difficult to parse a basic language about racket-peg HOT 7 CLOSED

#lang peg

braceLanguage <- item+;

item <- braceExpansion / simpleItem;
braceExpansion <- '{' braceLanguage (',' braceLanguage)* ','? '}';

simpleItem <- [^{,}]+;

#lang peg

(struct seq (a b) #:transparent);
(struct brace (l) #:transparent);
(struct idt (a) #:transparent);

braceLanguage <- v:item+ -> (let loop ((l v))
                              (if (null? (cdr l))
                                  (car l)
                                  (loop (cons (seq (car l) (cadr l)) (cddr l)))));

item <- braceExpansion / simpleItem;
braceExpansion <- ~'{' l:(braceLanguage (~',' braceLanguage)* ~(','?)) '}' -> l;

simpleItem <- v:[^{,}]+ -> (idt v);

#lang racket

;; list-monad.rkt

(provide liftm2 sequence
         concatenate string-concatenate)

(define (concatenate lists)
  (apply append lists))

(define (string-concatenate strings)
  (apply string-append strings))

(define (>>= m f)
  (apply append (map f m)))

(define (liftm2 f a b)
  (>>= a (lambda (x) (>>= b (lambda (y) (list (f x y)))))))

(define (sequence lsts)
  (if (null? lsts)
      (list '())
      (liftm2 cons (car lsts) (sequence (cdr lsts)))))

#lang peg

(require "list-monad.rkt");

text <- str:[^{},]+ -> (list str);
items <- xs:(text / choice)+ -> (map string-concatenate (sequence xs));
choice <- '{' x:items xs:(~',' items)* '}' -> (concatenate (cons x xs));