scm: add changed-object, use if-let/when-let

Add a changed-object (to complement changed), and some rework some of
the code with the new if-let/when-let macros.

NEWS.org

@@ -79,8 +79,9 @@
      ~fields~/~field~ (alists with metadata about database fields). See the SCM
      reference documentation for further details (1.14.3).
 
-   - add procedures ~date-object~ to get a message's sent-date as an SRFI-19
-     object; and ~utc-offset~ to get the UTC (timezone) offset for a date.
+   - add procedures ~date-object~ which is like ~date~, but returns message's
+     sent-date as an SRFI-19 object; and ~utc-offset~ to get the UTC (timezone)
+     offset for a date. Similarly, add ~changed-object~ to complement ~changed~
      (1.14.3)
 
 * 1.12 (released on February 24, 2024)

scm/mu-scm.scm

@@ -45,6 +45,7 @@
             utc-offset
 
 	    changed
+            changed-object
 
 	    message-id
 	    path
@@ -377,17 +378,21 @@ I.e., the offset from the UTC for the time the message was sent.
 (define-method (date-object (message <message>))
   "Get an SRFI-19 date object for MESSAGE's sent date.
 This includes the date and the timezone (if known). #f if not found."
-  (let ((unix-time (date message)) (offset (utc-offset message)))
-    (if unix-time
-        (time-utc->date (make-time 'time-utc 0 unix-time)
-                        (or offset 0))
-        #f)))
+  (when-let ((unix-time (date message)))
+    (time-utc->date (make-time 'time-utc 0 unix-time)
+                    (or (utc-offset message) 0))))
 
 (define-method (changed (message <message>))
   "Get the timestamp for the last change to MESSAGE.
 This is the number of seconds since epoch; #f if not found."
   (assoc-ref (message->alist message) 'changed))
 
+(define-method (changed-object (message <message>))
+  "Get an SRFI-19 date object for MESSAGE's last-change.
+No time-zone offset is defined. #f if not found."
+  (when-let ((unix-time (changed message)))
+    (time-utc->date (make-time 'time-utc 0 unix-time))))
+
 (define-method (path (message <message>))
   "Get the file-system path for MESSAGE or #f if not found."
   (assoc-ref (message->alist message) 'path))
@@ -400,11 +405,8 @@ A symbol, either 'high, 'low or 'normal, or #f if not found."
 (define-method (language (message <message>))
   "Get the ISO-639-1 language code for the MESSAGE as a symbol, if detected.
 Return #f otherwise."
-  (let ((lang (assoc-ref (message->alist message) 'language)))
-    (if lang
-	(string->symbol lang)
-	#f)))
-;; if-let would be nice!
+  (when-let ((lang (assoc-ref (message->alist message) 'language)))
+    (string->symbol lang)))
 
 (define-method (size (message <message>))
   "Get the size of the MESSAGE in bytes or #f if not available."
@@ -426,10 +428,9 @@ the empty list."
   "Get the oldest (first) reference for MESSAGE, or message-id if there are none.
 If neither are available, return #f.
 This is method is useful to determine the thread a message is in."
-  (let ((refs (references message)))
-    (if (and refs (not (null? refs)))
-	(car refs)
-	(message-id message))))
+  (if-let* ((refs (references message)) (_ (not (null? refs))))
+    (car refs)
+    (message-id message)))
 
 (define-method (mailing-list (message <message>))
   "Get the mailing-list id for MESSAGE or #f if not available."

scm/mu-scm.texi

@@ -840,16 +840,21 @@ Does this message include a calendar invitation? Returns @t{#t} or @code{#f}.
 
 @deffn {Scheme Procedure} changed message
 @end deffn
+@deffn {Scheme Procedure} changed-object message
+@end deffn
 Get the time of the message's last change (through @t{mu}), or @code{#f} if there
 is none. The time is expressed the data as the number of seconds since epoch,
 @t{time_t}.
 
 For example:
 @lisp
-(last-change msg)
+(changed msg)
 => 1703336567
 @end lisp
 
+@code{changed-object} returns a value as a SRFI-19 date object, similar to
+@code{date-object}, but without any time-zone offset.
+
 @deffn {Scheme Procedure} priority message
 @end deffn
 Get the message's priority. This is a symbol, either @t{high}, @t{normal} or