(module immutable-string mzscheme
  (require (lib "contract.ss"))
  
  ;; immutable-string-append: string string -> immutable-string
  ;; Appends two strings together, ensuring the result is an
  ;; immutable string.
  (define (immutable-string-append s1 s2)
    (cond
      [(= (string-length s1) 0)
       (string->immutable-string s2)]
      [(= (string-length s2) 0)
       (string->immutable-string s1)]
      [else
       (string->immutable-string (string-append s1 s2))]))
  
  
  ;; immutable-substring: string number number -> immutable-string
  ;; Gets the substring of a-str, ensuring the result is an
  ;; immutable string.
  (define immutable-substring
    (case-lambda
      [(a-str start)
       (immutable-substring a-str start (string-length a-str))]
      [(a-str start end)
       (cond
         [(and (= 0 start) (= end (string-length a-str)))
          (string->immutable-string a-str)]
         [else
          (string->immutable-string
           (substring a-str start end))])]))
  
  
  (define immutable-string/c (and/c immutable? string?))
  
  (provide/contract
   [immutable-string-append
    (string? string? . -> . immutable-string/c)]
   [immutable-substring
    (case-> (string? natural-number/c
                     . -> . immutable-string/c)
            (string? natural-number/c natural-number/c
                     . -> . immutable-string/c))]
   [immutable-string/c contract?]))