(srfi 124) implements weak pairs, not ephemerons

[dpk]

Ephemerons should be broken if the only remaining strong reference to their key is inside their datum:

> (import (srfi 124))
> (define x (list 'x))
> (define ex (make-ephemeron x x))
> (set! x #f)
> (##gc)
> (ephemeron-broken? ex)
#f ;; expected: #t

Compare Racket-Chez 9.5.9.9 (for Racket 8.7)

> (define x (list 'x))
> (define ex (ephemeron-cons x x))
> (set! x #f)
> (collect)
> ex
(#!bwp . #!bwp)

I attempted to fix this in Gerbil using a pair of wills, but as Andy Wingo discovered, this is wrong too.

I think true ephemerons cannot be expressed in terms of wills.

The best fix would be to change Gambit’s primitive weak-referencing system to ephemerons with finalizers (as in Haskell and XEmacs) instead of weak boxes with finalizers.