TypedStruct is a library for defining structs with a type without writing boilerplate code.

Rationale

To define a struct in Elixir, you probably want to define three things:

• the struct itself, with default values,
• the list of enforced keys,
• its associated type.

It ends up in something like this:

defmodule Person do
  @moduledoc """
  A struct representing a person.
  """

  @enforce_keys [:name]
  defstruct name: nil,
            age: nil,
            happy?: true,
            phone: nil

  @typedoc "A person"
  @type t() :: %__MODULE__{
          name: String.t(),
          age: non_neg_integer() | nil,
          happy?: boolean(),
          phone: String.t() | nil
        }
end

In the example above you can notice several points:

• the keys are present in both the defstruct and type definition,
• enforced keys must also be written in @enforce_keys,
• if a key has no default value and is not enforced, its type should be nullable.

If you want to add a field in the struct, you must therefore:

• add the key with its default value in the defstruct list,
• add the key with its type in the type definition.

If the field is not optional, you should even add it to @enforce_keys. This is way too much work for lazy people like me, and moreover it can be error-prone.

It would be way better if we could write something like this:

defmodule Person do
  @moduledoc """
  A struct representing a person.
  """

  use TypedStruct

  @typedoc "A person"
  typedstruct do
    field :name, String.t(), enforce: true
    field :age, non_neg_integer()
    field :happy?, boolean(), default: true
    field :phone, String.t()
  end
end

Thanks to TypedStruct, this is now possible :)

Usage

Setup

To use TypedStruct in your project, add this to your Mix dependencies:

{:typed_struct, "~> 0.1.4"}

If you do not plan to compile modules using TypedStruct at runtime, you can add runtime: false to the dependency tuple as TypedStruct is only used during compilation.

If you want to avoid mix format putting parentheses on field definitions, you can add to your .formatter.exs:

[
  ...,
  import_deps: [:typed_struct]
]

General usage

To define a typed struct, use TypedStruct, then define your struct within a typedstruct block:

defmodule MyStruct do
  # Use TypedStruct to import the typedstruct macro.
  use TypedStruct

  # Define your struct.
  typedstruct do
    # Define each field with the field macro.
    field :a_string, String.t()

    # You can set a default value.
    field :string_with_default, String.t(), default: "default"

    # You can enforce a field.
    field :enforced_field, integer(), enforce: true
  end
end

Each field is defined through the field/2 macro.

If you want to enforce all the keys by default, you can do:

defmodule MyStruct do
  use TypedStruct

  # Enforce keys by default.
  typedstruct enforce: true do
    # This key is enforced.
    field :enforced_by_default, term()

    # You can override the default behaviour.
    field :not_enforced, term(), enforce: false

    # A key with a default value is not enforced.
    field :not_enforced_either, integer(), default: 1
  end
end

You can also generate an opaque type for the struct:

defmodule MyOpaqueStruct do
  use TypedStruct

  # Generate an opaque type for the struct.
  typedstruct opaque: true do
    field :name, String.t()
  end
end

Documentation

To add a @typedoc to the struct type, just add the attribute above the typedstruct block:

@typedoc "A typed struct"
typedstruct do
  field :a_string, String.t()
  field :an_int, integer()
end

Reflexion

To enable the use of information defined by TypedStruct by other modules, each typed struct defines three functions:

• __keys__/0 - returns the keys of the struct
• __defaults__/0 - returns the default value for each field
• __types__/0 - returns the quoted type for each field

For instance:

iex(1)> defmodule Demo do
...(1)>   use TypedStruct
...(1)>
...(1)>   typedstruct do
...(1)>     field :a_field, String.t()
...(1)>     field :with_default, integer(), default: 7
...(1)>   end
...(1)> end
{:module, Demo,
<<70, 79, 82, 49, 0, 0, 8, 60, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0, 241,
  0, 0, 0, 24, 11, 69, 108, 105, 120, 105, 114, 46, 68, 101, 109, 111, 8, 95,
  95, 105, 110, 102, 111, 95, 95, 9, 102, ...>>, {:__types__, 0}}
iex(2)> Demo.__keys__()
[:a_field, :with_default]
iex(3)> Demo.__defaults__()
[a_field: nil, with_default: 7]
iex(4)> Demo.__types__()
[
  a_field: {:|, [],
  [
    {{:., [line: 5],
      [{:__aliases__, [line: 5, counter: -576460752303422524], [:String]}, :t]},
      [line: 5], []},
    nil
  ]},
  with_default: {:integer, [line: 6], []}
]

What do I get?

When defining an empty typedstruct block:

defmodule Example do
  use TypedStruct

  typedstruct do
  end
end

you get an empty struct with its module type t():

defmodule Example do
  @enforce_keys []
  defstruct []

  @type t() :: %__MODULE__{}
end

Each field call adds information to the struct, @enforce_keys and the type t().

A field with no options adds the name to the defstruct list, with nil as default. The type itself is made nullable:

defmodule Example do
  use TypedStruct

  typedstruct do
    field :name, String.t()
  end
end

becomes:

defmodule Example do
  @enforce_keys []
  defstruct name: nil

  @type t() :: %__MODULE__{
          name: String.t() | nil
        }
end

The default option adds the default value to the defstruct:

field :name, String.t(), default: "John Smith"

# Becomes
defstruct name: "John Smith"

When set to true, the enforce option enforces the key by adding it to the @enforce_keys attribute.

field :name, String.t(), enforce: true

# Becomes
@enforce_keys [:name]
defstruct name: nil

In both cases, the type has no reason to be nullable anymore by default. In one case the field is filled with its default value and not nil, and in the other case it is enforced. Both options would generate the following type:

@type t() :: %__MODULE__{
      name: String.t() # Not nullable
    }

Passing opaque: true replaces @type with @opaque in the struct type specification:

typedstruct opaque: true do
  field :name, String.t()
end

# Becomes

@opaque t() :: %__MODULE__{
          name: String.t()
        }