* conditional based on binding of a variable
@ 2024-04-11 22:34 Damien Mattei
2024-04-12 11:40 ` Damien Mattei
0 siblings, 1 reply; 5+ messages in thread
From: Damien Mattei @ 2024-04-11 22:34 UTC (permalink / raw)
To: guile-user
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
^ permalink raw reply [flat|nested] 5+ messages in thread* Re: conditional based on binding of a variable
2024-04-11 22:34 conditional based on binding of a variable Damien Mattei
@ 2024-04-12 11:40 ` Damien Mattei
2024-04-14 7:24 ` Damien Mattei
0 siblings, 1 reply; 5+ messages in thread
From: Damien Mattei @ 2024-04-12 11:40 UTC (permalink / raw)
To: guile-user
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
>
>
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: conditional based on binding of a variable
2024-04-12 11:40 ` Damien Mattei
@ 2024-04-14 7:24 ` Damien Mattei
2024-04-16 6:36 ` Damien Mattei
0 siblings, 1 reply; 5+ messages in thread
From: Damien Mattei @ 2024-04-14 7:24 UTC (permalink / raw)
To: guile-user
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
>>
>>
^ permalink raw reply [flat|nested] 5+ messages in thread* Re: conditional based on binding of a variable
2024-04-14 7:24 ` Damien Mattei
@ 2024-04-16 6:36 ` Damien Mattei
2024-04-16 6:41 ` Damien Mattei
0 siblings, 1 reply; 5+ messages in thread
From: Damien Mattei @ 2024-04-16 6:36 UTC (permalink / raw)
To: guile-user
this could not works because the defined-symbol? used in the macro
if-defined return false because at the expansion of the macro the symbol is
not defined.
Instead at run-time, this works:
#|kawa:21|# (define (foo2) (define x 7) (defined-symbol? x))
#|kawa:22|# (foo2)
#t
On Sun, Apr 14, 2024 at 9:24 AM Damien Mattei <damien.mattei@gmail.com>
wrote:
> 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
>>>
>>>
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: conditional based on binding of a variable
2024-04-16 6:36 ` Damien Mattei
@ 2024-04-16 6:41 ` Damien Mattei
0 siblings, 0 replies; 5+ messages in thread
From: Damien Mattei @ 2024-04-16 6:41 UTC (permalink / raw)
To: guile-user
scheme@(guile-user)> (define (foo2) (define x 7) (defined-symbol? x))
scheme@(guile-user)> (foo2)
$2 = #t
On Tue, Apr 16, 2024 at 8:36 AM Damien Mattei <damien.mattei@gmail.com>
wrote:
> this could not works because the defined-symbol? used in the macro
> if-defined return false because at the expansion of the macro the symbol is
> not defined.
>
> Instead at run-time, this works:
>
> #|kawa:21|# (define (foo2) (define x 7) (defined-symbol? x))
> #|kawa:22|# (foo2)
> #t
>
> On Sun, Apr 14, 2024 at 9:24 AM Damien Mattei <damien.mattei@gmail.com>
> wrote:
>
>> 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
>>>>
>>>>
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2024-04-16 6:41 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2024-04-11 22:34 conditional based on binding of a variable Damien Mattei
2024-04-12 11:40 ` Damien Mattei
2024-04-14 7:24 ` Damien Mattei
2024-04-16 6:36 ` Damien Mattei
2024-04-16 6:41 ` Damien Mattei
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