CPSC110 Pset7 solution

;​; DO NOT PUT ANYTHING PERSONALLY IDENTIFYING BEYOND YOUR CWL IN THIS​
​FILE.​
​;; YOUR CWLs WILL BE SUFFICIENT TO IDENTIFY YOU.​
​;;​
​(require spd/tags)​

​(@assignment psets/pset-07); Do not edit or remove this tag​

​(@problem 1)​

;​; There is a primitive type called 1String, defined as follows:​
​;;​
​;; 1String is String​
​;; interp. a 1String represents a single letter​
​;; CONSTRAINT: any 1String is only one letter long​

(​ @htdd ListOf1String)​
​;; ListOf1String is one of:​
​;; - empty​
​;; - (cons 1String ListOf1String)​
​;; interp. a sequence of letters​

(​ define L1 empty)​
​(define L2 (list "a" "B" "c"))​

(​ @htdd Operations)​
​;; Operations is one of:​
​;; - empty​
​;; - (cons "keep" Operations)​
​;; - (cons "space" Operations)​
​;; - (cons "remove" Operations)​
​;; interp. a sequence of operations to apply to a list of 1String​
​;; "keep" means keep the corresponding letter​
​;; "space" means replace the corresponding letter with a " "​
​;; "remove" means remove the corresponding letter​


(​ @htdf decoder)​
​(@signature ListOf1String Operations -> String)​
​;; produce string containing letters in l with operations applied​
​(check-expect (decoder empty empty) "")​
​(check-expect (decoder empty (list "keep")) "")​
​(check-expect (decoder empty (list "space")) "")​
​(check-expect (decoder empty (list "remove")) "")​
​(check-expect (decoder (list "h" "i") empty) "hi")​
​(check-expect (decoder (list "h" "o" "w") (list "keep" "keep" "keep"))​
​"how")​
​(check-expect (decoder (list "o" "h") (list "space" "space"))​

, "​ ")​
​(check-expect (decoder (list "a" "d" "d") (list "remove" "remove"))​
​"d")​
​(check-expect (decoder (list "i" "k" "l" "h" "a" "m" "k" "s" "a" "r" "m")​
​(list "keep" "remove" "remove" "space" "keep" "keep"​
​"space" "keep" "keep" "remove" "keep"))​
​"i am sam")​

​(check-expect (decoder (list "e" "s" "r" "a" "m" "d" "i" "f" "a" "m")​
​(list "remove" "keep" "remove" "keep" "keep" "space"​
​"keep" "space"))​
​"sam i am")​


​ |​
#
​CROSS PRODUCT OF TYPE-COMMENTS TABLE​

​l-> empty (cons 1String ListOf1String)​
​ops​


​empty (string-append (first l) ;(2)​
​(decoder (rest l)​
​empty))​


​(cons "keep" "" (1) (string-append (first l) ;(3)​
​Operations) (decoder (rest l)​
​(rest ops)))​


​(cons "space" (string-append " " ;(4)​
​Operations) (decoder (rest l)​
​(rest ops)))​


​(cons "remove" (decoder (rest l) (rest ops)) ;(5)​
​Operations)​
​|#​

​(@template-origin 2-one-of)​

​(define (decoder l ops)​
​(cond [(empty? l) ""] ;(1)​
​[(empty? ops) ;(2)​
​(string-append (first l)​
​(decoder (rest l)​
​empty))]​