function parseOperationType(lexer) {
  var operationToken = expect(lexer, _lexer.TokenKind.NAME);
  switch (operationToken.value) {
    case 'query':
      return 'query';
    case 'mutation':
      return 'mutation';
    // Note: subscription is an experimental non-spec addition.
    case 'subscription':
      return 'subscription';
  }

  throw unexpected(lexer, operationToken);
}
OperationType : one of query mutation subscription