fix: compilation error when no JSON libraries are listed in deps

lib/protox/define_message.ex

@@ -1,13 +1,10 @@
 defmodule Protox.DefineMessage do
   @moduledoc false
 
-  @default_json_library Jason
-
   alias Protox.Field
 
   def define(messages, opts \\ []) do
     keep_unknown_fields = Keyword.get(opts, :keep_unknown_fields, true)
-    json_library = Keyword.get(opts, :json_library, @default_json_library)
 
     for msg = %Protox.Message{} <- messages do
       fields = Enum.sort(msg.fields, &(&1.tag < &2.tag))
@@ -19,7 +16,7 @@ defmodule Protox.DefineMessage do
       unknown_fields_funs = make_unknown_fields_funs(unknown_fields, keep_unknown_fields)
       required_fields_fun = make_required_fields_fun(required_fields)
       fields_access_funs = make_fields_access_funs(fields)
-      json_funs = make_json_funs(msg.name, json_library)
+      json_funs = make_json_funs(msg.name, Protox.JsonLibrary.get_wrapper(opts))
       default_fun = make_default_funs(fields)
       syntax_fun = make_syntax_fun(msg.syntax)
       file_options_fun = make_file_options_fun(msg)
@@ -96,9 +93,31 @@ defmodule Protox.DefineMessage do
     end
   end
 
-  defp make_json_funs(msg_name, json_library) do
-    json_library_wrapper = Protox.JsonLibrary.get_wrapper(json_library)
+  defp make_json_funs(_msg_name, _json_library_wrapper = nil) do
+    quote do
+      @spec json_decode(iodata()) :: {:error, any()}
+      def json_decode(_input) do
+        {:error, Protox.JsonLibraryError.new()}
+      end
+
+      @spec json_decode!(iodata()) :: no_return()
+      def json_decode!(_input) do
+        raise Protox.JsonLibraryError.new()
+      end
+
+      @spec json_encode(struct()) :: {:error, any()}
+      def json_encode(_msg) do
+        {:error, Protox.JsonLibraryError.new()}
+      end
+
+      @spec json_encode!(struct()) :: no_return()
+      def json_encode!(_msg) do
+        raise Protox.JsonLibraryError.new()
+      end
+    end
+  end
 
+  defp make_json_funs(msg_name, json_library_wrapper) do
     quote do
       @spec json_decode(iodata()) :: {:ok, struct()} | {:error, any()}
       def json_decode(input) do

lib/protox/jason.ex

@@ -1,24 +1,27 @@
 defmodule Protox.Jason do
   @moduledoc false
-  @behaviour Protox.JsonLibrary
 
-  @impl true
-  def decode!(iodata) do
-    try do
-      Jason.decode!(iodata)
-    rescue
-      e in [Jason.DecodeError, Protocol.UndefinedError] ->
-        reraise Protox.JsonDecodingError.new(Exception.message(e)), __STACKTRACE__
+  if Code.ensure_loaded?(Jason) do
+    @behaviour Protox.JsonLibrary
+
+    @impl true
+    def decode!(iodata) do
+      try do
+        Jason.decode!(iodata)
+      rescue
+        e in [Jason.DecodeError, Protocol.UndefinedError] ->
+          reraise Protox.JsonDecodingError.new(Exception.message(e)), __STACKTRACE__
+      end
     end
-  end
 
-  @impl true
-  def encode!(term) do
-    try do
-      Jason.encode_to_iodata!(term)
-    rescue
-      e in Jason.EncodeError ->
-        reraise Protox.JsonEncodingError.new(Exception.message(e)), __STACKTRACE__
+    @impl true
+    def encode!(term) do
+      try do
+        Jason.encode_to_iodata!(term)
+      rescue
+        e in Jason.EncodeError ->
+          reraise Protox.JsonEncodingError.new(Exception.message(e)), __STACKTRACE__
+      end
     end
   end
 end

lib/protox/json_library.ex

@@ -3,6 +3,8 @@ defmodule Protox.JsonLibrary do
   The behaviour to implement when wrapping a JSON library.
   """
 
+  @default_json_library Jason
+
   @doc """
   Should wrap any exception of the underlying library in Protox.JsonDecodingError.
   """
@@ -14,11 +16,13 @@ defmodule Protox.JsonLibrary do
   @callback encode!(term()) :: iodata() | no_return()
 
   @doc false
-  def get_wrapper(json_library) do
+  def get_wrapper(opts) do
+    json_library = Keyword.get(opts, :json_library, @default_json_library)
+
     if Code.ensure_loaded?(json_library) do
       Module.concat(Protox, json_library)
     else
-      raise Protox.JsonLibraryError.new()
+      nil
     end
   end
 end

lib/protox/poison.ex

@@ -1,24 +1,27 @@
 defmodule Protox.Poison do
   @moduledoc false
-  @behaviour Protox.JsonLibrary
 
-  @impl true
-  def decode!(iodata) do
-    try do
-      Poison.decode!(iodata)
-    rescue
-      e in [Poison.DecodeError, Poison.ParseError] ->
-        reraise Protox.JsonDecodingError.new(Exception.message(e)), __STACKTRACE__
+  if Code.ensure_loaded?(Poison) do
+    @behaviour Protox.JsonLibrary
+
+    @impl true
+    def decode!(iodata) do
+      try do
+        Poison.decode!(iodata)
+      rescue
+        e in [Poison.DecodeError, Poison.ParseError] ->
+          reraise Protox.JsonDecodingError.new(Exception.message(e)), __STACKTRACE__
+      end
     end
-  end
 
-  @impl true
-  def encode!(term) do
-    try do
-      Poison.encode!(term)
-    rescue
-      e in Poison.EncodeError ->
-        reraise Protox.JsonEncodingError.new(Exception.message(e)), __STACKTRACE__
+    @impl true
+    def encode!(term) do
+      try do
+        Poison.encode!(term)
+      rescue
+        e in Poison.EncodeError ->
+          reraise Protox.JsonEncodingError.new(Exception.message(e)), __STACKTRACE__
+      end
     end
   end
 end