Walter Lewis via General Guile related discussions <guile-user@gnu.org> writes:
> (define-syntax unhygienic
> (syntax-rules ()
> ((_ the-pair fetch)
> (begin
> (define the-head (car the-pair))
> (define (fetch) the-head)))))
>
> (unhygienic '(1) fetch1)
> (unhygienic '(2) fetch2)
>
> (fetch1)
>
> ;; => 2
I just tested this and see the same problem:
(unhygienic '(a) name1)
(unhygienic '(b) name2)
(name1) ;; => b
(name2) ;; => b
So it’s reproduced. Thank you for the minimal example!
> It seems that the second call to unhygienic shadows the previous
> definition of the-head. I expected syntax-rules to generate fresh
> names for it each time.
>
> Is this expected, and if so, is there any way to generate an internal
> definition using syntax-rules? For my case, I was writing a macro to
> generate SRFI-9 record definitions, but I noticed some of the getters
> and setters which I intended to be private were shadowing each other.
I did not expect this.
To track this:
(define-syntax unhygienic
(syntax-rules ()
((_ the-pair fetch)
(begin
(define the-head (car the-pair))
(define (the-proc) the-head)
(define (fetch) the-head)
(display the-proc)))))
(display the-head)
;; => error Unbound variable: the-head (step out of the debugger with C-d)
(unhygienic '(a) name1)
;; => #<procedure the-proc-1d2f8a1cb6ff0af6 ()> (the proc has a long suffix,
looks good)
(display the-head)
;; => error: Unbound variable: the-head (looks good?)
(name1) ;; => a
(unhygienic '(b) name2)
;; #<procedure the-proc-1d2f8a1cb6ff0af6 ()> (the suffix of the proc is the
same as for the one in a?)
(name1) ;; => b
Maybe this is just, because the procedure is *the same*?
Let’s use fetch in the-proc:
(define-syntax unhygienic
(syntax-rules ()
((_ the-pair fetch)
(begin
(define the-head (car the-pair))
(define (the-proc) fetch)
(define (fetch) the-head)
(display the-proc)))))
(unhygienic '(a) name1)
;; => #<procedure the-proc-7f18b8db60e30e7 ()>
(unhygienic '(b) name2)
;; => #<procedure the-proc-ca383a2334aaa56 ()>
(name1)
;; => b
Now the proc is no longer the same, but the variable still collides.
Auto-completion in the REPL
the-head-<TAB>
;; => the-head-3f6c11022fffe02
the-head also has a suffix, as it should have, but there is only one.
Does the counter go wrong?
One more check:
(define-syntax unhygienic
(syntax-rules ()
((_ the-pair fetch)
(begin
(define the-head fetch)
(define (the-proc) fetch)
(define (fetch) the-head)
(display the-proc)))))
Now using fetch in the-head I get two different variables in the shell.
Maybe detection of external data is missing that the-pair is external?
Best wishes,
Arne
--
Unpolitisch sein
heißt politisch sein,
ohne es zu merken.
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