String expression evaluation example

str-expr.rkt

#lang racket


;; A str-expr is either:
;;
;;     1.  a plain string, or
;;     2.  (list '+ str-expr str-expr)
;;     3.  (list '* number str-expr)

;; evaluate: str-expr -> string
(define (evaluate expr)
  (cond
    [(string? expr)
     expr]
    [(eq? (first expr) '+)
     (string-append (evaluate (second expr))
                    (evaluate (third expr)))]
    [(eq? (first expr) '*)
     (define chunk (evaluate (third expr)))
     (define n (second expr))
     (apply string-append (for/list ([x n]) chunk))]))


(module+ test 
  (require rackunit)
  
  (check-equal? (evaluate '"hello") 
                "hello")
  
  (check-equal? (evaluate '(+ "hello" "world")) 
                "helloworld")
  
  (check-equal? (evaluate '(+ "rock" (+ (* 5 "p") "aper")))
                "rockpppppaper")
  
  (check-equal? (evaluate '(* 3 (+ "scis" "sors")))
                "scissorsscissorsscissors"))