Protocol controlling how a value is encoded to JSON.

deriving

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

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 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, [:foo, :bar, :baz]), opts)
  end
end

If we called @derive {Jason.Encoder, only: [: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, [: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

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.