From 08d2847767cb14aa45a68a2c30d03dea84d47f84 Mon Sep 17 00:00:00 2001
From: Alex Shinn <alexshinn@gmail.com>
Date: Mon, 30 Aug 2021 16:15:57 +0900
Subject: [PATCH] explicitly state that it is an error if the default values
 mutate the let-optionals list (issue #778)

---
 lib/chibi/optional.scm | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/chibi/optional.scm b/lib/chibi/optional.scm
index b310d8cc..97dc86f7 100644
--- a/lib/chibi/optional.scm
+++ b/lib/chibi/optional.scm
@@ -28,6 +28,9 @@
 ;;> elements of \var{ls} beyond the length of \var{ls}, otherwise any
 ;;> extra values are unused.
 ;;>
+;;> \var{ls} is evaluated only once.  It is an error if any
+;;> \var{default} mutates \var{ls}.
+;;>
 ;;> Typically used on the dotted rest list at the start of a lambda,
 ;;> \scheme{let-optionals} is more concise and more efficient than
 ;;> \scheme{case-lambda} for simple optional argument uses.