Subject: bug#76242: [PATCH] Clarify that mapconcat's FUNCTION can return nil

From: Hong Xu <[email-masked]>
Date: Wed, 12 Feb 2025 14:27:03 -0800

* src/fns.c (Fmapconcat): Explain that FUNCTION can return nil.
---
 src/fns.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/fns.c b/src/fns.c
index bee44b222c5e..f02a1b6838da 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -3400,9 +3400,11 @@ SEQUENCE may be a list, a vector, a bool-vector, or a string.
 Optional argument SEPARATOR must be a string, a vector, or a list of
 characters; nil stands for the empty string.
 
-FUNCTION must be a function of one argument, and must return a value
-  that is a sequence of characters: either a string, or a vector or
-  list of numbers that are valid character codepoints.  */)
+ FUNCTION must be a function of one argument, and must return either:
+
+   nil, which is treated as an empty string, or
+   a value that is a sequence of characters, which is either a string,
+   or a vector or list of numbers that are valid character codepoints. */)
   (Lisp_Object function, Lisp_Object sequence, Lisp_Object separator)
 {
   USE_SAFE_ALLOCA;
-- 
2.48.1