Re: More Guile 1.8 code failing in 2.x

[Richard Shann <richard.shann@virgin.net>]

On Wed, 2013-02-27 at 08:19 -0500, mark@markwitmer.com wrote:
> Richard Shann <richard.shann@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*))))))
>