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TypedStruct

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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

  typedstruct do
    @typedoc "A person"

    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, "~> 0.3.0"}

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 at build time.

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.

Options

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

If you often define submodules containing only a struct, you can avoid boilerplate code:

defmodule MyModule do
  use TypedStruct

  # You now have %MyModule.Struct{}.
  typedstruct module: Struct do
    field :field, term()
  end
end

Documentation

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

typedstruct do
  @typedoc "A typed struct"

  field :a_string, String.t()
  field :an_int, integer()
end

You can also document submodules this way:

typedstruct module: MyStruct do
  @moduledoc "A submodule with a typed struct."
  @typedoc "A typed struct in a submodule"

  field :a_string, String.t()
  field :an_int, integer()
end

Plugins

It is possible to extend the scope of TypedStruct by using its plugin interface, as described in TypedStruct.Plugin. For instance, to automatically generate lenses with the Lens library, you can use TypedStructLens and do:

defmodule MyStruct do
  use TypedStruct

  typedstruct do
    plugin TypedStructLens

    field :a_field, String.t()
    field :other_field, atom()
  end

  @spec change(t()) :: t()
  def change(data) do
    # a_field/0 is generated by TypedStructLens.
    lens = a_field()
    put_in(data, [lens], "Changed")
  end
end

Some available plugins

This list is not meant to be exhaustive, please search for “typed_struct” on hex.pm for other results. If you want your plugin to appear here, please open an issue.

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

generates the following type:

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

When passing module: ModuleName, the whole typedstruct block is wrapped in a module definition. This way, the following definition:

defmodule MyModule do
  use TypedStruct

  typedstruct module: Struct do
    field :field, term()
  end
end

becomes:

defmodule MyModule do
  defmodule Struct do
    @enforce_keys []
    defstruct field: nil

    @type t() :: %__MODULE__{
            field: term() | nil
          }
  end
end

Initial roadmap

  • Struct definition
  • Type definition (with nullable types)
  • Default values
  • Enforced keys (non-nullable types)
  • Plugin API

Plugin ideas

  • Default value type-checking (is it possible?)
  • Guard generation
  • Integration with Lens
  • Integration with Ecto

Related libraries

  • Domo: a library to validate structs that define a t() type, like the one generated by TypedStruct.
  • TypedEctoSchema: a library that provides a DSL on top of Ecto.Schema to achieve the same result as TypedStruct, with Ecto.

Contributing

Before contributing to this project, please read the CONTRIBUTING.md.

License

Copyright © 2018-2022 Jean-Philippe Cugnet and Contributors

This project is licensed under the MIT license.