/app # iex -S mix
Erlang/OTP 21 [erts-10.3.5.1] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe]

Interactive Elixir (1.7.4) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> defmodule TestStruct do
...(1)>   defstruct field_one: nil,
...(1)>             field_two: nil,
...(1)>             field_three: nil,
...(1)>             field_four: nil
...(1)>   use ExConstructor
...(1)> end
{:module, TestStruct,
 <<70, 79, 82, 49, 0, 0, 7, 224, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0, 249,
   0, 0, 0, 23, 17, 69, 108, 105, 120, 105, 114, 46, 84, 101, 115, 116, 83, 116,
   114, 117, 99, 116, 8, 95, 95, 105, 110, ...>>, :ok}
iex(2)> 
nil
iex(3)> TestStruct.new(%{"field_one" => "a", "fieldTwo" => "b", :field_three => "c", :FieldFour => "d"})
%TestStruct{field_four: nil, field_one: "a", field_three: "c", field_two: "b"}