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From: Maxime Devos <maximedevos@telenet.be>
To: Damien Mattei <damien.mattei@gmail.com>,
	Matt Wette <matt.wette@gmail.com>
Cc: guile-user <guile-user@gnu.org>
Subject: Re: Fwd: new function
Date: Tue, 21 Sep 2021 21:03:27 +0200	[thread overview]
Message-ID: <52130eefb713942a36aa5717ef4c916173567ce3.camel@telenet.be> (raw)
In-Reply-To: <CADEOadcb84OQxMRBPT6Bp9ejSB6MV6vs-wPs-Lqyvz10nhDRBg@mail.gmail.com>

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Damien Mattei schreef op di 21-09-2021 om 15:04 [+0200]:
> i have tested define-once
> http://www.gnu.org/software/guile/docs/master/guile.html/Top-Level.html
> (the defvar of Lisp)and idea are:
> -unfortunately it is considered by scheme as a define,so there is some
> context where it is not allowed in my code
> -seems to work fine at toplevel (as mentioned in doc) but strange behavior
> in a function, i did not understand really what happened but i got some
> #unspecified value.
> 
> here are my test code:
> cheme@(guile-user)> (define (foo2)
>   (define-once  x 1)
>   (if #t
>       (let ()
>         (define-once x 2)
>         ;;(set! x 2)
>         (display "x=")
>         (display x)
>         (newline))
>       'never)
>   (display x)
>   (newline))

Possibly you want (added a set? argument for demonstration):

(define (foo2 set?)
  (define x) ; define an (undefined or unbound, not sure about terminology) variable
  (if set?
      (let ()
        (set! x 2) ; change the value of x
        (display "x=")
        (display x)
        (newline))
      'never)
  (display x)
  (newline))

That should be portable and avoids global state.

scheme@(guile-user)> x
;;; <stdin>:20:0: warning: possibly unbound variable `x'
ice-9/boot-9.scm:1685:16: In procedure raise-exception:
Unbound variable: x
scheme@(guile-user) [1]> ,q

scheme@(guile-user) [1]> (foo2 #f)
#<unspecified> ; I expected an error as would result from ...

;; ... this ...
scheme@(guile-user)> (variable-ref (make-undefined-variable))
ice-9/boot-9.scm:1685:16: In procedure raise-exception:
In procedure variable-ref: Unbound variable: #<variable 7f6de46cde80 value: #<undefined>>

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [2]> 

;; instead  of #<unpecified> but whatever ...

scheme@(guile-user) [1]> (foo2 #t)
x=2
2
scheme@(guile-user) [1]> (foo2 #t)
x=2
2


scheme@(guile-user) [1]> (define x 3)
scheme@(guile-user) [1]> (foo2 #t)
x=2 ; foo2 doesn't use the global variable 'x'
2

scheme@(guile-user) [1]> x
$1 = 3

Does this seem reasonable to you?

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  reply	other threads:[~2021-09-21 19:03 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CADEOadfF+rR=fW0fS-PkQkDN8EHFB1sJnkeePMJyc23m9shLgA@mail.gmail.com>
2021-09-19  9:18 ` Fwd: new function Damien Mattei
2021-09-19 10:38   ` Maxime Devos
2021-09-19 14:45     ` Matt Wette
2021-09-19 14:56       ` Maxime Devos
2021-09-21 13:04       ` Damien Mattei
2021-09-21 19:03         ` Maxime Devos [this message]
2021-09-22  7:52           ` Damien Mattei
2021-09-22 18:03             ` Maxime Devos
2021-09-22 19:12           ` Taylan Kammer

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