Re: [patch] variable datums with syncase transformer

[Neil Jerram <neil@ossau.uklinux.net>]

Hi Stephen,

Following Ludovic's request for me to review this...

Stephen Compall <s11@member.fsf.org> writes:

> +;; The question syncase is asking when it asks `self-evaluating?' is
> +;; not really whether a given datum is self-evaluating, but whether it
> +;; should pass the datum through as-is to the evaluator.  With that in
> +;; mind, we wrap core `self-evaluating?' here so that syncase will
> +;; pass-through variables, meaning (variable? datum) => #t, to the
> +;; evaluator.  Without this, the system transformer will see a
> +;; variable datum as invalid syntax, which it is not to the core
> +;; transformer.
> +(define (self-evaluating? datum)
> +  (or ((@ (guile) self-evaluating?) datum)
> +      (variable? datum)))
> +

Given that self-evaluating? has no other purpose, I think it would be
preferable just to add "case scm_tc7_variable:" to the C code, along
with your excellent comment above.

I know that would technically be an incompatible change, but given
that I never heard mention of self-evaluating? before, I think we can
risk it.

(And someone can always write

 (define (compatible-self-evaluating? obj)
   (and (self-evaluating? obj)
        (not (variable? obj))))

if they need to.)

>  ;;; Load the preprocessed code
>  
>  (let ((old-debug #f)
> Index: test-suite/tests/syncase.test
> ===================================================================
> RCS file: /sources/guile/guile/guile-core/test-suite/tests/syncase.test,v
> retrieving revision 1.5
> diff -u -d -u -r1.5 syncase.test
> --- test-suite/tests/syncase.test	16 Apr 2006 23:27:14 -0000	1.5
> +++ test-suite/tests/syncase.test	14 Dec 2007 07:55:49 -0000
> @@ -34,3 +34,15 @@
>  
>  (pass-if "basic syncase macro"
>    (= (plus 1 2 3) (+ 1 2 3)))
> +
> +(pass-if "variable?s recognized as datums"
> +  (false-if-exception
> +   (begin (eq? car (eval '(@ (guile) car) (current-module)))
> +	  #t)))
> +
> +(define-syntax export-safe-plus
> +  (syntax-rules ()
> +    ((_ x ...) ((@ (guile) +) x ...))))
> +
> +(pass-if "variable?s passed through to evaluator"
> +  (= (export-safe-plus 1 2 3) (+ 1 2 3)))

These tests are fine (of course!).

Regards,
    Neil