;;;; This example illustrates the use of the design recipe for
;;;; functions consuming simple (atomic) data.

;; (Step 2) Contract/purpose/header
;; gas-cost : Number Symbol -> Number
;; computes total cost based on no. of gallons and grade
;; (define (gas-cost gallons octane) ...)

;; (Steps 4 and 5) Template, then body
(define (gas-cost gallons grade)
  (cond
    [(symbol=? grade 'regular) (* gallons 1.45)]
    [(symbol=? grade 'plus) (* gallons 1.55)]
    [(symbol=? grade 'premium) (* gallons 1.65)]
    ))

;; (Step 3) Examples and (Step 6) Tests
(gas-cost 1 'regular) "should be" 1.45
(gas-cost 5 'plus) "should be" 7.75
(gas-cost 10 'premium) "should be" 16.50