1. Write down a signature, a purpose statement, and a function header.

A signature is a comment that tells the readers of your design how many inputs your function consumes, from what collection of data they are drawn, and what kind of output data it produces.

A purpose statement is a comment that summarizes the purpose of the function in a single line. If you are ever in doubt about a purpose statement, write down the shortest possible answer to the question “what does the function compute?”

2. Illustrate the signature and the purpose statement with some tests. To construct a test, pick one piece of data from each input class from the signature and determine what you expect back.  Use a check-expect function.

3. Formulate how the function computes its results – develop a Scheme expression that uses Scheme’s primitive operations, other functions, and the variables.

Example

(define (area-of-disk r)
  (* 3.14 (* r r)))

;; Signature: area-of-ring : number number -> number

;; Purpose: to compute the area of a ring whose radius is
;; outer and whose hole has a radius of inner

;; Tests:
(check-expect (area-of-ring 5 3) 50.24)

;; Definition:
(define (area-of-ring outer inner)
  (- (area-of-disk outer)
     (area-of-disk inner)))

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