On Mon, 14 Sep 2020 at 14:18, Hendrik Boom <hend...@topoi.pooq.com> wrote:
> I would, ideally, only use hashcons on those cons-cells which had
> themselves
> been hashconsed, so eq? would suffice.
>
The challenge is checking to see whether a new allocation is required.
Checking `eq?` of the cdr suffices, but seldom is `eq?` appropriate for the
car, assuming you want `(eq? (hashcons (set) '()) (hashcons (set) '()))`
and similar to hold. Canonicalize looks, roughly, like
(define (canonicalize c)
(match c
[(cons a d) (if (cell-exists-in-weak-table-with-car-and-cdr? a d) ;; (X)
(extract-and-return-existing-cell a d)
(intern-and-return-cons-of a (canonicalize d)))]
[_ c]))
The line marked (X) will usually want to compare `a` with `equal?` and `d`
with `eq?`.
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