;; The first three lines of this file were inserted by DrScheme. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-beginner-reader.ss" "lang")((modname type_errors) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ())))

;; Using symbols to handle type errors

;; Passing a wrong type of data to a function - type error

;(* 'Yes 'No)


;; Contract: number -> number

;; Purpose: determine a yearly interest rate based on amount
;; The interest rate is 0.04 for amounts between 0 and
;; 1000 (incl), 0.045 for amounts between 1000 and 5000 (incl),
;; and 0.05 for larger amounts. 

;; Examples: (interest-rate 500) -> 0.04
;; (interest-rate 1000) -> 0.04
;; (interest-rate 10000) -> 0.05
(define (interest-rate amount)
  (cond
  [(< amount 0) 0.0]
  [(<= amount 1000) .04]
  [(<= amount 5000) .045]
  [(> amount 5000) 0.05]  
  ) 
)

;(interest-rate 'yes) ; a type error for <

;; A modified version that checks for wrong data type:

;; Contract: number -> number

;; Purpose: determine a yearly interest rate based on amount
;; The interest rate is 0.04 for amounts between 0 and
;; 1000 (incl), 0.045 for amounts between 1000 and 5000 (incl),
;; and 0.05 for larger amounts. 

;; Examples: (interest-rate 500) -> 0.04
;; (interest-rate 1000) -> 0.04
;; (interest-rate 10000) -> 0.05
(define (interest-rate2 amount)
  (cond
  [(not (number? amount)) 'AmountIsNotANumber]
  [(< amount 0) 'NegativeAmount]
  [(<= amount 1000) .04]
  [(<= amount 5000) .045]
  [(> amount 5000) 0.05]  
  ) 
)

(interest-rate2 -77)
;; expected value 
'NegativeAmount

(interest-rate2 true)
;; expected value 
'AmountIsNotANumber

(interest-rate2 2000)
;; expected value 
0.045

;; other functions that check for types
(boolean? true)
(boolean? 'true)

(number? -0.98)
(number? (+ 4 -77))
(number? 4/3)

(integer? 3.14)
(real? 3.14)
(integer? 8/2)

(exact? 3.14)
(exact? 8/2)
(exact? 1/3)
(exact? (sin 5))
(exact? (ceiling (sin 5)))

(symbol? 'yes)
(symbol? 5)
(symbol? "yes")