forma
[swe] verb. /fạ̊r:ma/
to adjust; adapt

forma

Applies typespecs to JSON-like data.

This module can parse JSON-like data (such as maps with key strings) into a more structured form by trying to map it to conform to a module's typespec.

This can generally be useful when interfacing with external data sources that provide you data as JSON or MessagePack, but that you wish to transform into either proper structs or richer data types without a native JSON representation (such as dates or sets) in your application.

It is heavily inspired by Go's way of dealing with JSON data.

defmodule User do
  defstruct [:id, :name, :age, :gender]

  @type t :: %__MODULE__{
    id: String.t,
    name: String.t,
    age: non_neg_integer(),
    gender: :male | :female | :other | :prefer_not_to_say
  }
end

Forma.parse(%{"id" => "1", "name" => "Fredrik", "age" => 30, "gender" => "male"}, User)
# => {:ok, %User{id: "1", name: "Fredrik", age: 30, gender: :male}}

Forma tries to figure out how to translate its input to a typespec. However, not all types have natural representations in JSON, for example dates, or don't want to expose their internals (opaque types).

If you're in control of the module defining the type, you can implement the __forma__/2 function to handle parsing input to your desired type

defmodule App.Date do
  @opaque t :: Date

  # first argument is the type to be parsed in this module
  def __forma__(:t, input) do
    case Date.from_iso8601(input) do
      {:ok, date} -> date
      {:error, reason} -> raise reason
    end
  end
end

If you're not in control of the module, you can pass a parser along as an optional argument,

defmodule LogRow do
  defstruct [:log, :timestamp]

  type t :: %__MODULE__{
    log: String.t,
    timestamp: NaiveDateTime.t
  }
end

date = fn input ->
  case NaiveDateTime.from_iso8601(input) do
    {:ok, datetime} -> datetime
    {:error, err} -> raise err
  end
end
parsers = %{{NaiveDateTime, :t} => date}
Forma.parse(%{"log" => "An error occurred", "timestamp" => "2015-01-23 23:50:07"}, LogRow, parsers)

The number of arguments to the parser functions depends on if the type is parameterized or not (MapSet.t vs MapSet.t(integer)).

Installation

If available in Hex, the package can be installed by adding forma to your list of dependencies in mix.exs:

def deps do
  [
    {:forma, "~> 0.7.1"}
  ]
end

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/forma.