[Chicken-users] Why does the JSON egg map JSON structs to Scheme vectors instead of alists?

[Vok Vojwo]

I am a bit confused by the way the JSON egg maps JSON structures to
Scheme values. The JSON egg maps a structure to a vector:

(use json)
(with-input-from-string "{\"pi\":3.14,\"e\":2.71}" json-read)
;; => #(("pi" . 3.14) ("e" . 2.71))

This makes it impossible to use the standard Scheme function assoc to
read the data.

The following functions fix the problem:

(define (json->alist arg)
  (cond
   ((vector? arg)
    (map (lambda (pair)
           (cons (car pair) (json->alist (cdr pair))))
         (vector->list arg)))
   ((list? arg)
    (list->vector (map json->alist arg)))
   (else arg)))

(define (alist->json arg)
  (cond
   ((list? arg)
    (list->vector (map (lambda (pair)
                         (cons (car pair) (alist->json (cdr pair))))
                       arg)))
   ((vector? arg)
    (map alist->json (vector->list arg)))
   (else arg)))

A small verification test:

(use http-client)

(define (with-input-from-url url chunk)
  (with-input-from-request url #f chunk))

(define json
  (with-input-from-url
   "http://www.google.com/calendar/feeds/address@hidden/public/full?alt=json";
   json-read))

(equal? json (alist->json (json->alist json)))
;; => #t

By using alists it is possible to use assoc to access the data:

(define (assoc* alist . path)
  (let assoc* ((path path)
               (alist alist))
    (if (null? path)
        alist
        (assoc* (cdr path)
                (cdr (assoc (car path) alist))))))

(assoc* (json->alist json) "feed" "title" "$t")
;; => "Official Google External Developer Events"

Is there any reason why the JSON egg creates vectors of pairs?

If not I would suggest to fix it to make it more schemish.