From 88fe78acc63adaff2d25e81b369f1395290e0e18 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Hugo=20H=C3=B6rnquist?= <hugo@lysator.liu.se>
Date: Wed, 18 Oct 2023 13:59:39 +0200
Subject: Add "unreachable".

---
 doc/ref/general/util-exceptions.texi | 11 +++++++++++
 module/hnh/util/exceptions.scm       |  5 +++++
 2 files changed, 16 insertions(+)

diff --git a/doc/ref/general/util-exceptions.texi b/doc/ref/general/util-exceptions.texi
index 34ba33f9..4c2b432d 100644
--- a/doc/ref/general/util-exceptions.texi
+++ b/doc/ref/general/util-exceptions.texi
@@ -38,3 +38,14 @@ signal SIGINT, which kills the program.
 @defun filter-stack pred? stack
 @c TODO document me
 @end defun
+
+
+@defun unreachable procedure fmt args
+Marks that a code path should be unreachable. Throws an exception
+which should never be caught (an @code{(exit -1)} would also have
+worked, but would make debugging harder).
+
+@var{procedure} is the procedure which reached an ``impossible''
+state, @var{fmt} a format string, and @var{args} arguments to that
+format string.
+@end defun
diff --git a/module/hnh/util/exceptions.scm b/module/hnh/util/exceptions.scm
index 1c3de8c7..9b6b25a8 100644
--- a/module/hnh/util/exceptions.scm
+++ b/module/hnh/util/exceptions.scm
@@ -11,6 +11,7 @@
            warning
            fatal
            filter-stack
+           unreachable
            ))
 
 
@@ -40,3 +41,7 @@
   (concatenate
    (for i in (iota (stack-length stk))
         (filter pred? (map binding-ref (frame-bindings (stack-ref stk i)))))))
+
+
+(define (unreachable procedure fmt args)
+  (scm-error 'unreachable procedure fmt args #f))
-- 
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