better error messages through assertions

[Ricardo Wurmus]

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