> Am So., 3. Aug. 2025 um 17:29 Uhr schrieb Marc Nieper-Wißkirchen
> <xxxxxx@gmail.com>:
>>
>> Maybe I don't understand the semantics of your ephemeron-case form.
>> Reading your first post, there does not seem to be a reference barrier
>> on the key in the broken path. Can you give a syntax-case definition
>> of ephemeron case?

(define-syntax ephemeron-case
  (syntax-rules (else)
    ((_ eph-expr
        ((k-id . v-id) expr_0 expr_1 ...)
        (else broken-expr_0 broken-expr_1 ...))
     (let* ((eph eph-expr)
            (k (ephemeron-key eph))
            (v (ephemeron-value eph))) ; why has this name been changed? grr
       (if (ephemeron-broken? eph)
           (begin broken-expr_0 broken-expr_1 ...)
           (let-syntax ((k-id (identifier-syntax k))
                        (v-id (identifier-syntax v)))
             (dynamic-wind
               (lambda () #f)
               (lambda () expr_0 expr_1 ...)
               (lambda () (reference-barrier k)))))))))

> On 3 Aug 2025, at 17:30, Marc Nieper-Wißkirchen <xxxxxx@gmail.com> wrote:
>
> PS And maybe give a simple example of how a user would typically use it.

(define (ephemeron-alist-ref ephal key)
  (let loop ((more ephal))
    (if (null? more)
        (assertion-violation 'ephemeron-alist-ref "nothing found for key" ephal)
        (ephemeron-case (car ephal)
          ((found-key . value)
           (if (eqv? found-key value)
               value
               (loop (cdr more))))
          (else (loop (cdr more)))))))

Daphne