From 9da1f49f890c754ee8687730ccd1fad8cb976804 Mon Sep 17 00:00:00 2001
From: Gordon Tisher <gordon@balafon.net>
Date: Wed, 9 Oct 2024 08:30:40 -0700
Subject: [PATCH] Fix `String/decode_utf8` returning a non-empty string when
 given an empty list (#726)

---
 src/fun/builtins.bend              | 4 +++-
 tests/golden_tests/io/utf8.bend    | 3 ++-
 tests/snapshots/io__utf8.bend.snap | 2 +-
 3 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/fun/builtins.bend b/src/fun/builtins.bend
index baa3a102..29d94f0e 100644
--- a/src/fun/builtins.bend
+++ b/src/fun/builtins.bend
@@ -561,7 +561,9 @@ Utf8/REPLACEMENT_CHARACTER : u24 = '\u{FFFD}'
 
 # Decodes a sequence of bytes to a String using utf-8 encoding.
 # Invalid utf-8 sequences are replaced with Utf8/REPLACEMENT_CHARACTER.
-String/decode_utf8 (bytes: (List u24)) : String =
+String/decode_utf8 (bytes: (List u24)) : String
+String/decode_utf8 [] = String/Nil
+String/decode_utf8 bytes =
   let (got, rest) = (Utf8/decode_character bytes)
   match rest {
     List/Nil: (String/Cons got String/Nil)
diff --git a/tests/golden_tests/io/utf8.bend b/tests/golden_tests/io/utf8.bend
index b3e16168..80618d6e 100644
--- a/tests/golden_tests/io/utf8.bend
+++ b/tests/golden_tests/io/utf8.bend
@@ -5,5 +5,6 @@ v2 = (to-and-back "(λf ((λx (f (x x))) (λx (f (x x)))))")
 v3 = (to-and-back "🌟")
 v4 = (to-and-back "Hello 🌎!")
 v5 = (to-and-back "𓆈 𓆉 𓆊 𓆋 𓅯")
+v6 = (String/decode_utf8 [])
 
-main = [v1, v2, v3, v4, v5]
+main = [v1, v2, v3, v4, v5, v6]
diff --git a/tests/snapshots/io__utf8.bend.snap b/tests/snapshots/io__utf8.bend.snap
index f4ec8ef3..19479507 100644
--- a/tests/snapshots/io__utf8.bend.snap
+++ b/tests/snapshots/io__utf8.bend.snap
@@ -3,4 +3,4 @@ source: tests/golden_tests.rs
 input_file: tests/golden_tests/io/utf8.bend
 ---
 Strict mode:
-["hi", "(λf ((λx (f (x x))) (λx (f (x x)))))", "🌟", "Hello 🌎!", "𓆈 𓆉 𓆊 𓆋 𓅯"]
+["hi", "(λf ((λx (f (x x))) (λx (f (x x)))))", "🌟", "Hello 🌎!", "𓆈 𓆉 𓆊 𓆋 𓅯", ""]