疑问 about syntax-parser HOT 3 CLOSED

export function createFourOperations(field: ChainFunction) {
  const addExpr = () => chain(term, exprTail)(ast => {
      return ast[0];
    });

  const exprTail = () => chain(optional(addOp, term, exprTail))();

  const term = () => chain(factor, termTail)(ast => {
      return ast[0];
    });

  const termTail = () => chain(optional(mulOp, factor, termTail))();

  const mulOp = () => chain(['*', '/', '%'])(ast => {
      return ast[0];
    });

  const addOp = () => chain(['+', '-'])(ast => {
      return ast[0];
    });

  const factor = () => chain([
      chain('(', addExpr, ')')(ast => {
        return ast[1];
      }),
      field
    ])(ast => {
      return ast[0];
    });

  return addExpr;
}

const fieldItem = () => chain(matchTokenType('word'))();
const field = () => createFourOperations(fieldItem)();