Subject: [PATCH] records: Raise a &fix-hint if a field has multiple values.

From: Maxime Devos <[email-masked]>
Date: Thu, 30 Sep 2021 11:32:29 +0200

* guix/records.scm (report-invalid-field-specifier): If
  'weird' is something like (field (record ...) extra ...), hint that 'extra
  ...' should probably be moved inside (record ...).
---
 guix/records.scm | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/guix/records.scm b/guix/records.scm
index ed94c83dac..db0c0a7ca0 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 <[email-masked]>
 ;;; Copyright © 2018 Mark H Weaver <[email-masked]>
+;;; Copyright © 2021 Maxime Devos <[email-masked]>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -21,10 +22,13 @@
   #: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)
   #:autoload (system base target) (target-most-positive-fixnum)
+  #:autoload (guix diagnostics) (&fix-hint)
   #:export (define-record-type*
             this-record
 
@@ -83,10 +87,35 @@ 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"
+         (let ((forms
                 (if parent-form
                     (list parent-form #'weird)
-                    (list #'weird)))))))
+                    (list #'weird))))
+           (syntax-case #'weird ()
+             ;; common mistake
+             ((field (record-name fields ...) extra-value extra-value* ...)
+              (raise-exception
+               (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’?"
+                                   (syntax->datum #'field)
+                                   (syntax->datum #'extra-value)
+                                   (syntax->datum #'(record-name fields ...)))))
+                (&syntax (form (car forms))
+                         (subform (and (not (null? (cdr forms)))
+                                       (cadr forms))))
+                (&fix-hint (hint (object->string
+                                  (syntax->datum
+                                   #'(field
+                                      (record-name fields ... extra-value
+                                                   extra-value* ...)))))))))
+             (_
+              (apply syntax-violation name
+                     "invalid field specifier" forms))))))))
 
   (define (report-duplicate-field-specifier name ctor)
     "Report the first duplicate identifier among the bindings in CTOR."

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-- 
2.33.0