From 70147fbefc3f54e6a012eb44c3b37f207778e1b1 Mon Sep 17 00:00:00 2001
From: Sai Asish Y <say.apm35@gmail.com>
Date: Sun, 23 Aug 2026 00:08:15 -0700
Subject: [PATCH 1/2] fix(vm): raise a clear error when encoding a decoded
 table

Signed-off-by: Sai Asish Y <say.apm35@gmail.com>
---
 lib/lua/vm/value.ex        | 60 ++++++++++++++++++++++++++------------
 test/lua/vm/value_test.exs | 10 +++++++
 2 files changed, 51 insertions(+), 19 deletions(-)

diff --git a/lib/lua/vm/value.ex b/lib/lua/vm/value.ex
index cd55225d..c6726164 100644
--- a/lib/lua/vm/value.ex
+++ b/lib/lua/vm/value.ex
@@ -277,25 +277,39 @@ defmodule Lua.VM.Value do
   end
 
   def encode(list, state, fun_wrapper) when is_list(list) do
-    if keyword_list?(list) do
-      {data, state} =
-        Enum.reduce(list, {%{}, state}, fn {k, v}, {data, state} ->
-          key = Atom.to_string(k)
-          {encoded_v, state} = encode(v, state, fun_wrapper)
-          {Map.put(data, key, encoded_v), state}
-        end)
-
-      State.alloc_table(state, data)
-    else
-      {data, state} =
-        list
-        |> Enum.with_index(1)
-        |> Enum.reduce({%{}, state}, fn {v, idx}, {data, state} ->
-          {encoded_v, state} = encode(v, state, fun_wrapper)
-          {Map.put(data, idx, encoded_v), state}
-        end)
-
-      State.alloc_table(state, data)
+    cond do
+      keyword_list?(list) ->
+        {data, state} =
+          Enum.reduce(list, {%{}, state}, fn {k, v}, {data, state} ->
+            key = Atom.to_string(k)
+            {encoded_v, state} = encode(v, state, fun_wrapper)
+            {Map.put(data, key, encoded_v), state}
+          end)
+
+        State.alloc_table(state, data)
+
+      # `decode/2` returns a table as a list of `{key, value}` pairs with
+      # binary/number keys. That shape has no positional encoding (each pair
+      # is a bare 2-tuple matching no `encode/3` clause), so it would crash
+      # with a raw `FunctionClauseError`. Refuse it with an actionable message
+      # instead, mirroring the struct and cyclic-tref clauses above.
+      decoded_pair_list?(list) ->
+        raise Lua.RuntimeException,
+              "cannot encode a list of {key, value} pairs into a Lua value. This is the " <>
+                "shape `decode/2` produces for a table, and it does not round-trip back " <>
+                "into the VM. Convert it to a map first (e.g. `Map.new/1`), which encodes " <>
+                "as a Lua table."
+
+      true ->
+        {data, state} =
+          list
+          |> Enum.with_index(1)
+          |> Enum.reduce({%{}, state}, fn {v, idx}, {data, state} ->
+            {encoded_v, state} = encode(v, state, fun_wrapper)
+            {Map.put(data, idx, encoded_v), state}
+          end)
+
+        State.alloc_table(state, data)
     end
   end
 
@@ -328,6 +342,14 @@ defmodule Lua.VM.Value do
   defp keyword_list?([]), do: true
   defp keyword_list?(_), do: false
 
+  # A non-empty list where every element is a `{key, value}` pair with a binary
+  # or number key: exactly what `decode/2` returns for a Lua table. The empty
+  # list is already handled by `keyword_list?/1`, so this never matches `[]`.
+  defp decoded_pair_list?([{k, _v} | rest]) when is_binary(k) or is_number(k), do: decoded_pair_list?(rest)
+
+  defp decoded_pair_list?([]), do: true
+  defp decoded_pair_list?(_), do: false
+
   # --- Decoding (Lua VM → Elixir) ---
 
   @doc """
diff --git a/test/lua/vm/value_test.exs b/test/lua/vm/value_test.exs
index 35da1881..0d163dcb 100644
--- a/test/lua/vm/value_test.exs
+++ b/test/lua/vm/value_test.exs
@@ -130,6 +130,16 @@ defmodule Lua.VM.ValueTest do
       inner_table = Map.fetch!(state.tables, inner_id)
       assert inner_table.data["x"] == 1
     end
+
+    test "refuses a decoded table (list of {key, value} pairs) with a clear error" do
+      assert_raise Lua.RuntimeException, ~r/list of \{key, value\} pairs/, fn ->
+        Value.encode([{"x", 1}, {"y", 2}], new_state())
+      end
+
+      assert_raise Lua.RuntimeException, ~r/list of \{key, value\} pairs/, fn ->
+        Value.encode([{1, "a"}, {2, "b"}], new_state())
+      end
+    end
   end
 
   describe "encode_list/2" do

From 508cd5c191b4040c25d48c329b7598d4d5c094f4 Mon Sep 17 00:00:00 2001
From: Sai Asish Y <say.apm35@gmail.com>
Date: Fri, 25 Sep 2026 09:53:24 -0700
Subject: [PATCH 2/2] use a heredoc for the decoded pair list error

Signed-off-by: Sai Asish Y <say.apm35@gmail.com>
---
 lib/lua/vm/value.ex | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/lib/lua/vm/value.ex b/lib/lua/vm/value.ex
index c6726164..3f3227de 100644
--- a/lib/lua/vm/value.ex
+++ b/lib/lua/vm/value.ex
@@ -294,11 +294,12 @@ defmodule Lua.VM.Value do
       # with a raw `FunctionClauseError`. Refuse it with an actionable message
       # instead, mirroring the struct and cyclic-tref clauses above.
       decoded_pair_list?(list) ->
-        raise Lua.RuntimeException,
-              "cannot encode a list of {key, value} pairs into a Lua value. This is the " <>
-                "shape `decode/2` produces for a table, and it does not round-trip back " <>
-                "into the VM. Convert it to a map first (e.g. `Map.new/1`), which encodes " <>
-                "as a Lua table."
+        raise Lua.RuntimeException, """
+        cannot encode a list of {key, value} pairs into a Lua value. This is the \
+        shape `decode/2` produces for a table, and it does not round-trip back \
+        into the VM. Convert it to a map first (e.g. `Map.new/1`), which encodes \
+        as a Lua table.\
+        """
 
       true ->
         {data, state} =