From ef26fd1d5ff07e15850eadb4b06936c4bcb453b1 Mon Sep 17 00:00:00 2001
From: Georgy Lukyanov <mail@geo2a.info>
Date: Fri, 28 Jul 2023 12:02:57 +0200
Subject: [PATCH] Allow matching two empty `KMap`s

---
 library/Booster/Pattern/Match.hs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/library/Booster/Pattern/Match.hs b/library/Booster/Pattern/Match.hs
index eb2d85708..ea04d9825 100644
--- a/library/Booster/Pattern/Match.hs
+++ b/library/Booster/Pattern/Match.hs
@@ -269,7 +269,12 @@ match1
             -- and, domain values, injections, maps: fail
             _other ->
                 failWith $ DifferentSymbols app subj
--- matching on maps unsupported
+----- KMap
+-- empty maps match trivially with an empty substitution
+match1
+    (KMap _ [] Nothing)
+    (KMap _ [] Nothing) = pure mempty
+-- matching on non-empty maps is not supported
 match1
     t1@KMap{}
     t2 = indeterminate t1 t2