Typed Variables and Functions

A quick introduction to types, including how to specify types of variables and functions.

Types of Everything

• Basic types: String, Number, Boolean, Image.
• Function: (-> [input types] [output type])
• Composite types: (Listof Number) - or even (Listof Any)

Specifying Types for Variables

There are two ways to specify a type: with inline or with the colon (:) on a separate line.

1. name : Type notation in place of the plain name

   (define WIDTH : Integer 400)

   Be careful to put spaces on either side of the colon. This method becomes [name : Type] when used in arguments to a function (see below).

2. (: name Type) on a separate line before name

   (: HEIGHT Integer)
   (define HEIGHT 300)

Specifying Types for Functions

You can use the [name : Type] or (: name Type) method to write the type of a function as well. You should include the output type, just like a signature.

1. [name : Type] method: A function that takes in a number and puts out a number plus 5:

   (define (f [x : Number]) : Number
     (+ x 5))

2. (: name Type) method: A function that takes in a string and puts out double its length:

   (: g (-> String Number))
   (define (g str)
     (* 2 (string-length str)))

3. A function that takes in a number and puts out a that many repetitions of “word” (literally):

   (define (h [num : Integer]) : (Listof String)
     (make-list num "word")