Build a product type

Includes:

• Struct
• Type definition
• Constructor function (for piping and defaults)
• Implicit defaults for simple values

Definition

For convenveniece, several variants of the DSL are available.

Standard

defmodule Player do
  # =============== #
  # Data Definition #
  # =============== #

  defdata do
    name       :: String.t()
    hit_points :: non_neg_integer()
    experience :: non_neg_integer()
  end

  # =================== #
  #    Rest of Module   #
  # (business as usual) #
  # =================== #

  @spec attack(t(), t()) :: {t(), t()}
  def attack(%{experience: xp} = player, %{hit_points: hp} = target) do
    {
      %{player | experience: xp + 50},
      %{target | hit_points: hp - 10}
    }
  end
end

#=> %Player{name: "Sir Bob", hit_points: 10, experience: 500}

Single Field Shorthand

Without any fields specified, Algae will default to a single field with the same name as the module (essentially a "wrapper type"). You must still provide the type for this field, however.

Embedded in another module:

defmodule Id do
  defdata any()
end

%Id{}
#=> %Id{id: nil}

Standalone:

defdata Wrapper :: any()

%Wrapper{}
#=> %Wrapper{wrapper: nil}

Constructor

A helper function, especially useful for piping. The order of arguments is the same as the order that they are defined in.

defmodule Person do
  defdata do
    name :: String.t()
    age  :: non_neg_integer()
  end
end

Person.new("Rachel Weintraub")
#=> %Person{
#     name: "Rachel Weintraub",
#     age:  0
#   }

Constructor Defaults

Fields will automatically default to a sensible value (a typical "zero" for that datatype). For example, non_neg_integer() will default to 0, and String.t() will default to "".

You may also overwrite these defaults with the \\\\ syntax.

defmodule Pet do
  defdata do
    name      :: String.t()
    leg_count :: non_neg_integer() \\\\ 4
  end
end

Pet.new("Crookshanks")
#=> %Pet{
#     name: "Crookshanks",
#     leg_count: 4
#   }

Pet.new("Paul the Psychic Octopus", 8)
#=> %Pet{
#     name: "Paul the Psychic Octopus",
#     leg_count: 8
#   }

This overwriting syntax is required for complex types:

defdata Grocery do
  item :: {String.t(), integer(), boolean()} \\\\ {"Apple", 4, false}
end

Grocery.new()
#=> %Grocery{
#     item: {"Apple", 4, false}
#   }

Overwrite Constructor

The new constructor function may be overwritten.

iex> defmodule Constant do
...>   defdata fun()
...>
...>   def new(value), do: %Constant{constant: fn _ -> value end}
...> end
...>
...> fourty_two = Constant.new(42)
...> fourty_two.constant.(33)
42

Empty Tag

An empty type (with no fields) is definable using the none() type

defmodule Nothing do
  defdata none()
end

Nothing.new()
#=> %Nothing{}

Specs

defsum([{:do, {:__block__, [any()], ast()}}]) :: ast()

Build a sum (coproduct) type from product types

defmodule Light do
  # ============== #
  # Sum Definition #
  # ============== #

  defsum do
    defdata Red    :: none()
    defdata Yellow :: none()
    defdata Green  :: none()
  end

  # =================== #
  #    Rest of Module   #
  # (business as usual) #
  # =================== #

  def from_number(1), do: %Light.Red{}
  def from_number(2), do: %Light.Yellow{}
  def from_number(3), do: %Light.Green{}
end

Light.new()
#=> %Light.Red{}

Embedded Products

Data with multiple fileds can be defined directly as part of a sum

defmodule Pet do
  defsum do
    defdata Cat do
      name :: String.t()
      claw_sharpness :: String.t()
    end

    defdata Dog do
      name :: String.t()
      bark_loudness :: non_neg_integer()
    end
  end
end

Default Constructor

The first defdata's constructor will be the default constructor for the sum

defmodule Maybe do
  defsum do
    defdata Nothing :: none()
    defdata Just    :: any()
  end
end

Maybe.new()
#=> %Maybe.Nothing{}

Tagged Unions

Sums join existing types with tags: new types to help distibguish the context that they are in (the sum type)

defdata Book  :: String.t() \\ "War and Peace"
defdata Video :: String.t() \\ "2001: A Space Odyssey"

defmodule Media do
  defsum do
    defdata Paper :: Book.t()
    defdata Film  :: Video.t() \\ Video.new("A Clockwork Orange")
  end
end

media = Media.new()
#=> %Paper{
#      paper: %Book{
#        book: "War and Peace"
#      }
#   }