I think I found a case where id-a binds id-b when (would-bind? id-a id-b)
returns #f:
(define-syntax bind-test0
(lambda (stx)
(define id #'x)
(define shifted-id (syntax-shift-phase-level id -1))
(displayln (would-bind? id shifted-id))
(syntax-case stx ()
[(_ e)
#`(let ([#,id e])
(displayln #,shifted-id))])))
(bind-test0 100) would print #f then 100. However, if shifted-ld and id are
swapped in (let ...), an undefined identifier error will be raised, which
is consistent with the return value of (would-bind? ...). Is there a bug in
how (let ...) handles identifiers with shifted phase levels?
Thanks,
Yongming
On Friday, July 26, 2019 at 7:07:49 AM UTC-4, Matthew Flatt wrote:
>
> At Thu, 25 Jul 2019 18:55:18 -0700 (PDT), Yongming Shen wrote:
> > Based on my understanding, (bound-identifier=? id-a id-b) only returns
> true
> > if id-a would bind id-b AND id-b would bind id-a. Also based on my
> > understanding, id-a will bind id-b doesn't imply that id-b will bind
> id-a.
> > So, if I only want to check whether id-a will bind id-b, which function
> > should I use?
>
> There's not a predicate like that right now (I guess only because we
> haven't needed it).
>
> Here's one way to implement the predicate:
>
> (define (would-bind? a b)
> (subset? (list->set (hash-ref (syntax-debug-info a) 'context))
> (list->set (hash-ref (syntax-debug-info b) 'context))))
>
> A built-in operation would work the same way, except that it could be
> more efficient by not serializing scopes to a list in debugging
> information and then converting the list back to a set.
>
>
> Matthew
>
>
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