Document Jason.Fragment (#166)

* Add a test

* Document Jason.Fragment

* Add something in the readme

README.md

@@ -135,6 +135,20 @@ Protocol.derive(Jason.Encoder, NameOfTheStruct, only: [...])
 Protocol.derive(Jason.Encoder, NameOfTheStruct)
 ```
 
+## Injecting an already encoded JSON inside a to-be-encoded structure
+
+If parts of the to-be-encoded structure are already JSON-encoded, you can
+use `Jason.Fragment` to mark the parts as already encoded, and avoid a
+decoding/encoding roundtrip.
+
+```elixir
+already_encoded_json = Jason.encode!(%{hello: "world"})
+Jason.encode!(%{foo: Jason.Fragment.new(already_encoded_json)})
+````
+
+This feature is especially useful if you need to cache a part of the JSON,
+or if it is already provided by another system (e.g. `jsonb_agg` with Postgres).
+
 ## License
 
 Jason is released under the Apache License 2.0 - see the [LICENSE](LICENSE) file.

lib/fragment.ex

@@ -1,4 +1,14 @@
 defmodule Jason.Fragment do
+  @moduledoc ~S"""
+  Provides a way to inject an already-encoded JSON structure into a
+  to-be-encoded structure in optimized fashion.
+
+  This avoids a decoding/encoding round-trip for the subpart.
+
+  This feature can be used for caching parts of the JSON, or delegating
+  the generation of the JSON to a third-party system (e.g. Postgres).
+  """
+
   defstruct [:encode]
 
   def new(iodata) when is_list(iodata) or is_binary(iodata) do

test/encode_test.exs

@@ -115,6 +115,11 @@ defmodule Jason.EncoderTest do
     assert to_json(multi_key_map) == ~s({"foo":"foo1","foo":"foo2"})
   end
 
+  test "Fragment" do
+    pre_encoded_json = Jason.encode!(%{hello: "world", test: 123})
+    assert to_json(%{foo: Jason.Fragment.new(pre_encoded_json)}) == ~s({"foo":{"hello":"world","test":123}})
+  end
+
   defmodule Derived do
     @derive Encoder
     defstruct name: ""