;; 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-intermediate-lambda-reader.ss" "lang")((modname foldr) (read-case-sensitive #t) (teachpacks ((lib "testing.ss" "teachpack" "htdp"))) (htdp-settings #8(#t constructor repeating-decimal #f #t none #f ((lib "testing.ss" "teachpack" "htdp")))))

;; The following predefined function foldr (stands for "fold right")
;; computes a value based on all elements of a list
;; ("folds" a list into a value)

;; the first argument is a function that's applied to the element and the result
;; of the recursive call, the second one is a value for an empty list (base case), 
;; the third one is the list

(foldr + 0 (list 1 2 3 4 5 6))
;; expected value 
21

(foldr * 1 (list 2 3 4))
;; expected value 
24

(foldr string-append "" (list "apple " "banana " "strawberry"))
;; expected value 
"apple banana strawberry"

;; fill in the arguments instead of ? according to the definitions:

;; apply foldr to write a function that takes a list of lists 
;; and "folds" them into one list
;;(foldr ? ? (list (list 2 3 4) (list 5 6) (list 7) empty))
;; expected value
(list 2 3 4 5 6 7)

;; apply foldr to write a function that takes a list of numbers
;; and finds the maximum value
;(foldr ? ? (list 1 2 3 5 0 -4))
;; expected value 
5
 
;; write your own example of using foldr. Explain (in comments) what 
;; it does

;; Write a function foldr1 that acts exactly like foldr 
;; Use above examples as your test cases (change the function 
;; name to foldr1)

;; start by writing a contract