feat: Tesla.Middleware.JSON: Add support for Elixir 1.18's JSON module (#747)

Author: adamu

lib/tesla/middleware/json.ex

@@ -12,6 +12,15 @@ defmodule Tesla.Middleware.JSON do
   mix deps.compile tesla
   ```
 
+  > #### Using built-in `JSON` from Elixir 1.18 {: .info}
+  >
+  > This middleware supports the built-in `JSON` module introduced in ELixir 1.18, but for historical
+  > reasons is it not the default. To use it, set it as the `:engine`:
+  >
+  >     {Tesla.Middleware.JSON, engine: JSON}
+  >
+  > For more advanced usage using custom encoders/decodes, provide the `:encode` and `:decode` anonymous functions instead.
+
   If you only need to encode the request body or decode the response body,
   you can use `Tesla.Middleware.EncodeJson` or `Tesla.Middleware.DecodeJson` directly instead.
 
@@ -24,6 +33,8 @@ defmodule Tesla.Middleware.JSON do
         # use jason engine
         Tesla.Middleware.JSON,
         # or
+        {Tesla.Middleware.JSON, engine: JSON}
+        # or
         {Tesla.Middleware.JSON, engine: JSX, engine_opts: [strict: [:comments]]},
         # or
         {Tesla.Middleware.JSON, engine: Poison, engine_opts: [keys: :atoms]},
@@ -39,7 +50,7 @@ defmodule Tesla.Middleware.JSON do
   - `:decode` - decoding function
   - `:encode` - encoding function
   - `:encode_content_type` - content-type to be used in request header
-  - `:engine` - encode/decode engine, e.g `Jason`, `Poison` or `JSX`  (defaults to Jason)
+  - `:engine` - encode/decode engine, e.g `JSON`, `Jason`, `Poison` or `JSX`  (defaults to Jason)
   - `:engine_opts` - optional engine options
   - `:decode_content_types` - list of additional decodable content-types
   """
@@ -172,7 +183,14 @@ defmodule Tesla.Middleware.JSON do
     if fun = opts[op] do
       fun.(data)
     else
-      engine = Keyword.get(opts, :engine, @default_engine)
+      engine =
+        case Keyword.fetch(opts, :engine) do
+          # Special case for JSON, which doesn't have encode/2 nor return {:ok, json}
+          {:ok, JSON} -> Tesla.Middleware.JSON.JSONAdapter
+          {:ok, engine} -> engine
+          :error -> @default_engine
+        end
+
       opts = Keyword.get(opts, :engine_opts, [])
 
       apply(engine, op, [data, opts])

lib/tesla/middleware/json/json_adapter.ex

@@ -0,0 +1,18 @@
+if Code.ensure_loaded?(JSON) do
+  defmodule Tesla.Middleware.JSON.JSONAdapter do
+    @moduledoc false
+    # An adapter for Elixir's built-in JSON module introduced in Elixir 1.18
+    # that adjusts for Tesla's assumptions about a JSON engine, which are not satisfied by
+    # Elixir's JSON module. The assumptions are:
+    # - the module provides encode/2 and decode/2 functions
+    # - the 2nd argument to the functions is opts
+    # - the functions return {:ok, json} or {:error, reason} - not the case for JSON.encode!/2
+    #
+    # We do not support custom encoders and decoders.
+    # The purpose of this adapter is to allow `engine: JSON` to be set easily. If more advanced
+    # customization is required, the `:encode` and `:decode` functions can be supplied to the
+    # middleware instead of the `:engine` option.
+    def encode(data, _opts), do: {:ok, JSON.encode!(data)}
+    def decode(binary, _opts), do: JSON.decode(binary)
+  end
+end

test/tesla/middleware/json/json_adapter_test.exs

@@ -0,0 +1,23 @@
+if Code.ensure_loaded?(JSON) do
+  defmodule Tesla.Middleware.JSON.JSONAdapterTest do
+    use ExUnit.Case, async: true
+
+    alias Tesla.Middleware.JSON.JSONAdapter
+
+    describe "encode/2" do
+      test "encodes as expected with default encoder" do
+        assert {:ok, ~S({"hello":"world"})} == JSONAdapter.encode(%{hello: "world"}, [])
+      end
+    end
+
+    describe "decode/2" do
+      test "returns {:ok, term} on success" do
+        assert {:ok, %{"hello" => "world"}} = JSONAdapter.decode(~S({"hello":"world"}), [])
+      end
+
+      test "returns {:error, reason} on failure" do
+        assert {:error, {:invalid_byte, _, _}} = JSONAdapter.decode("invalid_json", [])
+      end
+    end
+  end
+end

test/tesla/middleware/json_test.exs

@@ -299,4 +299,52 @@ defmodule Tesla.Middleware.JsonTest do
       assert env.body == %{"value" => 123}
     end
   end
+
+  if Code.ensure_loaded?(JSON) do
+    describe "Engine: JSON" do
+      defmodule JSONClient do
+        use Tesla
+
+        plug Tesla.Middleware.JSON, engine: JSON
+
+        adapter fn env ->
+          assert env.body == ~S({"hello":"world"})
+
+          {:ok,
+           %{
+             env
+             | status: 200,
+               headers: [{"content-type", "application/json"}],
+               body: ~s|{"value": 123}|
+           }}
+        end
+      end
+
+      test "encodes/decodes as expected" do
+        assert {:ok, env} = JSONClient.post("/json", %{hello: "world"})
+        assert env.body == %{"value" => 123}
+      end
+
+      defmodule Decoder do
+        use Tesla
+
+        plug Tesla.Middleware.DecodeJson, engine: JSON
+
+        adapter fn env ->
+          {:ok,
+           %{
+             env
+             | status: 200,
+               headers: [{"content-type", "application/json"}],
+               body: ~s|{"value": 123}|
+           }}
+        end
+      end
+
+      test "decodes as expected" do
+        assert {:ok, env} = Decoder.get("/json")
+        assert env.body == %{"value" => 123}
+      end
+    end
+  end
 end