pattern: /SELECT/i });
const From = createToken({ name: "From", pattern: /FROM/i });
const Where = createToken({ name: "Where", pattern: /WHERE/i });
const Comma = createToken({ name: "Comma", pattern: /,/ });
const Identifier = createToken({ name: "Identifier", pattern: /\w+/ });
const Integer = createToken({ name: "Integer", pattern: /0|[1-9]\d+/ });
const GreaterThan = createToken({ name: "GreaterThan", pattern: /</ });
const LessThan = createToken({ name: "LessThan", pattern: />/ });
const WhiteSpace = createToken({
  name: "WhiteSpace",
  pattern: /\s+/,
  group: Lexer.SKIPPED,
});

const allTokens = [
  WhiteSpace,
  Select,
  From,
  Where,
  Comma,
  Identifier,
  Integer,
  GreaterThan,
  LessThan,
];
const SelectLexer = new Lexer(allTokens);

// ----------------- parser -----------------
class SelectParserVersion1 extends CstParser {
  constructor(isInvokedByChildConstructor = false) {
    super(allTokens);

    const $ = this;

    $.RULE("selectStatement", () => {
      $.SUBRULE($.selectClause);
      $.SUBRULE($.fromClause);
      $.OPTION(() => {
        $.SUBRULE($.whereClause);
      });
    });

    $.RULE("selectClause", () => {
      $.CONSUME(Select);
      $.AT_LEAST_ONE_SEP({
        SEP: Comma,
        DEF: () => {
          $.CONSUME(Identifier);
        },
      });
    });

    // fromClause in version1 allows only a single column name.
    $.RULE("fromClause", () => {
      $.CONSUME(From);
      $.CONSUME(Identifier);
    });

    $.RULE("whereClause", () => {
      $.CONSUME(Where);
      $.SUBRULE($.expression);
    });

    $.RULE("expression", () => {
      $.SUBRULE($.atomicExpression);
      $.SUBRULE($.relationalOperator);
      $.SUBRULE2($.atomicExpression); // note the '2' suffix to distinguish
      // from the 'SUBRULE(atomicExpression)' 2 lines above.
    });

    $.RULE("atomicExpression", () => {
      $.OR([
        { ALT: () => $.CONSUME(Integer) },
        { ALT: () => $.CONSUME(Identifier) },
      ]);
    });

    $.RULE("relationalOperator", () => {
      $.OR([
        { ALT: () => $.CONSUME(GreaterThan) },
        { ALT: () => $.CONSUME(LessThan) },
      ]);
    });

    // the selfAnalysis must only be performed ONCE during grammar construction.
    // that invocation should be the in the LAST (bottom of the hierarchy) grammar.
    // of in inheritance chain.
    if (!isInvokedByChildConstructor) {
      // very important to call this after all the rules have been defined.
      // otherwise the parser may not work correctly as it will lack information
      // derived during the self analysis phase.
      this.performSelfAnalysis();
    }
  }
}

// V2 extends V1
class SelectParserVersion2 extends SelectParserVersion1 {
  constructor() {
    super(true);

    const $ = this;

    // "fromClause" production in version2 is overridden to allow multiple table names.
    this.fromClause = $.OVERRIDE_RULE("fromClause", () => {
      $.CONSUME(From);
      $.AT_LEAST_ONE_SEP({
        SEP: Comma,
        DEF: () => {
          $.CONSUME(Identifier);
        },
      });
    });

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

// ----------------- wrapping it all together -----------------

// reuse the same parser instances.
const version1Parser = new SelectParserVersion1();
const version2Parser = new SelectParserVersion2();

export function parseSelect(text, version) {
  const lexResult = SelectLexer.tokenize(text);

  let parser;

  // initialize a parser for the specific version version chosen.
  switch (version) {
    case 1:
      parser = version1Parser;
      break;
    case 2:
      parser = version2Parser;
      break;
    default:
      throw Error("no valid version chosen");
  }

  // setting a new input will RESET the parser instance's state.
  parser.input = lexResult.tokens;
  const cst = parser.selectStatement();

  return {
    value: cst,
    lexErrors: lexResult.errors,
    parseErrors: parser.errors,
  };
}