I suppose just putting in a hash isn't too bad.
#lang racket
(require racket/generator rackunit)
(define/contract (make-cartesian-generator gens)
((listof generator?) . -> . generator?)
(generator ()
(define solcache (make-hasheqv))
(let loop ([gens gens][genidx 0][sols empty])
(cond
[(empty? gens) (yield (reverse sols))]
[else
(match-define (cons gen rest) gens)
(cond
[(eq? (generator-state gen) 'done)
(for ([sol (in-list (reverse (hash-ref solcache genidx)))])
(loop rest (add1 genidx) (cons sol sols)))]
[else
(for ([sol (in-producer gen (void))])
(hash-update! solcache genidx (λ (vals) (cons sol vals))
null)
(loop rest (add1 genidx) (cons sol sols)))])]))))
(define g1 (generator () (yield 1) (yield 2) (void)))
(define g2 (generator () (yield 'a) (yield 'b) (void)))
(check-equal?
(for/list ([gs (in-producer (make-cartesian-generator (list g1 g2)) (void))])
gs)
'((1 a) (1 b) (2 a) (2 b)))
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