[bug#49671] guix: records: Improve error reporting.

Hi Guix!

This patch improves error reporting a bit when making mistakes in guix
records. This is motivated by a user getting "invalid field specifier"
for their whole services field in their os record. With this patches,
they would have seen:

multiple values in field specifier. Got 2 values associated with key
services. Values are:
(append (list (service ...) ...))
(modify-services %desktop-services ...)

Which would have hinted them at fixing the parenthesis. Or at least, it
would have saved us some time trying to count them :)

Here are the cases that are handled and the associated message:

(operating-system
  services)
guix system: error: services: invalid field specifier. The format of a
field is `(services value)'

(operating-system
  (services))
test.scm:2:2: error: (services): Value missing in field specifier. The
format of a field is `(services value)'.

(operating-system
  (services 1 2 3))
test.scm:2:2: error: (services 1 2 3): multiple values in field
specifier. Got 3 values associated with key services. Values are:
1
2
3

(operating-system
  ())
guix system: error: (): invalid field specifier. The format of a field
is `(field value)'

(operating-system
  ((services %desktop-services)))
test.scm:2:2: error: ((services %desktop-services)): invalid field
specifier. (services %desktop-services) is not a valid field name.


Of course, we can improve these error messages, and internationalize
them.

WDYT?

Hi Julien,

Julien Lepiller <julien@lepiller.eu> writes:

> Hi Guix!
>
> This patch improves error reporting a bit when making mistakes in guix
> records. This is motivated by a user getting "invalid field specifier"
> for their whole services field in their os record. With this patches,
> they would have seen:

After applying your patch, I get:

--8<---------------cut here---------------start------------->8---
guix/records.scm:108:19: warning: "multiple values in field specifier. Got ~a values associated with key ~a. Values are:~%~{~a~%~}": unsupported format option ~{, use (ice-9 format) instead
--8<---------------cut here---------------end--------------->8---

After adding `(ice-9 format)` to imports it works as expected. I see
this also applies to package records! This will be great for those
starting to package in Guix.

>
> multiple values in field specifier. Got 2 values associated with key
> services. Values are:
> (append (list (service ...) ...))
> (modify-services %desktop-services ...)
>
> Which would have hinted them at fixing the parenthesis. Or at least, it
> would have saved us some time trying to count them :)
>
> Here are the cases that are handled and the associated message:
>
> (operating-system
>   services)
> guix system: error: services: invalid field specifier. The format of a
> field is `(services value)'
>
> (operating-system
>   (services))
> test.scm:2:2: error: (services): Value missing in field specifier. The
> format of a field is `(services value)'.
>
> (operating-system
>   (services 1 2 3))
> test.scm:2:2: error: (services 1 2 3): multiple values in field
> specifier. Got 3 values associated with key services. Values are:
                                              ^ Wrap in `'?
> 1
> 2
> 3
>
> (operating-system
>   ())
> guix system: error: (): invalid field specifier. The format of a field
> is `(field value)'
>
> (operating-system
>   ((services %desktop-services)))
> test.scm:2:2: error: ((services %desktop-services)): invalid field
> specifier. (services %desktop-services) is not a valid field name.
             ^ Should this also be wrapped in `'?

Why do some of these messages lose their context and come from `guix
system` instead?

>
> Of course, we can improve these error messages, and internationalize
> them.
>
> WDYT?

[...]

> -         (apply syntax-violation name "invalid field specifier"
> -                (if parent-form
> +         (syntax-case #'weird ()
> +           (()                             ;the empty list
> +             (apply syntax-violation name
> +                    "invalid field specifier. The format of a field is `(field value)'"
>                      (list parent-form #'weird)
> -                    (list #'weird)))))))
> +                    (list #'weird)))

Why the extra `(list #'weird')`? AFAICT right now this is providing
`(list parent-form #:'weird)` as the parent form.  And since parent-form
is optional, shouldn't this be

--8<---------------cut here---------------start------------->8---
  (apply syntax-violation name
         "invalid field specifier. The format of a field is `(field value)'"
         (if parent-form (list parent-form #:'weird) (list weird)))
--8<---------------cut here---------------end--------------->8---

(and similar for the others)?

--
Sarah

Hello!

Julien Lepiller <julien@lepiller.eu> skribis:

> Here are the cases that are handled and the associated message:
>
> (operating-system
>   services)
> guix system: error: services: invalid field specifier. The format of a
> field is `(services value)'
>
> (operating-system
>   (services))
> test.scm:2:2: error: (services): Value missing in field specifier. The
> format of a field is `(services value)'.
>
> (operating-system
>   (services 1 2 3))
> test.scm:2:2: error: (services 1 2 3): multiple values in field
> specifier. Got 3 values associated with key services. Values are:
> 1
> 2
> 3
>
> (operating-system
>   ())
> guix system: error: (): invalid field specifier. The format of a field
> is `(field value)'
>
> (operating-system
>   ((services %desktop-services)))
> test.scm:2:2: error: ((services %desktop-services)): invalid field
> specifier. (services %desktop-services) is not a valid field name.
>
>
> Of course, we can improve these error messages, and internationalize
> them.

I like the idea!

I would prefer for error messages to be plain errors, without hints, so:

  test.scm:2:2: error: (services 1 2 3): multiple values in field specifier

(Not a sentence, no period.)

But I’d also like to have hints, and ideally all hints should be
reported consistently, via ‘&fix-hint’ or similar.

Thankfully, we can do that via (guix diagnostics) + SRFI-35/34:

  (raise (condition
           (&syntax (form 'x) (subform 'y))
           (&fix-hint (hint "Consider fixing this."))))

‘call-with-error-handling’ in (guix ui) might need to be adjusted.

The only downside is that (guix records) would now depend on another
Guix modules, which I tried to avoid so far so it could be more easily
reused elsewhere.  But that’s the price to pay to get consistent error
reports, and I think it’s okay.

Note that ‘tests/guix-system.sh’ and a couple of other files test exact
error reports, so we’ll have to make sure it still works and possibly
augment the tests.

WDYT?

Thanks,
Ludo’.