Re: Writing a procedure in different style

[Taylan Kammer]

On 12.12.2020 13:22, Zelphir Kaltstahl wrote:
> ~~~~
> (define find-in-tree*
>    (λ (peg-tree filter-proc)
>
>      (define traverse
>        (λ (subtree cont)
>          (simple-format (current-output-port)
>                         "working with subtree ~a\n"
>                         subtree)
>          (cond
>           [(null? subtree) (cont)]
>           [(pair? (first subtree))
>            (traverse (first subtree)
>                      (λ () (traverse (cdr subtree) cont)))]
>           [(filter-proc (first subtree))
>            (cons subtree (cont))]
>           [else
>            (traverse (cdr subtree) cont)])))
>
>      (traverse peg-tree (λ () '()))))
> ~~~~

From what I can tell, the continuation argument is essentially used as 
a way of queuing up further work that is to be done.  Adding an element 
is done by wrapping it in yet another procedure that adds some work, and 
popping an element is done by applying it.

This means it should be possible to change it into an explicit data 
structure (e.g. simply a list).  Here's an attempt, which I have not 
tested in any way:

  (define find-in-tree*
     (λ (peg-tree filter-proc)

       (define traverse
         (λ (subtree rest)
           (simple-format (current-output-port)
                          "working with subtree ~a\n"
                          subtree)
           (cond
            [(null? subtree)
             (if (null? rest)
                 '()
                 (traverse (car rest) (cdr rest))]
            [(pair? (first subtree))
             (traverse (first subtree)
                       (cons (cdr subtree) rest))]
            [(filter-proc (first subtree))
             (cons subtree
                   (traverse (car rest) (cdr rest)))]
            [else
             (traverse (cdr subtree) rest)])))

       (traverse peg-tree '())))

In summary:

- I rename 'cont' to 'rest'

- I change
  (cont)
  to
  (if (null? rest) '() (traverse (car rest) (cdr rest)))

- I change
  (λ () (traverse (cdr subtree) cont))
  to
  (cons (cdr subtree) rest)

- I change the initial seed of
  (λ () '())
  to
  '()

And I think theoretically it should behave the same.


Taylan