[PATCH] guix: records: Improve error reporting.

OpenSubmitted by Julien Lepiller.
Details
3 participants
  • Sarah Morgensen
  • Julien Lepiller
  • Ludovic Courtès
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unassigned
Severity
normal
J
J
Julien Lepiller wrote on 21 Jul 2021 01:40
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(address . guix-patches@gnu.org)
20210721014047.3878a0c7@tachikoma.lepiller.eu
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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?
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From 4cde5555682ca6a3c78cb274cc308087e51ea0e9 Mon Sep 17 00:00:00 2001
From: Julien Lepiller <julien@lepiller.eu>
Date: Wed, 21 Jul 2021 01:30:04 +0200
Subject: [PATCH] guix: records: Improve error reporting.

* guix/records.scm (report-invalid-field-specifier): Handle various
invalidity causes separately.
---
guix/records.scm | 37 ++++++++++++++++++++++++++++++++++---
1 file changed, 34 insertions(+), 3 deletions(-)

Toggle diff (58 lines)
diff --git a/guix/records.scm b/guix/records.scm
index 3d54a51956..3c3dd478a5 100644
--- a/guix/records.scm
+++ b/guix/records.scm
@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2018 Mark H Weaver <mhw@netris.org>
+;;; Copyright © 2021 Julien Lepiller <julien@lepiller.eu>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -83,10 +84,40 @@ error-reporting purposes."
          ;; WEIRD may be an identifier, thus lacking source location info, and
          ;; BINDINGS is a list, also lacking source location info.  Hopefully
          ;; PARENT-FORM provides source location info.
-         (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)))
+           (((field ...) _ ...)            ;a list whose first element is a list
+            (apply syntax-violation name
+                   (format #f "invalid field specifier. ~a is not a valid field name."
+                           (map syntax->datum #'(field ...)))
+                   (list parent-form #'weird)
+                   (list #'weird)))
+           ((field)                        ;a list with one element
+            (apply syntax-violation name
+                   (format #f "Value missing in field specifier. The format of \
+a field is `(~a value)'."
+                           (syntax->datum #'field))
+                   (list parent-form #'weird)
+                   (list #'weird)))
+           ((field value ...)              ;any other list
+            (apply syntax-violation name
+                   (format #f "multiple values in field specifier. Got ~a values \
+associated with key ~a. Values are:~%~{~a~%~}"
+                           (length #'(value ...)) (syntax->datum #'field)
+                           (map syntax->datum #'(value ...)))
+                    (list parent-form #'weird)
+                    (list #'weird)))
+           (field                          ;not a list
+             (apply syntax-violation name
+                    (format #f "invalid field specifier. The format of a field \
+is `(~a value)'"
+                            (syntax->datum #'field))
+                    (list parent-form #'weird)
+                    (list #'weird))))))))
 
   (define (report-duplicate-field-specifier name ctor)
     "Report the first duplicate identifier among the bindings in CTOR."
-- 
2.32.0
S
S
Sarah Morgensen wrote on 21 Jul 2021 21:21
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(name . Julien Lepiller)(address . julien@lepiller.eu)(address . 49671@debbugs.gnu.org)
86zgufscb0.fsf@mgsn.dev
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Hi Julien,

Julien Lepiller <julien@lepiller.eu> writes:

Toggle quote (7 lines)
> 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:

Toggle snippet (3 lines)
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

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.

Toggle quote (25 lines)
>
> 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 `'?
Toggle quote (13 lines)
> 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?

Toggle quote (6 lines)
>
> Of course, we can improve these error messages, and internationalize
> them.
>
> WDYT?

[...]

Toggle quote (10 lines)
> - (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

Toggle snippet (5 lines)
(apply syntax-violation name
"invalid field specifier. The format of a field is `(field value)'"
(if parent-form (list parent-form #:'weird) (list weird)))

(and similar for the others)?

--
Sarah
L
L
Ludovic Courtès wrote on 4 Aug 2021 17:19
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(name . Julien Lepiller)(address . julien@lepiller.eu)(address . 49671@debbugs.gnu.org)
87bl6dp78s.fsf@gnu.org
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Hello!

Julien Lepiller <julien@lepiller.eu> skribis:

Toggle quote (34 lines)
> 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’.
L
L
Ludovic Courtès wrote on 4 Oct 2021 16:26
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Re: bug#50914: [PATCH] records: Raise a &fix-hint if a field has multiple values.
(name . Maxime Devos)(address . maximedevos@telenet.be)
87r1d0ooh2.fsf@gnu.org
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Hi,

Maxime Devos <maximedevos@telenet.be> skribis:

Toggle quote (4 lines)
> * guix/records.scm (report-invalid-field-specifier): If
> 'weird' is something like (field (record ...) extra ...), hint that 'extra
> ...' should probably be moved inside (record ...).

Please see also https://issues.guix.gnu.org/49671.

Toggle quote (8 lines)
> + (condition
> + (&origin (origin name))
> + (&message (message
> + (format #f "field ‘~a’ should only have one \
> +value, but an extra value ‘~a’ was passed as well. Perhaps this extra \
> +value was supposed to be a field specifier, and needs to be moved inside \
> +the record ‘~a’?"

We’ll need i18n here and straight quotes.

Toggle quote (4 lines)
> + (&syntax (form (car forms))
> + (subform (and (not (null? (cdr forms)))
> + (cadr forms))))

No cadrcdr please. :-)

Toggle quote (6 lines)
> + (&fix-hint (hint (object->string
> + (syntax->datum
> + #'(field
> + (record-name fields ... extra-value
> + extra-value* ...)))))))))

The ‘hint’ field must be a string, typically the message you had above;
see other examples in the code.

TIA!

Ludo’.
J
J
Julien Lepiller wrote on 31 Oct 2021 03:06
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[PATCH] guix: records: Improve error reporting.
(address . 49671@debbugs.gnu.org)
20211031030635.520a3c6c@tachikoma.lepiller.eu
Download
Hi Guix!

Here is a new patch that attempts to better catch various causes of
syntax errors in records. I think I fixed all the concerns Ludo had,
and I draw some inspiration from https://issues.guix.gnu.org/50914for
using conditions.

Here are examples of what you get:

(operating-system
())

test.scm:1:0: error: invalid field specifier: ()
hint: The format of a field is `(field value)'



(operating-system
((services %base-services)))

test.scm:1:0: error: invalid field specifier: (services %base-services)
is not a valid field name



(operating-system
(services))

test.scm:1:0: error: missing value in field specifier
hint: The field is missing a value: `(services <value>)'.



(operating-system
(services (service tor-service-type) %base-services))

test.scm:1:0: error: multiple values in field specifier
hint: 2 values were associated with field `services'. We got the
following values:

(service tor-service-type)
%base-services

Perhaps the additional values were intended to be other field
specifiers. This usually indicates missing or misplaced parenthesis.



(operating-system
services %base-services)

test.scm:1:0: error: invalid field specifier: #<syntax services>
hint: The format of a field is `(field value)'



(not sure why the last one is wrapped in syntax...)
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From 9e08a887a08da3f0cc132d148b748eb2ce7db135 Mon Sep 17 00:00:00 2001
Message-Id: <9e08a887a08da3f0cc132d148b748eb2ce7db135.1635645523.git.julien@lepiller.eu>
From: Julien Lepiller <julien@lepiller.eu>
Date: Sun, 31 Oct 2021 02:58:14 +0100
Subject: [PATCH] guix: records: Improve error reporting.

* guix/records.scm (report-invalid-field-specifier): Handle various
invalidity causes separately.
---
guix/records.scm | 56 ++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 52 insertions(+), 4 deletions(-)

Toggle diff (86 lines)
diff --git a/guix/records.scm b/guix/records.scm
index ed94c83dac..9baa2c2f1d 100644
--- a/guix/records.scm
+++ b/guix/records.scm
@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2018 Mark H Weaver <mhw@netris.org>
+;;; Copyright © 2021 Julien Lepiller <julien@lepiller.eu>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -21,9 +22,13 @@ (define-module (guix records)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-9)
   #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-35)
+  #:use-module (ice-9 format)
   #:use-module (ice-9 match)
   #:use-module (ice-9 regex)
   #:use-module (ice-9 rdelim)
+  #:use-module (guix diagnostics)
+  #:use-module (guix ui)
   #:autoload (system base target) (target-most-positive-fixnum)
   #:export (define-record-type*
             this-record
@@ -83,10 +88,53 @@ (define-syntax record-error
          ;; WEIRD may be an identifier, thus lacking source location info, and
          ;; BINDINGS is a list, also lacking source location info.  Hopefully
          ;; PARENT-FORM provides source location info.
-         (apply syntax-violation name "invalid field specifier"
-                (if parent-form
-                    (list parent-form #'weird)
-                    (list #'weird)))))))
+         (let* ((weird-properties (source-properties #'weird))
+                (parent-properties (and parent-form (syntax-source parent-form)))
+                (location (source-properties->location
+                            (or (and (not (null? weird-properties)) weird-properties)
+                                (and (not (null? parent-properties)) parent-properties)
+                                '()))))
+           (syntax-case #'weird ()
+             (()                             ;the empty list
+              (raise-exception
+                (condition
+                  (&message (message (format #f (G_ "invalid field specifier: ~a")
+                                             #'weird)))
+                  (&error-location (location location))
+                  (&fix-hint (hint (G_ "The format of a field is `(field value)'"))))))
+             (((field ...) _ ...)            ;a list whose first element is a list
+              (raise-exception
+                (condition
+                  (&message (message (format #f (G_ "invalid field specifier: ~a \
+is not a valid field name")
+                                             (map syntax->datum #'(field ...)))))
+                  (&error-location (location location)))))
+             ((field)                        ;a list with one element
+              (raise-exception
+                (condition
+                  (&message (message (G_ "missing value in field specifier")))
+                  (&error-location (location location))
+                  (&fix-hint (hint (format #f (G_ "The field is missing a value: `(~a <value>)'.")
+                                           (syntax->datum #'field)))))))
+             ((field value ...)              ;any other list
+              (raise-exception
+                (condition
+                  (&message (message (G_ "multiple values in field specifier")))
+                  (&error-location (location location))
+                  (&fix-hint (hint (format #f (G_ "~a values were associated with \
+field `~a'.  We got the following values:~%@lisp~%~{~a~%~}~%@end lisp~%Perhaps the additional values
+were intended to be other field specifiers.  This usually indicates missing or \
+misplaced parenthesis.")
+                                           (length #'(value ...))
+                                           (syntax->datum #'field)
+                                           (map syntax->datum #'(value ...))))))))
+             (field                          ;not a list
+              (raise-exception
+                (condition
+                  (&message (message (format #f (G_ "invalid field specifier: ~a")
+                                             #'weird)))
+                  (&error-location (location location))
+                  (&fix-hint (hint (G_ "The format of a field is `(field value)'"))))))))))))
 
   (define (report-duplicate-field-specifier name ctor)
     "Report the first duplicate identifier among the bindings in CTOR."
-- 
2.33.1
J
J
Julien Lepiller wrote on 22 Nov 2021 03:40
Download
Re: [bug#49671] [PATCH v3] guix: records: Improve error reporting.
(address . 49671@debbugs.gnu.org)
20211122034022.152daf55@tachikoma.lepiller.eu
Download
Here is another improvement compared to v2. This time there are two
patches: the first adds support for &syntax in (guix ui), and will
print something like

in form: <pretty-printed-form>

where "in form" is in green.

The second patch is very similar to v2, but will now also raise a
&syntax condition, so it can be pretty-printed. The previous issue
where I printed #<syntax ...> is fixed, I simply forgot a syntax->datum.

WDYT?
Download
From d1d857ea0ca76f1a917382777f57871e50026df3 Mon Sep 17 00:00:00 2001
Message-Id: <d1d857ea0ca76f1a917382777f57871e50026df3.1637548566.git.julien@lepiller.eu>
From: Julien Lepiller <julien@lepiller.eu>
Date: Mon, 22 Nov 2021 02:54:06 +0100
Subject: [PATCH 1/2] guix: ui: Print syntax errors.

* guix/ui.scm (display-syntax): New procedure.
(call-with-error-handling, report-load-error): Print syntax errors.
---
guix/ui.scm | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)

Toggle diff (68 lines)
diff --git a/guix/ui.scm b/guix/ui.scm
index bd999103ff..79aab2db84 100644
--- a/guix/ui.scm
+++ b/guix/ui.scm
@@ -69,9 +69,11 @@ (define-module (guix ui)
   #:use-module (srfi srfi-31)
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-35)
+  #:use-module (ice-9 exceptions)
   #:autoload   (ice-9 ftw)  (scandir)
   #:use-module (ice-9 match)
   #:use-module (ice-9 format)
+  #:use-module (ice-9 pretty-print)
   #:use-module (ice-9 regex)
   #:autoload   (ice-9 popen) (open-pipe* close-pipe)
   #:autoload   (system repl repl)  (start-repl)
@@ -315,6 +317,19 @@ (define* (display-hint message #:optional (port (current-error-port)))
      (texi->plain-text message))
    port))
 
+(define %syntax-color (color BOLD GREEN))
+
+(define* (display-syntax form #:optional (port (current-error-port)))
+  "Display FORM, an sexp, to PORT"
+  (define colorize
+    (if (color-output? port)
+        (lambda (str)
+          (colorize-string str %syntax-color))
+        identity))
+
+  (display (colorize (G_ "in form: ")) port)
+  (pretty-print form port))
+
 (define* (report-unbound-variable-error args #:key frame)
   "Return the given unbound-variable error, where ARGS is the list of 'throw'
 arguments."
@@ -398,6 +413,9 @@ (define* (report-load-error file args #:optional frame)
                    (formatted-message-arguments obj)))
            (else
             (report-error (G_ "exception thrown: ~s~%") obj)))
+     (when (syntax-error? obj)
+       (let ((form (or (syntax-error-subform obj) (syntax-error-form obj))))
+         (display-syntax form)))
      (when (fix-hint? obj)
        (display-hint (condition-fix-hint obj))))
     ((key args ...)
@@ -801,6 +819,9 @@ (define (call-with-error-handling thunk)
                      (and (error-location? c) (error-location c))
                      (gettext (formatted-message-string c) %gettext-domain)
                      (formatted-message-arguments c))
+              (when (syntax-error? c)
+                (let ((form (or (syntax-error-subform c) (syntax-error-form c))))
+                  (display-syntax form)))
               (when (fix-hint? c)
                 (display-hint (condition-fix-hint c)))
               (exit 1))
@@ -826,6 +847,9 @@ (define (call-with-error-handling thunk)
               (report-error (and (error-location? c) (error-location c))
                             (G_ "~a~%")
                             (gettext (condition-message c) %gettext-domain))
+              (when (syntax-error? c)
+                (let ((form (or (syntax-error-subform c) (syntax-error-form c))))
+                  (display-syntax form)))
               (when (fix-hint? c)
                 (display-hint (condition-fix-hint c)))
               (exit 1)))
-- 
2.33.1
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From 2eebd1113a40fc6e7018975d3696546de584e4a0 Mon Sep 17 00:00:00 2001
Message-Id: <2eebd1113a40fc6e7018975d3696546de584e4a0.1637548566.git.julien@lepiller.eu>
In-Reply-To: <d1d857ea0ca76f1a917382777f57871e50026df3.1637548566.git.julien@lepiller.eu>
References: <d1d857ea0ca76f1a917382777f57871e50026df3.1637548566.git.julien@lepiller.eu>
From: Julien Lepiller <julien@lepiller.eu>
Date: Sun, 31 Oct 2021 02:58:14 +0100
Subject: [PATCH 2/2] guix: records: Improve error reporting.

* guix/records.scm (report-invalid-field-specifier): Handle various
invalidity causes separately.
---
guix/records.scm | 56 ++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 52 insertions(+), 4 deletions(-)

Toggle diff (86 lines)
diff --git a/guix/records.scm b/guix/records.scm
index ed94c83dac..eeb5908844 100644
--- a/guix/records.scm
+++ b/guix/records.scm
@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2018 Mark H Weaver <mhw@netris.org>
+;;; Copyright © 2021 Julien Lepiller <julien@lepiller.eu>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -21,9 +22,13 @@ (define-module (guix records)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-9)
   #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-35)
+  #:use-module (ice-9 exceptions)
   #:use-module (ice-9 match)
   #:use-module (ice-9 regex)
   #:use-module (ice-9 rdelim)
+  #:use-module (guix diagnostics)
+  #:use-module (guix ui)
   #:autoload (system base target) (target-most-positive-fixnum)
   #:export (define-record-type*
             this-record
@@ -83,10 +88,53 @@ (define-syntax record-error
          ;; WEIRD may be an identifier, thus lacking source location info, and
          ;; BINDINGS is a list, also lacking source location info.  Hopefully
          ;; PARENT-FORM provides source location info.
-         (apply syntax-violation name "invalid field specifier"
-                (if parent-form
-                    (list parent-form #'weird)
-                    (list #'weird)))))))
+         (let* ((weird-properties (source-properties #'weird))
+                (parent-properties (and parent-form (syntax-source parent-form)))
+                (form parent-form)
+                (location (source-properties->location
+                            (or (and (not (null? weird-properties)) weird-properties)
+                                (and (not (null? parent-properties)) parent-properties)
+                                '()))))
+           (syntax-case #'weird ()
+             (()                             ;the empty list
+              (raise-exception
+                (condition
+                  (&message (message (G_ "invalid field specifier.")))
+                  (&syntax (form form) (subform (syntax->datum #'weird)))
+                  (&error-location (location location))
+                  (&fix-hint (hint (G_ "The format of a field is `(field value)'"))))))
+             (((field ...) _ ...)            ;a list whose first element is a list
+              (raise-exception
+                (condition
+                  (&message (message (G_ "invalid field name.")))
+                  (&syntax (form form) (subform (map syntax->datum #'(field ...))))
+                  (&error-location (location location)))))
+             ((field)                        ;a list with one element
+              (raise-exception
+                (condition
+                  (&message (message (G_ "missing value in field specifier.")))
+                  (&syntax (form form) (subform (syntax->datum #'weird)))
+                  (&error-location (location location))
+                  (&fix-hint (hint (format #f (G_ "The field is missing a value: `(~a <value>)'.")
+                                           (syntax->datum #'field)))))))
+             ((field value ...)              ;any other list
+              (raise-exception
+                (condition
+                  (&message (message (G_ "multiple values in field specifier.")))
+                  (&syntax (form form) (subform (syntax->datum #'weird)))
+                  (&error-location (location location))
+                  (&fix-hint (hint (format #f (G_ "~a values were associated with \
+field `~a'.  Perhaps the additional values were intended to be other field \
+specifiers.  This usually indicates missing or misplaced parenthesis.")
+                                           (length #'(value ...))
+                                           (syntax->datum #'field)))))))
+             (field                          ;not a list
+              (raise-exception
+                (condition
+                  (&message (message (G_ "invalid field specifier.")))
+                  (&syntax (form form) (subform (syntax->datum #'weird)))
+                  (&error-location (location location))
+                  (&fix-hint (hint (G_ "The format of a field is `(field value)'"))))))))))))
 
   (define (report-duplicate-field-specifier name ctor)
     "Report the first duplicate identifier among the bindings in CTOR."
-- 
2.33.1
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