# [plt-scheme] From hash-table to generator

I have implemented sets on top of hash tables.
A SET is a
(define-struct set (ht))
where ht is hash-table from strings to #t.
I'd like to extend srfi-42 with a :set generator,
as an example:
> (list-ec (:set x (list->set 1 2 3))
x)
(1 2 3)
My first version converted the hash-table to a list
and expanded :set into a :list :
(define-syntax :set
(syntax-rules (index)
((:set cc var (index i) arg)
(:parallel cc (:set var arg) (:integers i)) )
((:set cc var arg)
(:list cc var (set->list arg)))))
where
(define (hash-table->list ht)
(let ([xs '()])
(hash-table-for-each ht
(lambda (x v)
(set! xs (cons x xs))))
xs))
(define (set->list s)
(hash-table->list (set-ht s)))
But allocating a list just to iterate through the elements
just feels wrong.
So I thought I'd see what happened, if I used generators
instead. First the hash-table is converted into a
generator, and then each element is generated one at a
time:
(define-syntax :set
(syntax-rules (index)
((:set cc var (index i) arg)
(:parallel cc (:set var arg) (:integers i)) )
((:set cc var arg)
(:do cc
(let ((g (hash-table->generator (set-ht arg)))))
((x (g)))
x
(let ((var x)))
#t
((g))))))
The problem is thus to write hash-table->generator efficiently.
As it turns out, the inversion via call/cc looses big time
to the convert-to-list approach.
So the next question, is: Is there a better way to turn
a hash-table into a generator?
--
Jens Axel Søgaard