The struct method takes a module and some params and coerces the params into the struct defined in the module. You need to define a function masonstruct/0 on the module which defines how to coerce the params, e.g.:

defmodule User do
  defstruct [ :age, :size, :gpa, :active, :role, :STATUS, :created_at ]

  def masonstruct do
    %{
      age: Integer,
      gpa: Float,
      active: Boolean,
      role: Atom,
      status: &coerce_status/1,
      createdAt: &({ :created_at, elem(DateTime.from_iso8601(&1), 1) })
    }
  end

  def coerce_status(value) do
    case value do
      "active" -> { :STATUS, :online }
      _ -> { :STATUS, value }
    end
  end
end

Simple coercion

Mason expects values to be strings by default and converts them to Integer, Boolean, Float or Atom.

iex> Mason.struct User, %{ age: "23", gpa: "4.0", active: "true", role: "admin" }
%User{
  STATUS: nil,
  active: true,
  age: 23,
  created_at: nil,
  gpa: 4.0,
  role: :admin,
  size: nil
}

Dynamic coercion

You can supply a function to do the coercion. It is also possible to map keys this way. The function takes the value of the field as argument.

Consider this, e.g.:

def masonstruct do
{
  createdAt: &({ :created_at, elem(DateTime.from_iso8601(&1), 1) })
}
end

iex> user = Mason.struct User, %{ createdAt: "2016-02-29T12:30:30+00:00" }
iex> user.created_at
#DateTime<2016-02-29 12:30:30Z>

Coercing lists

You can coerce into lists by supplying an array with the type. You can supply Elixir’s simple types as well as a function. Lists can be nested. Consider the following masonstruct definition:

def masonstruct do
{
  available: [ Boolean ],
  dates: [ &(elem(DateTime.from_iso8601(&1), 1)) ]
}


iex> Mason.struct User, %{ available: [ "true", "false"] }
%User{
  STATUS: nil,
  active: nil,
  age: nil,
  available: [true, false],
  created_at: nil,
  data: nil,
  dates: nil,
  gpa: nil,
  role: nil,
  roles: nil,
  size: nil,
  user_ids: nil
}

iex> user = Mason.struct User, %{ dates: [ "2016-02-29T12:30:30+00:00" ] }
iex> List.first user.dates
#DateTime<2016-02-29 12:30:30Z>