Implement JSON.Encoder if available (#215)

lib/decimal.ex

@@ -2095,3 +2095,12 @@ defimpl String.Chars, for: Decimal do
     Decimal.to_string(dec)
   end
 end
+
+# TODO: remove when we require Elixir 1.18
+if Code.ensure_loaded?(JSON.Encoder) and function_exported?(JSON.Encoder, :encode, 2) do
+  defimpl JSON.Encoder, for: Decimal do
+    def encode(decimal, _encoder) do
+      [?", Decimal.to_string(decimal), ?"]
+    end
+  end
+end

test/decimal_test.exs

@@ -934,4 +934,10 @@ defmodule DecimalTest do
       Decimal.sqrt(Decimal.new(d(3, 1, -1)))
     end
   end
+
+  if Version.match?(System.version(), ">= 1.18.0-rc") do
+    test "JSON.Encoder implementation" do
+      assert JSON.encode!(%{x: Decimal.new("1.0")}) == "{\"x\":\"1.0\"}"
+    end
+  end
 end