Protocol controlling how a value is encoded to JSON.

Deriving

The protocol allows leveraging the Elixir’s @derive feature to simplify protocol implementation in trivial cases. Accepted options are:

• :only - encodes only values of specified keys.
• :except - encodes all struct fields except specified keys.

By default all keys except the :__struct__ key are encoded.

Example

Let’s assume a presence of the following struct:

defmodule Test do
  defstruct [:foo, :bar, :baz]
end

If we were to call @derive Jason.Encoder just before defstruct, an implementaion similar to the follwing implementation would be generated:

defimpl Jason.Encoder, for: Test do
  def encode(value, opts) do
    Jason.Encode.map(Map.take(value, [:foo, :bar, :baz]), opts)
  end
end

If we called @derive {Jason.Encoder, only: [:foo]}, an implementation similar to the following implementation would be genrated:

defimpl Jason.Encoder, for: Test do
  def encode(value, opts) do
    Jason.Encode.map(Map.take(value, [:foo]), opts)
  end
end

If we called @derive {Jason.Encoder, except: [:foo]}, an implementation similar to the following implementation would be generated:

defimpl Jason.Encoder, for: Test do
  def encode(value, opts) do
    Jason.Encode.map(Map.take(value, [:bar, :baz]), opts)
  end
end

The actually generated implementations are more efficient computing some data during compilation similar to the macros from the Jason.Helpers module.

Explicit implementation

If you wish to implement the protocol fully yourself, it is advised to use functions from the Jason.Encode module to do the actual iodata generation - they are highly optimized and verified to always produce valid JSON.