From 0ab9170aa7e321524ad97cdb5ee0bc0d09c5d4b2 Mon Sep 17 00:00:00 2001
From: Tommi Reiman <tommi@iki.fi>
Date: Tue, 5 Sep 2023 18:37:39 +0300
Subject: [PATCH] allow nil as valid fallback value

---
 src/malli/core.cljc       |  4 ++--
 test/malli/core_test.cljc | 10 +++++++++-
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/src/malli/core.cljc b/src/malli/core.cljc
index 6cdd31ae4..8808e22f2 100644
--- a/src/malli/core.cljc
+++ b/src/malli/core.cljc
@@ -560,8 +560,8 @@
                               (let [x* (transformer x)]
                                 (if ((nth validators i) x*)
                                   (reduced x*)
-                                  (or acc x*))))
-                            nil transformers))
+                                  (if (-equals acc ::nil) x* acc))))
+                            ::nil transformers))
                          (fn [x]
                            (reduce-kv
                             (fn [x i validator] (if (validator x) (reduced ((nth transformers i) x)) x))
diff --git a/test/malli/core_test.cljc b/test/malli/core_test.cljc
index 367a1867f..2ef6fba5b 100644
--- a/test/malli/core_test.cljc
+++ b/test/malli/core_test.cljc
@@ -319,7 +319,15 @@
                             :string] 1 math)
                  (m/decode [:orn
                             ["kw-math" math-kw-string]
-                            ["string" :string]] 1 math)))))
+                            ["string" :string]] 1 math))))
+
+        (testing "first branch nil can be selected as a fallback"
+          (is (= nil (m/decode
+                      [:or
+                       [:keyword {:decode/math (constantly nil)}]
+                       :keyword]
+                      "kikka"
+                      (mt/transformer {:name :math}))))))
 
       (testing "top-level transformations are retained"
         (doseq [schema [[:or {:decode/string {:enter (fn [m] (update m :enter #(or % true)))