#lang racket

; Do boolean.rkt dodajemy pary
;
; Miejsca, ktore sie zmienily oznaczone sa przez !!!

; --------- ;
; Wyrazenia ;
; --------- ;

(struct const     (val)      #:transparent)
(struct binop     (op l r)   #:transparent)
(struct var-expr  (id)       #:transparent)
(struct let-expr  (id e1 e2) #:transparent)
(struct if-expr   (eb et ef) #:transparent)
(struct cons-expr (e1 e2)    #:transparent) ; <------------------- !!!
(struct car-expr  (e)        #:transparent) ; <------------------- !!!
(struct cdr-expr  (e)        #:transparent) ; <------------------- !!!
(struct is-pair   (e)        #:transparent)

(define (expr? e)
  (match e
    [(const n) (or (number? n) (boolean? n))]
    [(binop op l r) (and (symbol? op) (expr? l) (expr? r))]
    [(var-expr x) (symbol? x)]
    [(let-expr x e1 e2)
     (and (symbol? x) (expr? e1) (expr? e2))]
    [(if-expr eb et ef)
     (and (expr? eb) (expr? et) (expr? ef))]
    [(cons-expr e1 e2) (and (expr? e1) (expr? e2))] ; <----------- !!!
    [(car-expr e) (expr? e)] ; <---------------------------------- !!!
    [(cdr-expr e) (expr? e)] ; <---------------------------------- !!!
    [(is-pair e)  (expr? e)]
    [_ false]))

(define (parse q)
  (cond
    [(number? q) (const q)]
    [(eq? q 'true)  (const true)]
    [(eq? q 'false) (const false)]
    [(symbol? q) (var-expr q)]
    [(and (list? q) (eq? (length q) 3) (eq? (first q) 'cons)) ; <- !!!
     (cons-expr (parse (second q))
                (parse (third q)))]
    [(and (list? q) (eq? (length q) 2) (eq? (first q) 'car)) ; <-- !!!
     (car-expr (parse (second q)))]
    [(and (list? q) (eq? (length q) 2) (eq? (first q) 'cdr)) ; <-- !!!
     (cdr-expr (parse (second q)))]
    [(and (list? q) (eq? (length q) 3) (eq? (first q) 'let))
     (let-expr (first (second q))
               (parse (second (second q)))
               (parse (third q)))]
    [(and (list? q) (eq? (length q) 4) (eq? (first q) 'if))
     (if-expr (parse (second q))
              (parse (third q))
              (parse (fourth q)))]
    [(and (list? q) (eq? (length q) 3) (symbol? (first q)))
     (binop (first q)
            (parse (second q))
            (parse (third q)))]
    [(and (list? q) (eq? (length q) 2) (eq? (first q) 'pair?))
     (is-pair (parse (second q)))]))

(define (test-parse) (parse '(let [x (+ 2 2)] (+ x 1))))

; ---------- ;
; Srodowiska ;
; ---------- ;

(struct environ (xs))

(define env-empty (environ null))
(define (env-add x v env)
  (environ (cons (cons x v) (environ-xs env))))
(define (env-lookup x env)
  (define (assoc-lookup xs)
    (cond [(null? xs) (error "Unknown identifier" x)]
          [(eq? x (car (car xs))) (cdr (car xs))]
          [else (assoc-lookup (cdr xs))]))
  (assoc-lookup (environ-xs env)))

; --------- ;
; Ewaluacja ;
; --------- ;

(define (value? v)
  (or (number? v)
      (boolean? v)
      (and (pair? v) (value? (car v)) (value? (cdr v)))))

(define (op->proc op)
  (match op ['+ +] ['- -] ['* *] ['/ /] ['% modulo]
            ['= =] ['> >] ['>= >=] ['< <] ['<= <=]
            ['and (lambda (x y) (and x y))]
            ['or  (lambda (x y) (or  x y))]))

(define (eval-env e env)
  (match e
    [(const n) n]
    [(binop op l r) ((op->proc op) (eval-env l env)
                                   (eval-env r env))]
    [(let-expr x e1 e2)
     (eval-env e2 (env-add x (eval-env e1 env) env))]
    [(var-expr x) (env-lookup x env)]
    [(if-expr eb et ef) (if (eval-env eb env)
                            (eval-env et env)
                            (eval-env ef env))]
    [(cons-expr e1 e2) (cons (eval-env e1 env) ; <---------------- !!!
                             (eval-env e2 env))]
    [(car-expr e) (car (eval-env e env))] ; <--------------------- !!!
    [(cdr-expr e) (cdr (eval-env e env))] ; <------------------- !!!
    [(is-pair e) (cons? (eval-env e env))]))
(define (eval e) (eval-env e env-empty))

(define program
  '(car (if true (cons 1 2) false)))

(define (test-eval) (eval (parse program)))