From e0a1a678e27a6a3da5e12edfcc7323c5204f6d2a Mon Sep 17 00:00:00 2001
From: Stefan Kangas <stefankangas@gmail.com>
Date: Tue, 27 Sep 2022 18:16:51 +0200
Subject: [PATCH] Make format-spec support functions

---
 lisp/format-spec.el            | 15 +++++++++++++--
 test/lisp/format-spec-tests.el | 12 ++++++++++++
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/lisp/format-spec.el b/lisp/format-spec.el
index 45c19aebc8..31e1d861c2 100644
--- a/lisp/format-spec.el
+++ b/lisp/format-spec.el
@@ -59,6 +59,16 @@ format-spec
 leading zeros or truncating leading characters until it's ten
 characters wide\".
 
+If the value is a function, it will be evaluated only when
+needed.  For example:
+
+  (format-spec \"%n\"
+               \\=`((?n . ,(lambda ()
+                          (read-number \"Number: \")))))
+
+Note that it is better to make sure the lambda 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 +104,15 @@ format-spec
                  (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 (and (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)))))
diff --git a/test/lisp/format-spec-tests.el b/test/lisp/format-spec-tests.el
index 4a3cc74c33..f137f94aaa 100644
--- a/test/lisp/format-spec-tests.el
+++ b/test/lisp/format-spec-tests.el
@@ -148,6 +148,18 @@ format-spec
              (format-spec fmt '((?b . "asd") (?a . "fgh")))
              #("fgh%asdasd" 0 3 (a b) 3 4 (c d) 7 10 (e f))))))
 
+(ert-deftest format-spec/function ()
+  (let* (called
+         (spec `((?a . "foo")
+                 (?f . ,(lambda ()
+                          (setq called t)
+                          "bar")))))
+    (should (equal (format-spec "%a" spec) "foo"))
+    (should-not called)
+    (should (equal (format-spec "%f" spec) "bar"))
+    (should called)))
+
+
 (ert-deftest format-spec-unknown ()
   (should-error (format-spec "foo %b %z zot" '((?b . "bar"))))
   (should-error (format-spec "foo %b %%%z zot" '((?b . "bar"))))
-- 
2.30.2