Re: More Guile 1.8 code failing in 2.x

[Andy Wingo <wingo@pobox.com>]

Hi Richard,

On Wed 27 Feb 2013 15:22, Richard Shann <richard.shann@virgin.net> writes:

> On Wed, 2013-02-27 at 08:19 -0500, mark@markwitmer.com wrote:
>> Richard Shann <richard.shann@virgin.net> writes:
>> 
>> > (if (not (defined? 'ToggleTripleting::InsideTriplet))
>> >     (define ToggleTripleting::InsideTriplet #t))

I'm sorry we were not able to make this case work in Guile 2; it's quite
complicated actually.

>> 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

You could define it in a prelude somewhere:

  (cond-expand
   (guile-2) ; nothing
   (else ; guile < 2.0
    (define-macro (define-once sym exp)
      `(if (not (defined? ',sym)) (define ,sym ,exp)))))

Then you could change all uses of this idiom to use define-once.

Note that Guile 2.'s `define-once' is slightly different; it always
declares a local definition, and it uses the existing value only if
there is already a local definition.  In this way imported values get
overridden by define-once.

Here is a Guile 2-compatible version:

  (cond-expand
   (guile-2) ; nothing
   (else ; guile < 2.0
    (define-macro (define-once sym exp)
      `(define ,sym (if (module-locally-bound? ',sym) ,sym ,val)))))

Finally in Guile 2 there is also the `define!' procedure, which makes
top-level definitions programmatically.  Use it if you need it.  But if
you can, better to rewrite to use standard Scheme.

In general I recommend adding shims to Denemo to make it
forward-compatible -- like adding define-once if needed so that it uses
the Guile 2.0 idioms, even on 1.8.

Good luck, and let us know how it goes.

Andy
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