There's a lot of open issues
Strict Attributes is a gem that allows you to define strict attributes on your models.
 Project Readme

A Literal Ruby Gem [WIP]

While I’m very excited about RBS and Steep, they will need significant adoption by libraries before we can really use them in our apps.

In the meantime, I think you can get a long way by adding lightweight type checking to your main public interfaces (initializers and writer methods) for models, operations, jobs, components, etc.

Literal provides a few tools to help you define type-checked structs, data objects and value objects, as well as a mixin for your plain old Ruby objects.

What about performance?

Literal uses code generation at startup, rather than dynamic meta-programming, so the performance of a literal initializer, is equivalent to one you’d write by hand. The performance impact of most of the type-checking is negligible: a single case equality check against the given type.

Some of the advanced types, such as _Array and _Hash could have a significant performance impact for large collections, since they need to check every value at runtime. But you can disable expensive type-checking in production, while benefitting from using it in test and development environments.


Literal structs are similar to Ruby structs, but they have type-checked writers and initializers.

You can define a literal struct by subclassing Literal::Struct and calling the attribute macro for each attribute. The first argument is the name of the attribute, which must be a Symbol. The second argument is the type, which must respond to ===.

class Address < Literal::Struct
  attribute :house_number, Integer
  attribute :street, String

You can optionally pass reader: and writer: keyword arguments set to :public, :protected, :private or false.

Literal structs have public readers and writers by default.


Literal::Data is similar to a Ruby Data object. It is frozen and, therefore, immutable. Unlike a Literal::Struct, it has no writers.

Additionally, all unfrozen values given to a Literal::Data initializer will be duplicated and frozen for extra safety. You can define a Literal::Data object much the same as a Literal::Struct, but the attribute macro doesn’t accept the writer: argument.

class Measure < Literal::Data
  attribute :amount, Float
  attribute :unit, Symbol

Just like a Ruby Data object, you can make a copy of a Literal::Data object with specific changes using #with.


Literal::Value is like a Literal::Data, but specifically designed to enrich a single value — you could wrap a String as an EmailAddress or an Integer as a UserId.

We can define literal value classes like this:

EmailAddress = Literal::Value(String)
UserID = Literal::Value(Integer)

We could use this UserID class like this:

user_id =

The input will be type-checked. If it's not frozen already, it will be duplicated and frozen.

You can access the value by calling value on the object:

user_id.value # => 123

Because these value types are defined with an underlying type — in this case, Integer — the value objects also implement specific coercion methods. With an Integer value, you can call to_i to get the underlying value:

user_id.to_i # => 123

With the EmailAddress, we could call to_s or to_str.


The Literal::Attributes mixin allows you to define type-checked attribute accessors on your plain old Ruby objects using the same attribute macro.

By default, writers are private and readers are not defined. It also provides a default initializer which assigns all the keyword arguments to the corresponding attributes.

Here we have a user class with a name and an age. The name is a String and the age is between 18 and infinity.

class User
  extend Literal::Attributes

  attribute :name, String
  attribute :age, 18..

If we try to pass an invalid value to the initializer, we’ll get an error. Under the hood, the attributes are being assigned to instance variables by the same name. Internally, we can reference these instance variables directly:

def first_name = @name.split(/\s/).first

The type-checking is really designed for the public interface. Internally, I think it’s good practice to reference the instance variables directly. You can always use the writers (which are private by default) if you need to do type-checking, but that’s not usually necessary. We’re not trying to make the application type-safe. We can’t do that without a complete type system. What we’re doing is adding some helpful checks to the main public interfaces.


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You can create a Literal::Maybe with Literal::Maybe(Integer).new(1). This will return a Literal::Some. If you created it with nil instead, it would return a Literal::Nothing. Literal::Some and Literal::Nothing both implement the same interface, so you can treat them as Literal::Maybe.


When called on a Literal::Some, yields the value to the block and returns its result wrapped in a Literal::Some. When called on Literal::Nothing, returns self.


It's possible to end up with a Literal::Some(nil) when using map. To avoid this, use maybe instead.


Like map but if the result of the block is nil, returns Literal::Nothing instead.


account = Maybe(Account).new(get_account)
account.maybe(&:user).maybe(&:address).maybe(&:street).value_or { "No Street Address" }


Like map but expects the block to return a Literal::Maybe (either a Literal::Some or Literal::Nothing).


When called on Literal::Some, yields the value to the block and if the block returns truthy, returns self. Otherwise, returns Literal::Nothing. When called on Literal::Nothing, returns self.


When called on a Literal::Some, returns the underlying value. When called on Literal::Nothing yields the block.


Literal::Either(String, Integer).new("Hello") will return a Literal::Left, while Literal::Either(String, Integer).new(1) will return a Literal::Right.

If we call left on a Literal::Right, we'll get Literal::Nothing. However, if we call right on the Literal::Right, we'll get Some(Integer).

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Literal::Attributes, Literal::Struct, and Literal::Data all extend Literal::Types, which provide some advanced types including some generic-like collection types.

These types are implemented as methods that return an object with an === method designed to match a value to the specified type. From any context where Literal::Types is extended, you can reference them directly.

Documentation for Literal::Types

Writing your own type matchers

Values are compared to types using the type’s === operator. When there’s an error, the type’s inspect value is shown in the message. That means you can easily define your own type matchers as objects that respond to === and inspect. It’s worth taking a look at how the built in type matchers are defined here.

Procs as types

It just so happens that, Procs alias === to call, which means you can provide a Proc as a type and it will work as expected. The Proc will be called with the value and should return true or false.


Class with instance variables

Normal Ruby Class Dry::Initializer Literal::Attributes
5.226M ips 1.494M ips 3.399M ips


Normal Ruby Struct Dry::Struct Literal::Struct
4.100M ips 1.333M ips 3.029M ips


It’s worth noting that while Ruby’s built-in Data objects are themselves frozen, they do not freeze the given values. Dry::Struct::Value does freeze the given values and Literal::Data duplicates and freezes the given values if they’re not already frozen.

Normal Ruby Data Dry::Struct::Value Literal::Data
4.272M ips 286.696K ips 2.017M ips

Running tests

bundle exec gd