Make format-spec accept function substitutions

* lisp/format-spec.el (format-spec): Accept a function producing the
substitution for a character.
* doc/lispref/strings.texi (Custom Format Strings): Document the
above change.
* test/lisp/format-spec-tests.el (format-spec/function): New test.
Ref. https://lists.gnu.org/r/emacs-devel/2022-09/msg01875.html

1 files changed, 15 insertions, 2 deletions

diff --git a/lisp/format-spec.el b/lisp/format-spec.el
index 45c19aebc8b..60ff9f90864 100644
--- a/lisp/format-spec.el
+++ b/lisp/format-spec.el
@@ -59,6 +59,18 @@ value associated with ?b in SPECIFICATION, either padding it with
 leading zeros or truncating leading characters until it's ten
 characters wide\".
 
+the substitution for a specification character can also be a
+function, taking no arguments and returning a string to be used
+for the replacement.  It will only be called if FORMAT uses that
+character.  For example:
+
+  (format-spec \"%n\"
+               \\=`((?n . ,(lambda ()
+                          (read-number \"Number: \")))))
+
+Note that it is best to make sure the function is not quoted,
+like above, so that it is compiled by the byte-compiler.
+
 Any text properties of FORMAT are copied to the result, with any
 text properties of a %-spec itself copied to its substitution.
 
@@ -94,14 +106,15 @@ is returned, where each format spec is its own element."
                  (width (match-string 2))
                  (trunc (match-string 3))
                  (char (string-to-char (match-string 4)))
-                 (text (assq char specification)))
+                 (text (let ((res (cdr (assq char specification))))
+                         (if (functionp res) (funcall res) res))))
             (when (and split
                        (not (= (1- beg) split-start)))
               (push (buffer-substring split-start (1- beg)) split-result))
             (cond (text
                    ;; Handle flags.
                    (setq text (format-spec--do-flags
-                               (format "%s" (cdr text))
+                               (format "%s" text)
                                (format-spec--parse-flags flags)
                                (and width (string-to-number width))
                                (and trunc (car (read-from-string trunc 1)))))