Re: how to know if a variable is defined

[Damien Mattei <damien.mattei@gmail.com>]

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hello,

i ported my code from guile that use ,as you say it 'exceptions':

;; #|kawa:57|#
;; #|kawa:58|# (define r 2)
;; #|kawa:59|# (if-defined r 'defined (define r 7))
;;if-defined : where=#t
;; id=r
;; (if-defined r 'defined (define r 7))
;; defined
;; #|kawa:60|# r
;; 2
;; #|kawa:61|# (defined-symbol? r)
;; #t
;; #|kawa:62|# (defined-symbol? t)
;; /dev/tty:62:18: warning - no declaration seen for t
;; defined-symbol? : undefined
;; #f
;; #|kawa:63|# (let ((k 7)) (defined-symbol? k))
;; #t
;;

(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


;; #|kawa:86|# (define k 0)
;; #|kawa:87|# (let loop () (if (< k 4) (let () (display k) (newline) (<- k
(+ k 1)) (loop))))
;; if-defined : where=#t
;; id=k
;; 0
;; 1
;; 2
;; 3
;; #|kawa:88|# (let () (define k 0) (let loop () (if (< k 4) (let ()
(display k) (newline) (<- k (+ k 1)) (loop)))))
;; if-defined : where=#t
;; id=k
;; 0
;; 1
;; 2
;; 3
;; #|kawa:89|# (let () (define s 0) (let loop () (if (< s 4) (let ()
(display s) (newline) (<- s (+ s 1)) (loop)))))
;; if-defined : where=#t
;; id=s
;; 0
;; 1
;; 2
;; 3
;; #|kawa:90|# (let ((s 0)) (let loop () (if (< s 4) (let () (display s)
(newline) (<- s (+ s 1)) (loop)))))
;; if-defined : where=#t
;; id=s
;; 0
;; 1
;; 2
;; 3

(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))))))

i use the code in conjunction with this macro:
(define-syntax <-
   (lambda (stx)

   (syntax-case stx ()

        #`(if-defined var
     (set! var expr)
    (define var expr))))))

it works better than guile (guile fails on the let () tests...:
scheme@(guile-user)> (let ((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:19:72 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.

), as you can see the examples , but in some place of my code kawa fails
too, i made a simplified example from the whole code:

for example if i put all the stuff in a module test-defined.scm:

(module-name test-defined)

(export foo)



(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

(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))))))



(define-syntax <-
   (lambda (stx)

     (syntax-case stx ()

       ((_ var expr)

        #`(if-defined var
     (set! var expr)
     (define var expr))))))




(define (foo)
  (define expr2 '())

  (<- expr2 7))

then it fails:

#|kawa:1|# (require test-defined)defined-symbol? : undefined
if-defined : where=#f
id=expr2
(require test-defined)
/Users/mattei/Scheme-PLUS-for-Kawa/test-defined.scm:95:7: duplicate
declaration of 'expr2'
/Users/mattei/Scheme-PLUS-for-Kawa/test-defined.scm:93:11: (this is the
previous declaration of 'expr2')

it finds that expr2 is not defined,then expand a (define expr2 ...) but
then cause an error of duplicate declaration of expr2


On Thu, Apr 11, 2024 at 3:01 AM Per Bothner <per@bothner.com> wrote:

>
>
> On 4/10/24 14:49, Damien Mattei via Kawa wrote:
> > hello,
> >
> > is there a way to know if a variable is defined in Kawa?
> >
> > (like 'identifier-binding' in Racket)
>
> If you need to know if a variable is bound in the dynamic environment,
> you could perhaps use eval wrapped in an exception handler.
>
> There are probably better ways, but I don't remember off-hand.
>
> Can't think of anything to use if you need information about lexical
> bindings.
> I supposed you could implemented identifier-binding or something similar.
>
> --
>         --Per Bothner
> per@bothner.com   http://per.bothner.com/
>