section-4.1.3.rkg

#lang sicp

; ex 4.11 & ex 4.12

(define (make-frame variable values)
  (cons '*frame* (map cons variable values)))

(define (frame-variables frame) (map car (cdr frame)))

(define (frame-values frame) (map cdr (cdr frame)))

(define (add-binding-to-frame! var val frame)
  (set-cdr! frame (cons (cons var val) (cdr frame))))

(define (extend-environment vars vals base-env)
  (if (= (length vars) (length vals))
      (cons (make-frame vars vals) base-env)
      (if (< (length vars) (length vals))
          (error "Too many arguments supplied" vars vals)
          (error "Too few arguments supplied" vars vals))))

(define (scan var frame)
  (assoc var (cdr frame)))

; ex 4.13
;
; Only make the bound variable's value in the first frame to be
; '*unassigned*.

(define (unbound? exp) (tagged-list? exp 'make-unbound!))

(define (unbound-var exp) (cadr exp))

(define (eval-make-unbound! exp env)
  (let* ([frame (first-frame env)]
         [pair (scan (unbound-var exp) frame)])
    (if pair
        (set-cdr! pair '*unassigned*)
        (error "Unbound variable: MAKE-UNBOUND!" (unbound-var exp)))))

; ex 4.14
;
; Simply put, if map is primitive, the proc of map will be called in the
; implementation language. However, the proc of map is the representation of
; procedure in the implemented language; in the implementation language, it's
; just a list, which cannot be called.
;
; Take (map (lambda (n) n) '(1 2 3)) as an example. It becomes
;
; 1. (apply map '((procedure ...) (1 2 3)) which becomes
; 2. (map '(procedure ...) '(1 2 3)) which crashes because '(procedure ...) is
;    not callable.

; TODO ex 4.15