Typed Racket: From Dynamic to Static | Schema Programming Part 13

Racket is dynamic by default. But as projects grow, types become useful for documentation and safety. Typed Racket is a sister language that adds a static type system.

The Language

Change #lang racket to #lang typed/racket.

#lang typed/racket

(: add (-> Number Number Number))
(define (add x y)
  (+ x y))

(add 5 10) ; Works
; (add "hello" 5) ; Compile-time Type Error!

Type Annotations

Use the (: id type) syntax (the "colon" form) to annotate definitions.

• Number
• String
• Boolean
• (Listof Number)
• Any

Gradual Typing

The killer feature of Typed Racket is interop. You can have untyped code call typed code and vice-versa. Contracts are automatically generated at the boundary to ensure safety!

Polymorphism

(: my-map (All (A B) (-> (-> A B) (Listof A) (Listof B))))
(define (my-map f L)
  (if (null? L)
      '()
      (cons (f (first L)) (my-map f (rest L)))))

Summary

Typed Racket allows you to start fast (dynamic) and harden later (static) without rewriting your entire codebase in a different language.