Hi Guix,
today on IRC someone reported an ugly error message when reconfiguring
their system:
--8<---------------cut here---------------start------------->8---
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):
--8<---------------cut here---------------end--------------->8---
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.
--
Ricardo
