The Fourth Interpreter about the traceproc

程序的过程追踪能力体现着一个独特的优势，本文在First Interpreter,Second Interpreter,Third Interpreter基础上实现新的解释器功能。

实现效果:

    enter: x = #(struct:num-val 30)
    exting: result = #(struct:num-val 29)

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lang.scm修改程序追踪部分

      ;;增加了trace-proc
          (expression ("traceproc" "(" identifier ")" expression) traceproc-exp)

至于proc的语法并不需要做任何的修改。

data-structures.scm 进行部分修改

1. 在修改trace-proc实现的情况下还得修改proc?

原先的proc?实现

  ;; proc? : SchemeVal -> Bool
  ;; procedure : Var * Exp * Env -> Proc
  (define-datatype proc proc?
    (procedure
     (var symbol?)
     (body expression?)
     (env environment?)))

改进后的proc?实现,注意trace 是boolean?类型。

  ;; proc? : SchemeVal -> Bool
  ;; procedure : Var * Exp * Env -> Proc
  (define-datatype proc proc?
    (procedure
     (var symbol?)
     (body expression?)
     (env environment?)
     (trace boolean?)))

interp.scm 修改程序追踪部分

由于proc?的类型实现发生了变化，所以需要进行对应的修改apply-procedure和value-of。

apply-procedure的修改

原先的apply-procedure实现：

(define apply-procedure
    (lambda (proc1 val)
      (cases proc proc1
        (procedure (var body saved-env)
          (value-of body (extend-env var val saved-env))))))

改进后的apply-procedure增加了trace?的判断

;; apply-procedure : Proc * ExpVal -> ExpVal
  ;; Page: 79
  (define apply-procedure
    (lambda (proc1 val)
      (cases proc proc1
        (procedure (var body saved-env trace?)
                  (if trace? (printf "enter: ~a = ~a\n" var val) '**proc**)
                  (let ((value (value-of body (extend-env var val saved-env))))
                    (if trace? (printf "exting: result = ~a\n" value) '**proc**)
                    value)))))

value-of 的修改

原先的value-of实现：

  (proc-exp (var body)
                 (proc-val (procedure var body env)))

新改进的的value-of 实现:

    (proc-exp (var body)
                 (proc-val (procedure var body env #f)))
    (traceproc-exp (var body)
                  (proc-val (procedure var body env #t)))

测试追踪修改的结果

(run "proc(x) -(x, 1)")
(run "(proc(x) -(x,1)  30)")
(run "let f = traceproc (x) -(x,1) in (f 30)")
(run "(proc(f)(f 30)  proc(x)-(x,1))")
(run "let x = 3 in -(x,1)")

    (proc-val (procedure 'x (diff-exp (var-exp 'x) (const-exp 1)) (list (list 'i (num-val 1)) (list 'v (num-val 5)) (list 'x (num-val 10))) #f))
    (num-val 29)
    enter: x = #(struct:num-val 30)
    exting: result = #(struct:num-val 29)
    (num-val 29)
    (num-val 29)
    (num-val 2)

老阮不狂谁会得？出门一笑大江横！