From e21e68ae910628d61f325601e4c9483c6181581a Mon Sep 17 00:00:00 2001
From: Chris Xu <git@chrisxu.me>
Date: Mon, 19 Jan 2026 08:49:40 +0100
Subject: [PATCH 1/8] fix: remove parantheses to make except work
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

The expression `[:__struct__] -- except` evaluates to `[:__struct__]`
(since except typically doesn't contain `:__struct__)`, so the final
result is `Map.keys(struct) -- [:__struct__]` — the except fields are
never removed.

This especially happens when deriving from Ecto schemas and ignoring the
`__meta__` like this:
`@derive {Toon.Encoder, except: [:__meta__]}`

This solves it by removing the parentheses. The PR adds necessary tests
to make sure this is fixed.
---
 lib/toon/encoder.ex           |  2 +-
 mix.exs                       |  2 +-
 test/fixtures/test_structs.ex |  4 ++++
 test/toon/encoder_test.exs    | 20 ++++++++++++++++++++
 4 files changed, 26 insertions(+), 2 deletions(-)
 create mode 100644 test/fixtures/test_structs.ex
 create mode 100644 test/toon/encoder_test.exs

diff --git a/lib/toon/encoder.ex b/lib/toon/encoder.ex
index d3aa8b7..f19ca1f 100644
--- a/lib/toon/encoder.ex
+++ b/lib/toon/encoder.ex
@@ -87,7 +87,7 @@ defimpl Toon.Encoder, for: Any do
         only
 
       except = Keyword.get(opts, :except) ->
-        Map.keys(struct) -- ([:__struct__] -- except)
+        Map.keys(struct) -- [:__struct__ | except]
 
       true ->
         Map.keys(struct) -- [:__struct__]
diff --git a/mix.exs b/mix.exs
index 0adb77b..60bb432 100644
--- a/mix.exs
+++ b/mix.exs
@@ -44,7 +44,7 @@ defmodule Toon.MixProject do
     ]
   end
 
-  defp elixirc_paths(:test), do: ["lib"]
+  defp elixirc_paths(:test), do: ["lib", "test/fixtures"]
   defp elixirc_paths(_), do: ["lib"]
 
   def application do
diff --git a/test/fixtures/test_structs.ex b/test/fixtures/test_structs.ex
new file mode 100644
index 0000000..63d0453
--- /dev/null
+++ b/test/fixtures/test_structs.ex
@@ -0,0 +1,4 @@
+defmodule Toon.Fixtures.UserWithExcept do
+  @derive {Toon.Encoder, except: [:password]}
+  defstruct [:name, :email, :password]
+end
diff --git a/test/toon/encoder_test.exs b/test/toon/encoder_test.exs
new file mode 100644
index 0000000..9bac673
--- /dev/null
+++ b/test/toon/encoder_test.exs
@@ -0,0 +1,20 @@
+defmodule Toon.EncoderTest do
+  use ExUnit.Case, async: true
+
+  alias Toon.Fixtures.UserWithExcept
+
+  describe "fields_to_encode/2 with except option" do
+    @user_attrs %{name: "Alice", email: "a@b.com", password: "secret"}
+
+    test "excludes specified fields from encoding" do
+      user = struct(UserWithExcept, @user_attrs)
+
+      encoded = user |> Toon.Encoder.encode([]) |> IO.iodata_to_binary()
+      {:ok, decoded} = Toon.decode(encoded)
+
+      assert Map.has_key?(decoded, "name") == true
+      assert Map.has_key?(decoded, "email") == true
+      assert Map.has_key?(decoded, "password") == false
+    end
+  end
+end

From 03f1d5de5bb5e902b5144a159f4cec6af0069c97 Mon Sep 17 00:00:00 2001
From: Chris Xu <git@chrisxu.me>
Date: Tue, 20 Jan 2026 10:43:04 +0100
Subject: [PATCH 2/8] fix: dispatch to Toon.Encoder protocol for structs in
 normalize/1

Previously, structs would fall through to the map clause in normalize/1,
which attempted to enumerate them and failed with "Enumerable not implemented".

This fix adds a struct clause before the map clause that dispatches to the
Toon.Encoder protocol, mirroring how Jason handles struct encoding.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
---
 lib/toon/shared/utils.ex      |  7 +++++++
 test/fixtures/test_structs.ex | 11 +++++++++++
 test/toon/encoder_test.exs    | 24 +++++++++++++++++++++++-
 3 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/lib/toon/shared/utils.ex b/lib/toon/shared/utils.ex
index 87d2f39..060c505 100644
--- a/lib/toon/shared/utils.ex
+++ b/lib/toon/shared/utils.ex
@@ -194,6 +194,13 @@ defmodule Toon.Utils do
     Enum.map(value, &normalize/1)
   end
 
+  # # Structs - dispatch to Toon.Encoder protocol
+  def normalize(%{__struct__: _} = struct) do
+    struct
+    |> Toon.Encoder.encode([])
+    |> IO.iodata_to_binary()
+  end
+
   def normalize(value) when is_map(value) do
     Map.new(value, fn {k, v} ->
       {to_string(k), normalize(v)}
diff --git a/test/fixtures/test_structs.ex b/test/fixtures/test_structs.ex
index 63d0453..dd2a651 100644
--- a/test/fixtures/test_structs.ex
+++ b/test/fixtures/test_structs.ex
@@ -2,3 +2,14 @@ defmodule Toon.Fixtures.UserWithExcept do
   @derive {Toon.Encoder, except: [:password]}
   defstruct [:name, :email, :password]
 end
+
+defmodule Toon.Fixtures.CustomDate do
+  @moduledoc "Test struct with explicit Toon.Encoder implementation"
+  defstruct [:year, :month, :day]
+end
+
+defimpl Toon.Encoder, for: Toon.Fixtures.CustomDate do
+  def encode(%{year: y, month: m, day: d}, _opts) do
+    "#{y}-#{String.pad_leading(to_string(m), 2, "0")}-#{String.pad_leading(to_string(d), 2, "0")}"
+  end
+end
diff --git a/test/toon/encoder_test.exs b/test/toon/encoder_test.exs
index 9bac673..08ece4d 100644
--- a/test/toon/encoder_test.exs
+++ b/test/toon/encoder_test.exs
@@ -1,7 +1,7 @@
 defmodule Toon.EncoderTest do
   use ExUnit.Case, async: true
 
-  alias Toon.Fixtures.UserWithExcept
+  alias Toon.Fixtures.{CustomDate, UserWithExcept}
 
   describe "fields_to_encode/2 with except option" do
     @user_attrs %{name: "Alice", email: "a@b.com", password: "secret"}
@@ -17,4 +17,26 @@ defmodule Toon.EncoderTest do
       assert Map.has_key?(decoded, "password") == false
     end
   end
+
+  describe "Toon.Utils.normalize/1 dispatches to Toon.Encoder for structs" do
+    test "dispatches to explicit Toon.Encoder implementation" do
+      date = %CustomDate{year: 2024, month: 1, day: 15}
+
+      # Direct encoder call
+      encoded_directly = date |> Toon.Encoder.encode([]) |> IO.iodata_to_binary()
+
+      # normalize/1 should produce identical output
+      assert Toon.Utils.normalize(date) == encoded_directly
+    end
+
+    test "dispatches to @derive Toon.Encoder" do
+      user = %UserWithExcept{name: "Bob", email: "bob@test.com", password: "secret"}
+
+      # Direct encoder call
+      encoded_directly = user |> Toon.Encoder.encode([]) |> IO.iodata_to_binary()
+
+      # normalize/1 should produce identical output
+      assert Toon.Utils.normalize(user) == encoded_directly
+    end
+  end
 end

From 85f9f1af2422d53a41a8b656bf6310a29c2fe826 Mon Sep 17 00:00:00 2001
From: Chris Xu <git@chrisxu.me>
Date: Tue, 20 Jan 2026 14:29:46 +0100
Subject: [PATCH 3/8] fix(encoder): return normalized maps from derived
 encoders

Prevents double-escaping of nested structs by having derived encoders
return maps instead of TOON strings. The actual TOON encoding now
happens once at the top level.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
---
 lib/toon/encoder.ex           | 11 +++----
 lib/toon/shared/utils.ex      | 14 +++++---
 test/fixtures/test_structs.ex | 10 ++++++
 test/nested_struct_test.exs   | 60 +++++++++++++++++++++++++++++++++++
 test/toon/encoder_test.exs    | 28 ++++++++++------
 5 files changed, 102 insertions(+), 21 deletions(-)
 create mode 100644 test/nested_struct_test.exs

diff --git a/lib/toon/encoder.ex b/lib/toon/encoder.ex
index f19ca1f..2e7156a 100644
--- a/lib/toon/encoder.ex
+++ b/lib/toon/encoder.ex
@@ -42,13 +42,10 @@ defimpl Toon.Encoder, for: Any do
 
     quote do
       defimpl Toon.Encoder, for: unquote(module) do
-        def encode(struct, opts) do
-          map =
-            struct
-            |> Map.take(unquote(fields))
-            |> Map.new(fn {k, v} -> {to_string(k), v} end)
-
-          Toon.Encode.encode!(map, opts)
+        def encode(struct, _opts) do
+          struct
+          |> Map.take(unquote(fields))
+          |> Map.new(fn {k, v} -> {to_string(k), Toon.Utils.normalize(v)} end)
         end
       end
     end
diff --git a/lib/toon/shared/utils.ex b/lib/toon/shared/utils.ex
index 060c505..81251d2 100644
--- a/lib/toon/shared/utils.ex
+++ b/lib/toon/shared/utils.ex
@@ -194,11 +194,17 @@ defmodule Toon.Utils do
     Enum.map(value, &normalize/1)
   end
 
-  # # Structs - dispatch to Toon.Encoder protocol
+  # Structs - dispatch to Toon.Encoder protocol
   def normalize(%{__struct__: _} = struct) do
-    struct
-    |> Toon.Encoder.encode([])
-    |> IO.iodata_to_binary()
+    result = Toon.Encoder.encode(struct, [])
+
+    # If encoder returns iodata (string), convert it to binary
+    # If encoder returns a map (from @derive), use it directly
+    case result do
+      binary when is_binary(binary) -> binary
+      map when is_map(map) -> map
+      iodata -> IO.iodata_to_binary(iodata)
+    end
   end
 
   def normalize(value) when is_map(value) do
diff --git a/test/fixtures/test_structs.ex b/test/fixtures/test_structs.ex
index dd2a651..8a1bc90 100644
--- a/test/fixtures/test_structs.ex
+++ b/test/fixtures/test_structs.ex
@@ -13,3 +13,13 @@ defimpl Toon.Encoder, for: Toon.Fixtures.CustomDate do
     "#{y}-#{String.pad_leading(to_string(m), 2, "0")}-#{String.pad_leading(to_string(d), 2, "0")}"
   end
 end
+
+defmodule Toon.Fixtures.Person do
+  @derive Toon.Encoder
+  defstruct [:name, :age]
+end
+
+defmodule Toon.Fixtures.Company do
+  @derive Toon.Encoder
+  defstruct [:name, :ceo]
+end
diff --git a/test/nested_struct_test.exs b/test/nested_struct_test.exs
new file mode 100644
index 0000000..ddd576e
--- /dev/null
+++ b/test/nested_struct_test.exs
@@ -0,0 +1,60 @@
+defmodule Toon.NestedStructTest do
+  use ExUnit.Case, async: true
+
+  alias Toon.Fixtures.{Person, Company}
+
+  describe "nested struct encoding" do
+    test "encodes and decodes nested structs correctly" do
+      person = %Person{name: "John", age: 30}
+      company = %Company{name: "Acme", ceo: person}
+
+      # Encode the nested struct
+      encoded = Toon.encode!(company)
+
+      # Should produce TOON format
+      assert is_binary(encoded)
+
+      # Decode and verify
+      {:ok, decoded} = Toon.decode(encoded)
+
+      # Verify structure
+      assert decoded["name"] == "Acme"
+      assert decoded["ceo"]["name"] == "John"
+      assert decoded["ceo"]["age"] == 30
+    end
+
+    test "normalizes nested structs to maps" do
+      person = %Person{name: "Jane", age: 25}
+      company = %Company{name: "TechCo", ceo: person}
+
+      # normalize should convert nested structs to maps
+      normalized = Toon.Utils.normalize(company)
+
+      assert is_map(normalized)
+      assert normalized["name"] == "TechCo"
+      assert is_map(normalized["ceo"])
+      assert normalized["ceo"]["name"] == "Jane"
+      assert normalized["ceo"]["age"] == 25
+    end
+
+    test "handles deeply nested structs" do
+      person1 = %Person{name: "Alice", age: 35}
+      company1 = %Company{name: "StartupA", ceo: person1}
+
+      person2 = %Person{name: "Bob", age: 40}
+      company2 = %Company{name: "StartupB", ceo: person2}
+
+      # Create a list of companies (testing nested structs in lists)
+      data = %{"companies" => [company1, company2]}
+
+      encoded = Toon.encode!(data)
+      {:ok, decoded} = Toon.decode(encoded)
+
+      assert length(decoded["companies"]) == 2
+      assert decoded["companies"] |> Enum.at(0) |> Map.get("name") == "StartupA"
+      assert decoded["companies"] |> Enum.at(0) |> Map.get("ceo") |> Map.get("name") == "Alice"
+      assert decoded["companies"] |> Enum.at(1) |> Map.get("name") == "StartupB"
+      assert decoded["companies"] |> Enum.at(1) |> Map.get("ceo") |> Map.get("name") == "Bob"
+    end
+  end
+end
diff --git a/test/toon/encoder_test.exs b/test/toon/encoder_test.exs
index 08ece4d..3b86837 100644
--- a/test/toon/encoder_test.exs
+++ b/test/toon/encoder_test.exs
@@ -9,12 +9,18 @@ defmodule Toon.EncoderTest do
     test "excludes specified fields from encoding" do
       user = struct(UserWithExcept, @user_attrs)
 
-      encoded = user |> Toon.Encoder.encode([]) |> IO.iodata_to_binary()
-      {:ok, decoded} = Toon.decode(encoded)
+      # Derived encoder now returns a normalized map
+      encoded_map = Toon.Encoder.encode(user, [])
 
-      assert Map.has_key?(decoded, "name") == true
-      assert Map.has_key?(decoded, "email") == true
-      assert Map.has_key?(decoded, "password") == false
+      assert Map.has_key?(encoded_map, "name") == true
+      assert Map.has_key?(encoded_map, "email") == true
+      assert Map.has_key?(encoded_map, "password") == false
+
+      # Can still be encoded to TOON format
+      toon_string = Toon.encode!(encoded_map)
+      {:ok, decoded} = Toon.decode(toon_string)
+
+      assert decoded == encoded_map
     end
   end
 
@@ -22,21 +28,23 @@ defmodule Toon.EncoderTest do
     test "dispatches to explicit Toon.Encoder implementation" do
       date = %CustomDate{year: 2024, month: 1, day: 15}
 
-      # Direct encoder call
+      # Explicit encoder still returns iodata/string
       encoded_directly = date |> Toon.Encoder.encode([]) |> IO.iodata_to_binary()
+      assert encoded_directly == "2024-01-15"
 
-      # normalize/1 should produce identical output
+      # normalize/1 should produce identical output for explicit encoders
       assert Toon.Utils.normalize(date) == encoded_directly
     end
 
     test "dispatches to @derive Toon.Encoder" do
       user = %UserWithExcept{name: "Bob", email: "bob@test.com", password: "secret"}
 
-      # Direct encoder call
-      encoded_directly = user |> Toon.Encoder.encode([]) |> IO.iodata_to_binary()
+      # Derived encoder returns normalized map
+      encoded_map = Toon.Encoder.encode(user, [])
 
       # normalize/1 should produce identical output
-      assert Toon.Utils.normalize(user) == encoded_directly
+      assert Toon.Utils.normalize(user) == encoded_map
+      assert encoded_map == %{"name" => "Bob", "email" => "bob@test.com"}
     end
   end
 end

From 1d6ab2350613c6fde719867833a9b8e4fcace4eb Mon Sep 17 00:00:00 2001
From: Chris Xu <git@chrisxu.me>
Date: Tue, 20 Jan 2026 14:38:18 +0100
Subject: [PATCH 4/8] chore: fix credo issues and update encoder spec

- Add @moduledoc false to test fixture modules
- Fix alias ordering in nested_struct_test.exs
- Update @spec to allow map() return from derived encoders

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
---
 lib/toon/encoder.ex           | 2 +-
 test/fixtures/test_structs.ex | 3 +++
 test/nested_struct_test.exs   | 2 +-
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/lib/toon/encoder.ex b/lib/toon/encoder.ex
index 2e7156a..f6663a3 100644
--- a/lib/toon/encoder.ex
+++ b/lib/toon/encoder.ex
@@ -32,7 +32,7 @@ defprotocol Toon.Encoder do
 
   Returns IO data that can be converted to a string.
   """
-  @spec encode(t, keyword()) :: iodata()
+  @spec encode(t, keyword()) :: iodata() | map()
   def encode(value, opts)
 end
 
diff --git a/test/fixtures/test_structs.ex b/test/fixtures/test_structs.ex
index 8a1bc90..46087f8 100644
--- a/test/fixtures/test_structs.ex
+++ b/test/fixtures/test_structs.ex
@@ -1,4 +1,5 @@
 defmodule Toon.Fixtures.UserWithExcept do
+  @moduledoc false
   @derive {Toon.Encoder, except: [:password]}
   defstruct [:name, :email, :password]
 end
@@ -15,11 +16,13 @@ defimpl Toon.Encoder, for: Toon.Fixtures.CustomDate do
 end
 
 defmodule Toon.Fixtures.Person do
+  @moduledoc false
   @derive Toon.Encoder
   defstruct [:name, :age]
 end
 
 defmodule Toon.Fixtures.Company do
+  @moduledoc false
   @derive Toon.Encoder
   defstruct [:name, :ceo]
 end
diff --git a/test/nested_struct_test.exs b/test/nested_struct_test.exs
index ddd576e..dd8304c 100644
--- a/test/nested_struct_test.exs
+++ b/test/nested_struct_test.exs
@@ -1,7 +1,7 @@
 defmodule Toon.NestedStructTest do
   use ExUnit.Case, async: true
 
-  alias Toon.Fixtures.{Person, Company}
+  alias Toon.Fixtures.{Company, Person}
 
   describe "nested struct encoding" do
     test "encodes and decodes nested structs correctly" do

From 3c22b13d563e854f4bad24176e98ae20ca0bcdbd Mon Sep 17 00:00:00 2001
From: Chris Xu <git@chrisxu.me>
Date: Tue, 20 Jan 2026 14:55:12 +0100
Subject: [PATCH 5/8] ci: add OTP 28 and use mix quality.ci

- Add OTP 28 to test matrix with exclusions for Elixir < 1.18
- Replace separate format/credo steps with mix quality.ci
- Add PLT caching for faster dialyzer runs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
---
 .github/workflows/ci.yml | 40 ++++++++++++++++++++++++----------------
 1 file changed, 24 insertions(+), 16 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ae91634..f0197bc 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -16,14 +16,21 @@ jobs:
     strategy:
       matrix:
         elixir: ['1.15', '1.16', '1.17', '1.18', '1.19']
-        otp: ['25', '26', '27']
+        otp: ['25', '26', '27', '28']
         exclude:
-          # Elixir 1.15 doesn't support OTP 27
+          # Elixir 1.15 doesn't support OTP 27+
           - elixir: '1.15'
             otp: '27'
-          # Elixir 1.16 doesn't support OTP 27
+          - elixir: '1.15'
+            otp: '28'
+          # Elixir 1.16 doesn't support OTP 27+
           - elixir: '1.16'
             otp: '27'
+          - elixir: '1.16'
+            otp: '28'
+          # Elixir 1.17 doesn't support OTP 28
+          - elixir: '1.17'
+            otp: '28'
           # Elixir 1.19 doesn't support OTP 25
           - elixir: '1.19'
             otp: '25'
@@ -63,10 +70,6 @@ jobs:
   quality:
     name: Code Quality
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        elixir: ['1.19']
-        otp: ['27']
 
     steps:
       - name: Checkout code
@@ -75,8 +78,8 @@ jobs:
       - name: Set up Elixir
         uses: erlef/setup-beam@v1
         with:
-          elixir-version: ${{matrix.elixir}}
-          otp-version: ${{matrix.otp}}
+          elixir-version: '1.19'
+          otp-version: '27'
 
       - name: Restore dependencies cache
         uses: actions/cache@v4
@@ -84,18 +87,23 @@ jobs:
           path: |
             deps
             _build
-          key: ${{ runner.os }}-mix-${{ matrix.elixir }}-${{ matrix.otp }}-${{ hashFiles('**/mix.lock') }}
+          key: ${{ runner.os }}-mix-1.19-27-${{ hashFiles('**/mix.lock') }}
           restore-keys: |
-            ${{ runner.os }}-mix-${{ matrix.elixir }}-${{ matrix.otp }}-
+            ${{ runner.os }}-mix-1.19-27-
+
+      - name: Restore PLT cache
+        uses: actions/cache@v4
+        with:
+          path: priv/plts
+          key: ${{ runner.os }}-plt-1.19-27-${{ hashFiles('**/mix.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-plt-1.19-27-
 
       - name: Install dependencies
         run: mix deps.get
 
-      - name: Check formatting
-        run: mix format --check-formatted
-
-      - name: Run Credo
-        run: mix credo --strict
+      - name: Run quality checks
+        run: mix quality.ci
 
   coverage:
     name: Test Coverage

From 1c2b79802046448104b350b91f17a0dcdd64a1bf Mon Sep 17 00:00:00 2001
From: Chris Xu <git@chrisxu.me>
Date: Tue, 20 Jan 2026 14:59:03 +0100
Subject: [PATCH 6/8] style: restore matrix syntax for consistency

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
---
 .github/workflows/ci.yml | 48 ++++++++++++++++++++++------------------
 1 file changed, 26 insertions(+), 22 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f0197bc..cc4c11f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -15,25 +15,25 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        elixir: ['1.15', '1.16', '1.17', '1.18', '1.19']
-        otp: ['25', '26', '27', '28']
+        elixir: ["1.15", "1.16", "1.17", "1.18", "1.19"]
+        otp: ["25", "26", "27", "28"]
         exclude:
           # Elixir 1.15 doesn't support OTP 27+
-          - elixir: '1.15'
-            otp: '27'
-          - elixir: '1.15'
-            otp: '28'
+          - elixir: "1.15"
+            otp: "27"
+          - elixir: "1.15"
+            otp: "28"
           # Elixir 1.16 doesn't support OTP 27+
-          - elixir: '1.16'
-            otp: '27'
-          - elixir: '1.16'
-            otp: '28'
+          - elixir: "1.16"
+            otp: "27"
+          - elixir: "1.16"
+            otp: "28"
           # Elixir 1.17 doesn't support OTP 28
-          - elixir: '1.17'
-            otp: '28'
+          - elixir: "1.17"
+            otp: "28"
           # Elixir 1.19 doesn't support OTP 25
-          - elixir: '1.19'
-            otp: '25'
+          - elixir: "1.19"
+            otp: "25"
 
     steps:
       - name: Checkout code
@@ -70,6 +70,10 @@ jobs:
   quality:
     name: Code Quality
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        elixir: ["1.19"]
+        otp: ["28"]
 
     steps:
       - name: Checkout code
@@ -78,8 +82,8 @@ jobs:
       - name: Set up Elixir
         uses: erlef/setup-beam@v1
         with:
-          elixir-version: '1.19'
-          otp-version: '27'
+          elixir-version: ${{ matrix.elixir }}
+          otp-version: ${{ matrix.otp }}
 
       - name: Restore dependencies cache
         uses: actions/cache@v4
@@ -87,17 +91,17 @@ jobs:
           path: |
             deps
             _build
-          key: ${{ runner.os }}-mix-1.19-27-${{ hashFiles('**/mix.lock') }}
+          key: ${{ runner.os }}-mix-${{ matrix.elixir }}-${{ matrix.otp }}-${{ hashFiles('**/mix.lock') }}
           restore-keys: |
-            ${{ runner.os }}-mix-1.19-27-
+            ${{ runner.os }}-mix-${{ matrix.elixir }}-${{ matrix.otp }}-
 
       - name: Restore PLT cache
         uses: actions/cache@v4
         with:
           path: priv/plts
-          key: ${{ runner.os }}-plt-1.19-27-${{ hashFiles('**/mix.lock') }}
+          key: ${{ runner.os }}-plt-${{ matrix.elixir }}-${{ matrix.otp }}-${{ hashFiles('**/mix.lock') }}
           restore-keys: |
-            ${{ runner.os }}-plt-1.19-27-
+            ${{ runner.os }}-plt-${{ matrix.elixir }}-${{ matrix.otp }}-
 
       - name: Install dependencies
         run: mix deps.get
@@ -110,8 +114,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        elixir: ['1.19']
-        otp: ['27']
+        elixir: ["1.19"]
+        otp: ["27"]
 
     steps:
       - name: Checkout code

From c37aabb68ee403eccc05e1d5490c9bb5aae614fb Mon Sep 17 00:00:00 2001
From: Chris Xu <git@chrisxu.me>
Date: Tue, 20 Jan 2026 15:51:22 +0100
Subject: [PATCH 7/8] fix(dialyzer): resolve all dialyzer warnings

- Remove dead code patterns that could never match:
  - detect_root_type([]) in structural_parser.ex
  - append_lines(writer, [], _depth) in objects.ex
- Define precise types to fix supertype warnings:
  - Add @type decoded in decode.ex
  - Add @type validated in options.ex
  - Use nonempty_list() in arrays.ex and strings.ex specs
- Fix protocol fallback no_return warning:
  - Remove @spec from Toon.Encoder.Any encode/2 functions
  - Add .dialyzer_ignore.exs for intentional no_return behavior
- Add edge case tests to verify behavior before dead code removal

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
---
 .dialyzer_ignore.exs                 |  8 +++++
 README.md                            |  1 +
 lib/toon/decode/decode.ex            |  7 +++--
 lib/toon/decode/structural_parser.ex |  2 --
 lib/toon/encode/arrays.ex            |  3 +-
 lib/toon/encode/objects.ex           | 20 ++++--------
 lib/toon/encode/options.ex           | 11 ++++++-
 lib/toon/encode/strings.ex           |  2 +-
 lib/toon/encoder.ex                  |  2 --
 mix.exs                              |  3 +-
 test/toon/edge_cases_test.exs        | 47 ++++++++++++++++++++++++++++
 11 files changed, 82 insertions(+), 24 deletions(-)
 create mode 100644 .dialyzer_ignore.exs
 create mode 100644 test/toon/edge_cases_test.exs

diff --git a/.dialyzer_ignore.exs b/.dialyzer_ignore.exs
new file mode 100644
index 0000000..e210d92
--- /dev/null
+++ b/.dialyzer_ignore.exs
@@ -0,0 +1,8 @@
+# Dialyzer warnings to ignore
+#
+# Protocol fallback implementation that intentionally raises.
+# This is expected behavior - the Any implementation raises Protocol.UndefinedError
+# when a struct doesn't have an explicit Toon.Encoder implementation.
+[
+  {"lib/toon/encoder.ex", :no_return}
+]
diff --git a/README.md b/README.md
index 5e28423..5169ac9 100644
--- a/README.md
+++ b/README.md
@@ -2,6 +2,7 @@
 
 [![Hex.pm](https://img.shields.io/hexpm/v/toon.svg)](https://hex.pm/packages/toon)
 [![Documentation](https://img.shields.io/badge/docs-hexdocs-blue.svg)](https://hexdocs.pm/toon)
+[![Coverage Status](https://coveralls.io/repos/github/xu-chris/toon_ex/badge.svg?branch=main)](https://coveralls.io/github/xu-chris/toon_ex?branch=main)
 
 **TOON (Token-Oriented Object Notation)** encoder and decoder for Elixir.
 
diff --git a/lib/toon/decode/decode.ex b/lib/toon/decode/decode.ex
index 812abe4..1d88351 100644
--- a/lib/toon/decode/decode.ex
+++ b/lib/toon/decode/decode.ex
@@ -8,6 +8,9 @@ defmodule Toon.Decode do
   alias Toon.Decode.{Options, StructuralParser}
   alias Toon.DecodeError
 
+  @typedoc "Decoded TOON value"
+  @type decoded :: nil | boolean() | binary() | number() | list() | map()
+
   @doc """
   Decodes a TOON format string to Elixir data.
 
@@ -94,7 +97,7 @@ defmodule Toon.Decode do
       iex> Toon.Decode.decode!("count: 42")
       %{"count" => 42}
   """
-  @spec decode!(String.t(), keyword()) :: term()
+  @spec decode!(String.t(), keyword()) :: decoded()
   def decode!(string, opts \\ []) when is_binary(string) do
     case decode(string, opts) do
       {:ok, result} -> result
@@ -104,7 +107,7 @@ defmodule Toon.Decode do
 
   # Private functions
 
-  @spec do_decode(String.t(), map()) :: term()
+  @spec do_decode(String.t(), map()) :: decoded()
   defp do_decode(string, opts) do
     # Use structural parser for full TOON support
     case StructuralParser.parse(string, opts) do
diff --git a/lib/toon/decode/structural_parser.ex b/lib/toon/decode/structural_parser.ex
index 03f75fb..11a8468 100644
--- a/lib/toon/decode/structural_parser.ex
+++ b/lib/toon/decode/structural_parser.ex
@@ -157,8 +157,6 @@ defmodule Toon.Decode.StructuralParser do
     end
   end
 
-  defp detect_root_type([]), do: {:object, nil}
-
   # Parse root primitive value (single value without key)
   defp parse_root_primitive([%{content: content}], _opts) do
     # For root primitives, we parse directly without parser combinator
diff --git a/lib/toon/encode/arrays.ex b/lib/toon/encode/arrays.ex
index e4b82a6..c8766c7 100644
--- a/lib/toon/encode/arrays.ex
+++ b/lib/toon/encode/arrays.ex
@@ -49,7 +49,8 @@ defmodule Toon.Encode.Arrays do
       iex> IO.iodata_to_binary(result)
       "items[0]:"
   """
-  @spec encode_empty(String.t(), String.t() | nil) :: [iodata()]
+  @spec encode_empty(String.t(), String.t() | nil) ::
+          nonempty_list(nonempty_list(binary() | nonempty_list(binary())))
   def encode_empty(key, length_marker \\ nil) do
     marker = format_length_marker(0, length_marker)
     [[Strings.encode_key(key), "[", marker, "]", Constants.colon()]]
diff --git a/lib/toon/encode/objects.ex b/lib/toon/encode/objects.ex
index 1ecd621..e2c7610 100644
--- a/lib/toon/encode/objects.ex
+++ b/lib/toon/encode/objects.ex
@@ -17,7 +17,7 @@ defmodule Toon.Encode.Objects do
       iex> Toon.Encode.Objects.encode(map, 0, opts)
 
   """
-  @spec encode(map(), non_neg_integer(), map()) :: iodata()
+  @spec encode(map(), non_neg_integer(), map()) :: [iodata()]
   def encode(map, depth, opts) when is_map(map) do
     writer = Writer.new(opts.indent)
 
@@ -103,22 +103,14 @@ defmodule Toon.Encode.Objects do
 
   # Private helpers
 
-  defp append_lines(writer, [], _depth), do: writer
-
-  defp append_lines(writer, lines, depth) when is_list(lines) do
+  defp append_lines(writer, [header | data_rows], depth) do
     # For arrays, the first line is the header at current depth
     # Subsequent lines (data rows for tabular format) should be one level deeper
-    case lines do
-      [header | data_rows] ->
-        writer = Writer.push(writer, header, depth)
-
-        Enum.reduce(data_rows, writer, fn row, acc ->
-          Writer.push(acc, row, depth + 1)
-        end)
+    writer = Writer.push(writer, header, depth)
 
-      [] ->
-        writer
-    end
+    Enum.reduce(data_rows, writer, fn row, acc ->
+      Writer.push(acc, row, depth + 1)
+    end)
   end
 
   defp append_iodata(writer, iodata, _base_depth) do
diff --git a/lib/toon/encode/options.ex b/lib/toon/encode/options.ex
index 0eccd17..13efad6 100644
--- a/lib/toon/encode/options.ex
+++ b/lib/toon/encode/options.ex
@@ -5,6 +5,15 @@ defmodule Toon.Encode.Options do
 
   alias Toon.Constants
 
+  @typedoc "Validated encoding options"
+  @type validated :: %{
+          indent: pos_integer(),
+          delimiter: String.t(),
+          length_marker: String.t() | nil,
+          key_order: term(),
+          indent_string: String.t()
+        }
+
   @options_schema [
     indent: [
       type: :pos_integer,
@@ -90,7 +99,7 @@ defmodule Toon.Encode.Options do
       iex> Toon.Encode.Options.validate!(indent: 4)
       %{indent: 4, delimiter: ",", length_marker: nil, indent_string: "    "}
   """
-  @spec validate!(keyword()) :: map()
+  @spec validate!(keyword()) :: validated()
   def validate!(opts) when is_list(opts) do
     case validate(opts) do
       {:ok, validated} ->
diff --git a/lib/toon/encode/strings.ex b/lib/toon/encode/strings.ex
index d7a7133..b3f2aaa 100644
--- a/lib/toon/encode/strings.ex
+++ b/lib/toon/encode/strings.ex
@@ -24,7 +24,7 @@ defmodule Toon.Encode.Strings do
       iex> Toon.Encode.Strings.encode_string("line1\\nline2") |> IO.iodata_to_binary()
       ~s("line1\\\\nline2")
   """
-  @spec encode_string(String.t(), String.t()) :: iodata()
+  @spec encode_string(String.t(), String.t()) :: binary() | nonempty_list(binary())
   def encode_string(string, delimiter \\ ",") when is_binary(string) do
     if safe_unquoted?(string, delimiter) do
       string
diff --git a/lib/toon/encoder.ex b/lib/toon/encoder.ex
index f6663a3..dbab951 100644
--- a/lib/toon/encoder.ex
+++ b/lib/toon/encoder.ex
@@ -51,7 +51,6 @@ defimpl Toon.Encoder, for: Any do
     end
   end
 
-  @spec encode(struct(), keyword()) :: no_return()
   def encode(%_{} = struct, _opts) do
     raise Protocol.UndefinedError,
       protocol: @protocol,
@@ -71,7 +70,6 @@ defimpl Toon.Encoder, for: Any do
       """
   end
 
-  @spec encode(term(), keyword()) :: no_return()
   def encode(value, _opts) do
     raise Protocol.UndefinedError,
       protocol: @protocol,
diff --git a/mix.exs b/mix.exs
index 60bb432..94fd15e 100644
--- a/mix.exs
+++ b/mix.exs
@@ -26,7 +26,8 @@ defmodule Toon.MixProject do
           :underspecs,
           :unmatched_returns,
           :unknown
-        ]
+        ],
+        ignore_warnings: ".dialyzer_ignore.exs"
       ],
       aliases: aliases()
     ]
diff --git a/test/toon/edge_cases_test.exs b/test/toon/edge_cases_test.exs
new file mode 100644
index 0000000..674b226
--- /dev/null
+++ b/test/toon/edge_cases_test.exs
@@ -0,0 +1,47 @@
+defmodule Toon.EdgeCasesTest do
+  @moduledoc """
+  Tests for edge cases to ensure behavior is preserved after removing dead code.
+  """
+  use ExUnit.Case, async: true
+
+  describe "empty input decoding" do
+    test "empty string decodes to empty map" do
+      assert {:ok, %{}} = Toon.decode("")
+    end
+
+    test "whitespace-only string decodes to empty map" do
+      assert {:ok, %{}} = Toon.decode("   ")
+      assert {:ok, %{}} = Toon.decode("\n\n")
+      assert {:ok, %{}} = Toon.decode("  \n  \n  ")
+    end
+  end
+
+  describe "empty data encoding" do
+    test "empty map encodes to empty string" do
+      assert "" = Toon.encode!(%{})
+    end
+
+    test "empty list encodes with zero-length header" do
+      result = Toon.encode!(%{"items" => []})
+      assert result =~ "items[0]:"
+    end
+
+    test "nested empty structures" do
+      # Empty map inside map - produces key with colon
+      assert "nested:" = Toon.encode!(%{"nested" => %{}})
+
+      # Empty list inside map
+      result = Toon.encode!(%{"items" => []})
+      assert result =~ "[0]:"
+    end
+  end
+
+  describe "decode and encode roundtrip for edge cases" do
+    test "empty map roundtrip" do
+      original = %{}
+      encoded = Toon.encode!(original)
+      {:ok, decoded} = Toon.decode(encoded)
+      assert decoded == original
+    end
+  end
+end

From 4b742b545266a13fabb5b8a37a01459115669285 Mon Sep 17 00:00:00 2001
From: Kentaro Kuribayashi <kentarok@gmail.com>
Date: Wed, 28 Jan 2026 16:43:55 +0900
Subject: [PATCH 8/8] Update README.md

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 5169ac9..c48e0a4 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 
 [![Hex.pm](https://img.shields.io/hexpm/v/toon.svg)](https://hex.pm/packages/toon)
 [![Documentation](https://img.shields.io/badge/docs-hexdocs-blue.svg)](https://hexdocs.pm/toon)
-[![Coverage Status](https://coveralls.io/repos/github/xu-chris/toon_ex/badge.svg?branch=main)](https://coveralls.io/github/xu-chris/toon_ex?branch=main)
+[![Coverage Status](https://coveralls.io/repos/github/kentaro/toon_ex/badge.svg?branch=main)](https://coveralls.io/github/xu-chris/toon_ex?branch=main)
 
 **TOON (Token-Oriented Object Notation)** encoder and decoder for Elixir.