Recursive type_alias in sorbet

How would one declare a type alias that can contain a reference to itself? For example to mark a function returning a JSON-like structure.

Trying:

Foo = T.type_alias { T.any(Integer, T::Array[Foo]) }

Gives the following error:

Type alias Foo is involved in a cycle

As of version 0.5.x, this is not possible, as type aliases don't support cycles.

One option is to lower the typedness of your result, and just make it:

Foo = T.type_alias { T.any(Integer, T::Array[T.untyped]) }

If you really want to have type checking, you can express something like this by wrapping the result in a T::Struct. The objects will no longer be as flat as originally, though (see in sorbet.run):

class Foo < T::Struct
  prop :content, T.any(Integer, T::Array[Foo])
end

myFoo = Foo.new(content: 1)
content = myFoo.content
case content
when Integer
  puts "It was an integer!"
  T.reveal_type(content)
when Array
  puts "It was a Foo!"
  T.reveal_type(content)
else
  T.absurd(content)
end

Edit (2022-02): The documentation on type_alias has been updated to address recursive definitions. It includes an example for JSON-like structures in sorbet.run.