records: `alist->record' supports multiple-field occurrences.

* guix/records.scm (alist->record): Add `multiple-value-keys'
  parameter.  Update docstring, and honor it.
* tests/records.scm ("alist->record"): New record.

guix/records.scm

@@ -198,9 +198,19 @@ (define-record-type type
                                                          #'((field options ...)
                                                             ...))))))))))
 
-(define (alist->record alist make keys)
-  "Apply MAKE to the values associated with KEYS in ALIST."
-  (let ((args (map (cut assoc-ref alist <>) keys)))
+(define* (alist->record alist make keys
+                        #:optional (multiple-value-keys '()))
+  "Apply MAKE to the values associated with KEYS in ALIST.  Items in KEYS that
+are also in MULTIPLE-VALUE-KEYS are considered to occur possibly multiple
+times in ALIST, and thus their value is a list."
+  (let ((args (map (lambda (key)
+                     (if (member key multiple-value-keys)
+                         (filter-map (match-lambda
+                                      ((k . v)
+                                       (and (equal? k key) v)))
+                                     alist)
+                         (assoc-ref alist key)))
+                   keys)))
     (apply make args)))
 
 (define (object->fields object fields port)

tests/records.scm

@@ -158,6 +158,12 @@ (define-record-type* <foo> foo make-foo
     (list (recutils->alist p)
           (recutils->alist p))))
 
+(test-equal "alist->record" '((1 2) b c)
+  (alist->record '(("a" . 1) ("b" . b) ("c" . c) ("a" . 2))
+                 list
+                 '("a" "b" "c")
+                 '("a")))
+
 (test-end)