Hey thanks for the review :)
On Fri 15 Nov 2019 10:03, Ludovic Courtès writes:
> 0. Do I get it right that ‘throw’ and ‘catch’ are not “deprecated” in
> the sense of (ice-9 deprecated) and are instead simply not the
> “preferred” exception mechanism?
Correct. I think we could envision deprecating them in some future but
not within the next couple years at least.
> 1. I see things like:
>
> +(define (make-condition type . field+value)
> + "Return a new condition of type TYPE with fields initialized as specified
> +by FIELD+VALUE, a sequence of field names (symbols) and values."
> + (unless (exception-type? type)
> +(scm-error 'wrong-type-arg "make-condition" "Not a condition type: ~S"
> + (list type) #f))
>
> and:
>
> + (unless (symbol? key)
> +(throw 'wrong-type-arg "throw" "Wrong type argument in position ~a:
> ~a"
> + (list 1 key) (list key)))
>
> I guess we could add a specific ‘&type-exception’ exception or similar,
> which would allow us to improve error reporting (that can come later, of
> course.)
Yes. So right now Guile is in a bit of a transitional state -- it still
signals 99.9% of errors via `throw'. Probably we want to change to have
structured exceptions for almost all of these. To preserve
compatibility we would probably need to mix in an
&exception-with-kind-and-args to all of these exceptions, or otherwise
augment `exception-kind' and `exception-args' to synthesize these values
when appropriate.
> Guix has ‘&location’ error conditions, which I’ve found useful when
> combined with other error conditions in cases where location info from
> the stack isn’t useful:
>
> https://git.savannah.gnu.org/cgit/guix.git/tree/guix/utils.scm#n832
>
> I wonder if (ice-9 exceptions) should provide something like that.
Neat :) Yes sure. I think, any exception type can be added to (ice-9
exceptions) -- there's little "name cost" like there is in boot-9.
Which reminds me, I want to make boot-9 do a (resolve-module '(ice-9
exceptions)) so that the more capable make-exception-from-throw always
gets installed.
> 2. What are you thoughts regarding exposing structured exceptions to C?
> I’ve always been frustrated by ‘system-error’ :-). Guix has a hack to
> augment ‘system-error’ with information about the offending file name:
>
> https://git.savannah.gnu.org/cgit/guix.git/tree/guix/ui.scm#n520
>
> If the POSIX bindings would emit a structured ‘&system-error’ record,
> that’d be pretty cool.
I don't know :) Right now raise-exception is marked SCM_INTERNAL.
Probably it should be public. There is no public C API for any of this
new functionality, as it stands; a TODO.
Regarding exception objects, there are two questions: one, how to create
exceptions of specific kinds; I suspect scm_make_system_error () would
be fine, and probably you want scm_make_exception to be able to mix
various exceptions. Second question, do we want to expose accessors
too? It can be a lot of API surface and I am a bit wary of it. But,
perhaps it is the right thing. I do not know.
> 3. I wonder if we could take advantage of the new ‘&message’ exception
> to start i18n of error messages. It might be as simple as telling
> xgettext to recognize ‘make-exception-with-message’ as a keyword, though
> currently there are few calls to ‘make-exception-with-message’ followed
> by a literal.
Eventually this will get called by `error', I think; Guile's R7RS layer
in wip-r7rs defines `error' as being:
(define (error message . irritants)
(raise-exception
(let ((exn (make-exception-with-message message)))
(if (null? irritants)
exn
(make-exception exn
(make-exception-with-irritants irritants))
But yes this is definitely something to think about it.
> 4. Is ‘&warning’ actually used? Is the goal to make it continuable?
> That sounds great.
Any exception can be raised in a continuable way. Whether a raise is
continuable or not depends on the value of the #:continuable? keyword to
raise-exception. I think that's the intention of &warning but I don't
really have instincts about how it might be used. Guile defines it
because it's in R6RS, but how it will be used is an open question :)
> Bah, you give us a present and I reply with an additional wishlist.
> ;-)
:) I hope that the exceptions work can serve as a foundation for
further incremental, compatible improvement.
Cheers,
Andy
