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.mat...@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 > >
