# TypedStruct
[](https://travis-ci.com/ejpcmac/typed_struct)
[](https://hex.pm/packages/typed_struct)
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:
```elixir
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:
```elixir
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:
```elixir
{:typed_struct, "~> 0.2.1"}
```
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`:
```elixir
[
...,
import_deps: [:typed_struct]
]
```
### General usage
To define a typed struct, use
[`TypedStruct`](https://hexdocs.pm/typed_struct/TypedStruct.html), then define
your struct within a `typedstruct` block:
```elixir
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`](https://hexdocs.pm/typed_struct/TypedStruct.html#field/2) macro.
### Options
If you want to enforce all the keys by default, you can do:
```elixir
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:
```elixir
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:
```elixir
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:
```elixir
typedstruct do
@typedoc "A typed struct"
field :a_string, String.t()
field :an_int, integer()
end
```
You can also document submodules this way:
```elixir
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`](https://hexdocs.pm/typed_struct/TypedStruct.Plugin.html).
For instance, to automatically generate lenses with the
[Lens](https://github.com/obrok/lens) library, you can use
[`TypedStructLens`](https://github.com/ejpcmac/typed_struct_lens) and do:
```elixir
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
* [`typed_struct_lens`](https://github.com/ejpcmac/typed_struct_lens) –
Integration with the [Lens](https://github.com/obrok/lens) library.
* [`typed_struct_legacy_reflection`](https://github.com/ejpcmac/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](https://hex.pm/packages?search=typed_struct) 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:
```elixir
defmodule Example do
use TypedStruct
typedstruct do
end
end
```
you get an empty struct with its module type `t()`:
```elixir
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:
```elixir
defmodule Example do
use TypedStruct
typedstruct do
field :name, String.t()
end
end
```
becomes:
```elixir
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`:
```elixir
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.
```elixir
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:
```elixir
@type t() :: %__MODULE__{
name: String.t() # Not nullable
}
```
Passing `opaque: true` replaces `@type` with `@opaque` in the struct type
specification:
```elixir
typedstruct opaque: true do
field :name, String.t()
end
```
generates the following type:
```elixir
@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:
```elixir
defmodule MyModule do
use TypedStruct
typedstruct module: Struct do
field :field, term()
end
end
```
becomes:
```elixir
defmodule MyModule do
defmodule Struct do
@enforce_keys []
defstruct field: nil
@type t() :: %__MODULE__{
field: term() | nil
}
end
end
```
## [Contributing](CONTRIBUTING.md)
Before contributing to this project, please read the
[CONTRIBUTING.md](CONTRIBUTING.md).
## Initial roadmap
* [x] Struct definition
* [x] Type definition (with nullable types)
* [x] Default values
* [x] Enforced keys (non-nullable types)
* [x] Plugin API
## Plugin ideas
* [ ] Default value type-checking (is it possible?)
* [ ] Guard generation
* [x] Integration with [Lens](https://github.com/obrok/lens)
* [ ] Integration with [Ecto](https://github.com/elixir-ecto/ecto)
## License
Copyright © 2018-2020 Jean-Philippe Cugnet and Contributors
This project is licensed under the [MIT license](LICENSE).
