Re: conditional based on binding of a variable

[Damien Mattei]

i tried this, that works in some case:

;; scheme@(guile-user)> (defined-symbol? d)
;; ;;; <stdin>:20:17: warning: possibly unbound variable `d'
;; $3 = #f
(define-syntax defined-symbol?
  (syntax-rules ()
    ((_ x) (call-with-current-continuation
   (lambda (exit)
     (with-exception-handler
      (lambda (e)
(display "defined-symbol? : undefined") (newline)
(exit #f)) ; eval failed => not defined
      (lambda ()
(eval x (interaction-environment))
#t))))))) ; eval suceeded => defined


;; scheme@(guile-user)> (define r 2)
;; scheme@(guile-user)> (if-defined r 'defined (define r 7))
;; if-defined : where=#t
;; $16 = defined
;; scheme@(guile-user)> (if-defined t3 'defined (define t3 7))
;; if-defined : where=#f
;; scheme@(guile-user)> t3
;; $17 = 7
;; scheme@(guile-user)> (if-defined z (list z) 'not-defined)
;; if-defined : where=#f
;; $18 = not-defined
(define-syntax if-defined
  (lambda (stx)
    (syntax-case stx ()
      ((_ id iftrue iffalse)
       (let ((where (defined-symbol? #'id))) ;;(quote id))))
(display "if-defined : where=") (display where) (newline)
(display "id=") (display #'id) (newline)
(if where #'iftrue #'iffalse))))))

but fais with those examples:

scheme@(guile-user)> (define (foo) (define x 1) (if-defined x 'def
'not_def))
defined-symbol? : undefined
if-defined : where=#f
id=#<syntax:unknown file:37:39 x>
scheme@(guile-user)> (define (bar) (let ((x 1)) (if-defined x 'def
'not_def)))
defined-symbol? : undefined
if-defined : where=#f
id=#<syntax:unknown file:38:39 x>


On Fri, Apr 12, 2024 at 1:40 PM Damien Mattei <damien.mattei@gmail.com>
wrote:

> indeed ,seems not a thing feasible ,unless in Racket where it works great:
>
>
> https://stackoverflow.com/questions/3267351/how-do-i-test-whether-a-variable-is-defined-before-referencing-it?noredirect=1&lq=1
>
> On Fri, Apr 12, 2024 at 12:34 AM Damien Mattei <damien.mattei@gmail.com>
> wrote:
>
>>
>> hello,
>>
>> i'm searching the equivalent in scheme , guile or r6rs of this code for
>> Racket:
>>
>> ; Tests
>> ;; (if-defined z (list z) 'not-defined) ; -> not-defined
>>
>> ;; (if-defined t (void) (define t 5))
>> ;; t ; -> 5
>>
>> ;; (define x 3)
>> ;; (if-defined x (void) (define x 6))
>> ;; x ; -> 3
>> (define-syntax (if-defined stx)
>>   (syntax-case stx ()
>>     [(_ id iftrue iffalse)
>>      (let ([where (identifier-binding #'id)])
>>        (display "if-defined : where=") (display where) (newline)
>>        (display "id=") (display #'id) (newline)(newline)
>>        (if where #'iftrue #'iffalse))]))
>>
>> it is based on identifier-binding
>>
>> i tried defined? in guile but it is not reliable other than toplevel,
>> then i wrote this code based on exception and test the evaluation of the
>> variable in environment:
>>
>> ;; scheme@(guile-user)> (defined-symbol? d)
>> ;; ;;; <stdin>:20:17: warning: possibly unbound variable `d'
>> ;; $3 = #f
>> (define-syntax defined-symbol?
>>   (syntax-rules ()
>>     ((_ x) (call-with-current-continuation
>>    (lambda (exit)
>>      (with-exception-handler
>>       (lambda (e)
>> (display "defined-symbol? : undefined") (newline)
>> (exit #f)) ; eval failed => not defined
>>       (lambda ()
>> (eval x (interaction-environment))
>> #t))))))) ; eval suceeded => defined
>>
>> used in conjunction with:
>>
>> ;; scheme@(guile-user)> (define r 2)
>> ;; scheme@(guile-user)> (if-defined r 'defined (define r 7))
>> ;; if-defined : where=#t
>> ;; $16 = defined
>> ;; scheme@(guile-user)> (if-defined t3 'defined (define t3 7))
>> ;; if-defined : where=#f
>> ;; scheme@(guile-user)> t3
>> ;; $17 = 7
>> ;; scheme@(guile-user)> (if-defined z (list z) 'not-defined)
>> ;; if-defined : where=#f
>> ;; $18 = not-defined
>> (define-syntax if-defined
>>   (lambda (stx)
>>     (syntax-case stx ()
>>       ((_ id iftrue iffalse)
>>        (let ((where (defined-symbol? #'id))) ;;(quote id))))
>> (display "if-defined : where=") (display where) (newline)
>> (display "id=") (display #'id) (newline)
>> (if where #'iftrue #'iffalse))))))
>>
>> but in some case it fails again
>>
>> the goal is to be able to use this macro:
>>
>> (define-syntax <-
>>
>>   (lambda (stx)
>>
>>     (syntax-case stx ()
>>
>>       ((_ var expr)
>>
>>        #`(if-defined var
>>     (set! var expr)
>>     (define var expr))))))
>>
>> that define a variable when not defined or just set! it
>>
>> it then fails in some case:
>>
>> (let () (define k 0) (let loop () (if (< k 4) (let () (display k)
>> (newline) (<- k (+ k 1)) (loop)))))
>> defined-symbol? : undefined
>> if-defined : where=#f
>> id=#<syntax:unknown file:22:80 k>
>> #<unspecified>
>> ice-9/boot-9.scm:1685:16: In procedure raise-exception:
>> In procedure +: Wrong type argument in position 1: #<unspecified>
>>
>> Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
>>
>> but works in this case:
>>
>> scheme@(guile-user)> (define k 0)
>> scheme@(guile-user)> (let loop () (if (< k 4) (let () (display k)
>> (newline) (<- k (+ k 1)) (loop))))
>> if-defined : where=#t
>> id=#<syntax:unknown file:25:59 k>
>> 0
>> 1
>> 2
>> 3
>>
>> i know it is hard and perheaps not possible but if someone has any idea?
>>
>> Damien
>>
>>