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Re: better error messages through assertions
From: |
Maxim Cournoyer |
Subject: |
Re: better error messages through assertions |
Date: |
Fri, 25 Feb 2022 13:55:46 -0500 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) |
Hello Ricardo,
Ricardo Wurmus <rekado@elephly.net> writes:
> Hi Guix,
>
> today on IRC someone reported an ugly error message when reconfiguring
> their system:
>
> Backtrace:
> 18 (primitive-load "/home/me/.config/guix/current/bin/…")
> In guix/ui.scm:
> 2209:7 17 (run-guix . _)
> 2172:10 16 (run-guix-command _ . _)
> In ice-9/boot-9.scm:
> 1752:10 15 (with-exception-handler _ _ #:unwind? _ # _)
> In guix/status.scm:
> 822:3 14 (_)
> 802:4 13 (call-with-status-report _ _)
> In guix/scripts/system.scm:
> 1256:4 12 (_)
> In ice-9/boot-9.scm:
> 1752:10 11 (with-exception-handler _ _ #:unwind? _ # _)
> In guix/store.scm:
> 658:37 10 (thunk)
> 1320:8 9 (call-with-build-handler #<procedure 7fecaf8570c0 at g…> …)
> 2123:24 8 (run-with-store #<store-connection 256.99 7fecb75c7230> …)
> In guix/scripts/system.scm:
> 827:2 7 (_ _)
> 703:7 6 (_ #<store-connection 256.99 7fecb75c7230>)
> In gnu/system.scm:
> 1227:19 5 (operating-system-derivation _)
> In gnu/services.scm:
> 1091:6 4 (instantiate-missing-services _)
> In srfi/srfi-1.scm:
> 460:18 3 (fold #<procedure 7fecb73c0960 at gnu/services.scm:109…> …)
> In gnu/services.scm:
> 1092:27 2 (_ (#<<service> type: #<service-type gdm 7fecbd17f6…> …) …)
> In ice-9/boot-9.scm:
> 1685:16 1 (raise-exception _ #:continuable? _)
> 1685:16 0 (raise-exception _ #:continuable? _)
>
> ice-9/boot-9.scm:1685:16: In procedure raise-exception:
> In procedure struct-vtable: Wrong type argument in position 1 (expecting
> struct):
>
> As you can probably tell easily by looking at this message, the
> “service” field of the operating system configuration looked something
> like this:
>
> (services (append (list a b c %desktop-services) #;oops))
>
> instead of this
>
> (services (append (list a b c) %desktop-services))
>
> This is because INSTANTIATE-MISSING-SERVICES — and FOLD-SERVICES, and
> many more — assumes that it is only passed a plain list of services. It
> then proceeds to call SERVICE-KIND on what may or may not be a service.
>
> I think we should add simple type checks, something like this:
>
> (define (listof pred)
> (lambda (thing)
> (and (list? thing) (every pred thing))))
> …
> (define (assert-type type-check thing message)
> (or (false-if-exception (type-check thing))
> (report-error (G_ "type error: …\n" message))))
>
> ;; Use ASSERT-TYPE in an example procedure.
> (define (do-something-with-services services)
> (assert-type (listof service?) services
> "SERVICES must be a list of <service> values.")
>
> ;; Do things…
> (map service-kind services))
>
> What do you think? There are many different ways of implementing this
> (a new variant of DEFINE that also accepts a type declaration, an assert
> like above, a fancier assert that composes a helpful error message by
> itself, a separate type declaration that is looked up only when the
> corresponding procedure is called in a certain context, etc), but I’d
> first like to know if there is consensus that we want something like
> this.
I hear we now have "field sanitizers" on Guix records; without having
dug the details, it seems to be we could add a predicate validating the
input there? The nice thing about it is that it'd be a one place
change, instead of asserts to sprinkle around various places.
Thanks,
Maxim