A Literal Ruby Gem [WIP]

Types

Literal uses Ruby-native types. Any method that responds to ===(value) is considered a type. Note, this is how Ruby’s case statements and pattern matching work. It’s also how Array#any? and Array#all? work. Essentially all Ruby objects are types and just work the way you’d expect them to. A few examples:

• On a Range, ===(value) checks if the value is within the range.
• On a Regexp, ===(value) checks if the value matches the pattern.
• On a Class, ===(value) checks if the value is an instance of the class.
• On a Proc, ===(value) calls the proc with the value.
• On a String, ===(value) checks if the value is equal to the string.
• On the class String, ===(value) checks if the value is a string.

Literal extends this idea with the concept of generics or parameterised types. A generic is a method that returns an object that respond to ===(value).

If we want to check that a given value is an Array, we could do this:

Array === [1, 2, 3]

But what if we want to check that it’s an Array of Integers? Literal provides a library of special types that can be composed for this purpose. In this case, we can use the type _Array(Integer).

_Array(Integer) === [1, 2, 3]

These special types are defined on the Literal::Types module. To access them in a class, you can extend this module. To access them on an instance, you can include the module. If you want to use them globally, you can extend the module at the root.

extend Literal::Types

This is recommended for applications, but not for libraries, as we don’t want to pollute the global namespace from library code.

Literal::Properties, Literal::Object, Literal::Struct and Literal::Data already extend Literal::Types, so you don’t need to extend Literal::Types yourself if you’re only using literal types for literal properties.

Properties

Literal::Properties is a mixin that allows you to define the structure of an object. Properties are defined using the prop method.

The first argument is the name of the property as a Symbol. The second argument is the type of the property. Remember, the type can be any object that responds to ===(value).

The third argument is optional. You can set this to :*, :**, :&, or :positional to change the kind of initializer parameter.

class Person
  extend Literal::Properties

  prop :name, String
  prop :age, Integer
end

You can also use keyword arguments to define readers and writers. These can be set to false, :public, :protected, or :private and default to false.

class Person
  extend Literal::Properties

  prop :name, String, reader: :public
  prop :age, Integer, writer: :protected
end

Properties are required by deafult. To make them optional, set the type to a that responds to ===(nil) with true. Literal::Types provides a special types for this purpose. Let’s make the age optional by setting its type to a _Nilable(Integer):

class Person
  extend Literal::Properties

  prop :name, String
  prop :age, _Nilable(Integer)
end

Alternatively, you can give the property a default value. This default value must match the type of the property.

class Person
  extend Literal::Properties

  prop :name, String, default: "John Doe"
  prop :age, _Nilable(Integer)
end

Note, the above example will fail unless you have frozen string literals enabled. (Which, honestly, you should.) Default values must be frozen. If you can’t use a frozen value, you can pass a proc instead.

class Person
  extend Literal::Properties

  prop :name, String, default: -> { "John Doe" }
  prop :age, _Nilable(Integer)
end

The proc will be called to generate the default value.

You can also pass a block to the prop method. This block will be called with the value of the property when it’s set, which is useful for coercion.

class Person
  extend Literal::Properties

  prop :name, String
  prop :age, Integer do |value|
    value.to_i
  end
end

Coercion takes place prior to type-checking, so you can safely coerce a value to a different type in the block.

You can use properties that conflict with ruby keywords. Literal will handle everything for you automatically.

class Person
  extend Literal::Properties

  prop :class, String, :positional
  prop :end, Integer
end

If you’d prefer to subclass than extend a module, you can use the Literal::Object class instead. Literal::Object literally extends Literal::Properties.

class Person < Literal::Object
  prop :name, String
  prop :age, Integer
end

Structs

Literal::Struct is like Literal::Object, but it also provides a few extras.

Structs implement == so you can compare one struct to another. They also implement hash. Structs also have public readers and writers by default.

class Person < Literal::Struct
  prop :name, String
  prop :age, Integer
end

Data

Literal::Data is like Literal::Struct, but you can’t define writers. Additionally, objects are frozen after initialization. Additionally any non-frozen properties are duplicated and frozen.

class Person < Literal::Data
  prop :name, String
  prop :age, Integer
end