TypedStruct

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:

defmodulePersondo@moduledoc""" A struct representing a person. """@enforce_keys[:name]defstructname: nil,age: nil,happy?: true,phone: nil@typedoc"A person"@typet()::%__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:

defmodulePersondo@moduledoc""" A struct representing a person. """useTypedStructtypedstructdo@typedoc"A person"field:name,String.t(),enforce: truefield:age,non_neg_integer()field:happy?,boolean(),default: truefield:phone,String.t()endend

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:

defmoduleMyStructdo# Use TypedStruct to import the typedstruct macro.useTypedStruct# Define your struct.typedstructdo# 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: trueendend

Each field is defined through the field/2 macro.

Options

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

defmoduleMyStructdouseTypedStruct# Enforce keys by default.typedstructenforce: truedo# 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: 1endend

You can also generate an opaque type for the struct:

defmoduleMyOpaqueStructdouseTypedStruct# Generate an opaque type for the struct.typedstructopaque: truedofield:name,String.t()endend

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

defmoduleMyModuledouseTypedStruct# You now have %MyModule.Struct{}.typedstructmodule: Structdofield:field,term()endend

Documentation

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

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

You can also document submodules this way:

typedstructmodule: MyStructdo@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:

defmoduleMyStructdouseTypedStructtypedstructdopluginTypedStructLensfield:a_field,String.t()field:other_field,atom()end@specchange(t())::t()defchange(data)do# a_field/0 is generated by TypedStructLens.lens=a_field()put_in(data,[lens],"Changed")endend

Some available plugins

• typed_struct_lens – Integration with the Lens library.
• typed_struct_legacy_reflection – Re-enables the legacy reflection functions from TypedStruct 0.1.x.

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:

defmoduleExampledouseTypedStructtypedstructdoendend

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

defmoduleExampledo@enforce_keys[]defstruct[]@typet()::%__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:

defmoduleExampledouseTypedStructtypedstructdofield:name,String.t()endend

becomes:

defmoduleExampledo@enforce_keys[]defstructname: nil@typet()::%__MODULE__{name: String.t()|nil}end

The default option adds the default value to the defstruct:

field:name,String.t(),default: "John Smith"# Becomesdefstructname: "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]defstructname: 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:

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

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

typedstructopaque: truedofield:name,String.t()end

generates the following type:

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

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

defmoduleMyModuledouseTypedStructtypedstructmodule: Structdofield:field,term()endend

becomes:

defmoduleMyModuledodefmoduleStructdo@enforce_keys[]defstructfield: nil@typet()::%__MODULE__{field: term()|nil}endend

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.