[plt-scheme] What is the type of the "hello_world" function in Typed Scheme?

As an experiment, I would like to write the "hello_world" function in
Typed Scheme, but am having difficulty with the type annotation.

This is the function in the regular #scheme Module language:

#lang scheme

(define hello_world
  (display "Hello, world!\n"))

However, although I browsed through the following Typed Scheme manual:

Typed Scheme: Scheme with Static Types
http://docs.plt-scheme.org/typed-scheme/

it wasn't clear how to define a function that does not take any input.

Does anybody know how to define the above-mentioned function in Typed
Scheme?

-- Benjamin L. Russell