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T.self_type

TODO: This page is still a fragment. Contributions welcome!

Warning: This feature is experimental and has known limitations. It may not work as expected or change without notice.

T.self_type

This type can be used in return types to indicate that calling this method on will return the same type as the type of the receiver (the receiver is the thing we call a method on i.e., x in x.foo). For instance, #dup returns T.self_type. No matter what class you call it on, you will get back the same type.

# typed: true

class Parent
  extend T::Sig

  sig {returns(T.self_type)}
  def foo
    self
  end
end

class Child < Parent; end

T.reveal_type(Parent.new.foo) # Revealed type: Parent
T.reveal_type(Child.new.foo) # Revealed type: Child

module Mixin
  extend T::Sig

  sig {returns(T.self_type)}
  def bar
    self
  end
end

class UsesMixin
  extend Mixin
end

T.reveal_type(UsesMixin.bar) # Revealed type: T.class_of(UsesMixin)

Certain advanced use cases of T.self_type are not supported:

class Generic < Parent
  extend T::Generic
  TM = type_member

  sig {returns(Generic[T.self_type])} # error: Only top-level T.self_type is supported
  def bad
    Generic[T.untyped].new
  end
end

Note that T.self_type declares that the type should be exactly what the receiver of the method is. It is not useful for declaring the type of factory methods. For these types of methods, use T.attached_class instead:

class Parent
  # sig {returns(T.self_type)} # WRONG
  sig {returns(T.attached_class)}
  def self.make
    new
  end
end

class Child < Parent; end

T.reveal_type(Parent.make) # => `Parent`
T.reveal_type(Child.make)  # => `Child`

If the above example was declared using T.self_type, Sorbet would expect to see the make method return a value of type T.class_of(Parent), but new returns a value of type Parent (an instance of the class, not the class object itself).