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Re: [patch] variable datums with syncase transformer

From: Neil Jerram
Subject: Re: [patch] variable datums with syncase transformer
Date: Wed, 05 Mar 2008 20:55:33 +0000
User-agent: Gnus/5.110006 (No Gnus v0.6) Emacs/21.4 (gnu/linux)

address@hidden (Ludovic Courtès) writes:

> Note that `@' and `@@' rely on an interesting property of the evaluator:
> you type `(@ (guile) car)', `@' returns a *variable*, but what you get
> is a *procedure*, because in the meantime, the evaluator automagically
> performed a `variable-ref'.  This is not very elegant in my opinion.

However, I think it means that if the variable is then changed, the
code that included the (@ ...) expression will get the new value next
time it is executed.

This means that `(@ module identifier)' acts like a plain old
`identifier', in the sense that it gives you a location - whose value
may change over time - rather than the fixed value of the identifier
at the time the code was first executed.

I think that's a feature, and it's probable that someone somewhere is
relying on it.

> With that in mind, I propose instead the following patch, which also
> fixes your problem AFAICS:
> --- /home/ludo/src/guile/1.8/guile-core/ice-9/boot-9.scm.~1.356.2.10.~        
> 2007-09-01 19:11:00.000000000 +0200
> +++ /home/ludo/src/guile/1.8/guile-core/ice-9/boot-9.scm      2008-03-05 
> 17:25:15.000000000 +0100
> @@ -2988,7 +2988,7 @@
>    (let ((var (module-variable (resolve-interface mod-name) var-name)))
>      (if (not var)
>       (error "no such public variable" (list '@ mod-name var-name)))
> -    var))
> +    (variable-ref var)))
>  ;; The '@@' macro is like '@' but it can also access bindings that
>  ;; have not been explicitely exported.
> @@ -2997,7 +2997,7 @@
>    (let ((var (module-variable (resolve-module mod-name) var-name)))
>      (if (not var)
>       (error "no such variable" (list '@@ mod-name var-name)))
> -    var))
> +    (variable-ref var)))

If I've understood it correctly, I think this patch would change
existing behaviour, by returning a fixed value rather than a location.

Could we not look at fixing (ice-9 syncase) instead, so that it
understands variables?


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