On Wed, 2013-02-27 at 08:19 -0500, m...@markwitmer.com wrote:
> Richard Shann <richard.sh...@virgin.net> writes:
>
> > We have one more construct being refused as we upgrade to guile 2.0 in
> > GNU/Denemo
> >
> > (if (not (defined? 'ToggleTripleting::InsideTriplet))
> > (define ToggleTripleting::InsideTriplet #t))
> >
> > It is intended to set up a flag which toggles between true and false on
> > each call. If already set up, the flag is not altered.
> >
> > Apparently (I haven't been able to check) Guile 2.0 gives an error.
> >
> > #f definition in expression context, where definitions are not
> > allowed, ((line . 2) (column . 4) (filename . #f)) (define
> > ToggleTripleting::InsideTriplet #t) #f)
> >
> > Can anyone suggest what could replace that? I would prefer to to replace
> > it with something that has the same semantics, rather than setting up
> > some alternative method of handling the situation - registering all such
> > variables on program startup, or some such - as at the moment this code
> > only evaluated if used and doesn't require any mechanism to get
> > initialized, other than this construct.
> >
> > Richard Shann
> >
> >
> >
> >
>
> Hi Richard,
>
> You can probably just replace your code with
>
> (define-once ToggleTripleting::InsideTriplet #t)
Thanks for this - it seems define-once is not defined in guile 1.8
however, so while some systems only have guile 2.0 (Fedora) and others
only have guile 1.8 (Debian stable) I would need something more.
I wonder if I can test if define-once is defined and if not evaluate a
string which has the dis-allowed syntax in it?
(if (defined? 'define-once)
(define-once ToggleTripleting::InsideTriplet #t)
(eval-string "(if (not (defined? 'ToggleTripleting::InsideTriplet))
(define ToggleTripleting::InsideTriplet #t))"))
or will the guile 2.0 compiler try and compile the string being passed to
eval-string?
Richard
>
> (define-once ...) works like defvar in Common Lisp and will not
> redefine a variable once it has been defined. You can read about it at
> the bottom of the node "Top Level Variable Definitions" in the Guile
> manual.
>
> I'm still learning the ins and outs of procedural macros, but I think
> that if you need to include (define ...) in a conditional like that for
> more complicated purposes, you can use syntax-case like this:
>
> (define-syntax conditional-define
> (lambda (x)
> (syntax-case x ()
> ((conditional-define arg val)
> (let ((arg-datum (syntax->datum #'arg)))
> (if (not (defined? arg-datum))
> #'(define arg val)
> #'*unspecified*))))))
>
