Generics

Generics allow to parameterize a type based on other type. Consider a Box type:

class MyBox(T)
  def initialize(@value : T)
  end

  def value
    @value
  end
end

int_box = MyBox(Int32).new(1)
int_box.value # => 1 (Int32)

string_box = MyBox(String).new("hello")
string_box.value # => "hello" (String)

another_box = MyBox(String).new(1) # Error, Int32 doesn't match String

Generics are specially useful for implementing collection types. Array, Hash, Set are generic type. Pointer too.

More than one type argument is allowed:

class MyDictionary(K, V)
end

Only single letter names are allowed as names of type arguments.

Type variables inference

Type restrictions in a generic type's constructor are free variables when type arguments were not specified, and then are used to infer them. For example:

MyBox.new(1)       # : MyBox(Int32)
MyBox.new("hello") # : MyBox(String)

In the above code we didn't have to specify the type arguments of MyBox, the compiler inferred them following this process:

• MyBox.new(value) delegates to initialize(@value : T)
• T doesn't exist, so it's used as a free var
• Because MyBox is actually MyBox(T), and T is both a free variable and a type argument, T becomes the type of the passed value

In this way generic types are less tedious to work with.

Generic structs and modules

Structs and modules can be generic too. When a module is generic you include it like this:

module Moo(T)
  def t
    T
  end
end

class Foo(U)
  include Moo(U)

  def initialize(@value : U)
  end
end

foo = Foo.new(1)
foo.t # Int32

Note that in the above example T becomes Int32 because Foo.new(1) makes U become Int32, which in turn makes T become Int32 via the inclusion of the generic module.

Generic types inheritance

Generic classes and structs can be inherited. When inheriting you can specify an instance of the generic type, or delegate type variables:

class Parent(T)
end

class Int32Child < Parent(Int32)
end

class GenericChild(T) < Parent(T)
end