I had the impression that you want to create the hashtable when it’s not there
and use it when it is:
#lang racket
(define-syntax (set/define stx)
(syntax-case stx ()
[(_ id key val)
(syntax-local-value #'id (lambda () #f))
(with-syntax ([h (car (syntax-local-value #'id (lambda () #f)))])
#`(begin
(hash-set! h 'key val)
h))]
[(_ id key val)
#'(begin
(define-syntax id (cons #'h 0))
(define h (make-hash))
(set/define id key val))]))
(set/define h a 1)
(set/define h a 2)
To make sure you can see the hashtable, I also return it after every
set/define.
> On Apr 19, 2019, at 4:17 PM, zeRusski <vladilen.ko...@gmail.com> wrote:
>
> Here's what I've been trying and failing to do in Racket. The smallest
> example I
> could think of is a macro that sets a key in a hash-table, so basically this
> transformation:
>
> (set/define ht 'key 42)
> ;; =>
> (hash-set! ht 'key 42)
>
>
> but if ht there is an unbound identifier it must bind it to a fresh hash
> table. So
> basically introduce a (define ht (make-hash)) before setting the key. Assume
> we
> run in a context where define is allowed.
>
> Please, don't ask why I want something like this, I just do. So far tricks I
> could
> use in other lisps failed in Racket. Here's one silly idea: catch unbound
> identifier exn. You can do it as per below or in the handler itself but it
> doesn't
> matter cause that define is local (I think) and doesn't happen in the macro
> use
> context.
>
> (require (only-in syntax/macro-testing convert-compile-time-error))
>
> (define (unbound-id-error? err)
> (and (exn:fail:syntax? err)
> (regexp-match #rx"unbound identifier" (exn-message err))))
>
> (define-syntax-rule (set/define id key val)
> (unless (let/ec k
> (with-handlers ((unbound-id-error? (λ (_) (k #f))))
> (convert-compile-time-error
> (hash-set! id key val))))
> (displayln "escaped")
> (define id (make-hash))
> (hash-set! id key val)))
>
> (set/define ht 'key 42)
> ;; =>
> ;; runs but appears that the (define ht ..) doesn't happen at top-level
>
> This is already all sorts of ugly and it doesn't even work.
>
> Another idea is to replace #%top for the extent of that transformer, perform
> local-expand (or some equivalent) so that #%top expansion does the job. I've
> no
> idea how to do that, but I'm sure Racket could be persuaded. Incidentally, I'm
> curious how to create such local transformers e.g. something like
> (let-transformer((#%top ...)) body).
>
> Even if I knew how to do the above (local-expand #'(set/define ht 'key 42)
> '())
> run at compile time doesn't seem to wrap unbound ht in #%top. I thought it
> should?
>
> So then, two questions:
>
> 1. What's the Racket way of getting what I want?
>
> 2. Is there a way to torture the above code into submission? Put differently
> is
> there a way to ensure (define id (make-hash)) runs in appropriate context?
>
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