It's not an answer to your broader question, but I suspect what you
actually want here is
#+begin_src racket
;; inside syntax-parse
(datum->syntax this-syntax '<~)
#+end_src
Notice that the second argument to datum->syntax should be a symbol here,
rather than a syntax object. If you provide a syntax object as you did in
your original code, datum->syntax simply returns that syntax unchanged.
I don't expect using syntax-local-introduce will work reliably; before the
change to the scope sets model of hygiene in Racket 6.3 it was another way
to accomplish this purpose, but no longer. For example, it breaks when the
macro introducing the binding and its use are in different modules. Below,
(m introduced) works in a but fails in b.
#+begin_src racket
#lang racket
(module a racket
(define-syntax (m stx)
(syntax-case stx ()
[(_ body)
(with-syntax ([introduced (syntax-local-introduce #'introduced)])
#'(let ([introduced 5])
body))]))
(m introduced)
(provide m))
(module b racket
(require (submod ".." a))
(m introduced))
#+end_src
If you haven't read the papers describing Racket's hygiene algorithm and
module system, you might find them helpful. The documentation isn't enough
on its own, unfortunately.
For hygiene: http://www.cs.utah.edu/plt/scope-sets/
For phasing: https://www.cs.utah.edu/plt/publications/macromod.pdf
On Monday, May 6, 2019 at 10:53:33 AM UTC-4, zeRusski wrote:
>
> I wrote a macro which introduced an implicit binding <~ so that it could
> be used
> in expressions at the use-site. Initially did it with
>
> #+begin_src racket
> ;; inside syntax-parse
> (datum->syntax this-syntax #'<~)
> #+end_src
>
> followed by macro introduced expr that binds it, then the use-site
> macro-input
> that uses it. Think (let/ec <~ macro-input-body).
>
> Worked just fine when tested at top-level or module begin or in expression
> position, but then suddenly broke when I wrote another define-like macro
> whose
> body expanded into the macro above. Turns out scopes of <~ at use-site and
> one I
> introduced in a macro didn't match, at least that's what I surmount from
> the
> message below. I was originally going to ask if someone could teach me to
> read
> these messages, but then I found ~syntax-debug-info~ in docs :) and IIUC
> the
> message below tells me there are two identifier bindings where the error
> occurs
> whose scope-sets share some scopes namely "common scopes ...", but neither
> one's
> scope-set is a subset of the other hence the error. Am I reading it right?
>
> #+begin_src racket
> ; /Users/russki/Code/tilda/prelude/tilda.rkt:303:20: <~: unbound identifier
> ; in: <~
> ; context...:
> ; #(2212719 use-site) #(2212754 intdef) #(2212808 local)
> ; #(2212809 intdef) [common scopes]
> ; other binding...:
> ; local
> ; #(2212718 macro) [common scopes]
> ; common scopes...:
> ; #(2198084 module) #(2198091 module tilda) #(2212726 local)
> ; #(2212727 intdef) #(2212737 local) #(2212738 intdef) #(2212741 local)
> ; #(2212742 intdef) #(2212745 local) #(2212746 intdef) #(2212749 local)
> ; #(2212750 intdef) #(2212753 local)
> #+end_src
>
> I fixed the above with some guesswork that amounted to replacing
> datum->syntax
> with
>
> #+begin_src racket
> (syntax-local-introduce #'<~)
> #+end_src
>
> which IIUC simply flips the scopes so now <~ is use-site and may as well
> be part
> of the macro input. Right?
>
> Suddenly I find myself playing games with hygiene and not really knowing
> the
> rules.
>
> Are there any tutorials that show you how to use things documented in
> Syntax
> Transformers chapter of the Reference?
>
> How do you debug these scope games?
>
> How do you introduce or capture identifier bindings (break hygiene)?
>
> Can you temporarily unbind an identifier (for the extent of some expr), so
> basically remove or trim some scopes from identifiers that occur in macro
> input? I
> suppose there are several possible cases here:
> - trim or replace scopes of ids whose sets match those at use-site,
> guessing this
> won't unbind "shadowing" identifiers (let or define introduced in your
> macro
> input) i.e. those with extra scopes in addition to use-site,
> - how do we deal with those, could we trim ids whose scope sets are
> supersets of
> use-site?
> - assuming I know how to do the above, do I walk the syntax tree and trim
> those
> scopes every time I find matching id or is there a better way?
>
> At this point I'd like to better understand how to manipulate sets of
> scopes and
> verify the result. Could someone kindly teach me or point out good reads or
> examples?
>
> Thanks
>
>
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