chickadee » srfi-130 » string-fold

These are the fundamental iterators for strings.

The string-fold operator maps the kons procedure across the string from left to right

(... (kons s[2] (kons s[1] (kons s[0] knil))))

In other words, string-fold obeys the (tail) recursion

(string-fold kons knil s start end) =
    (string-fold kons (kons s[start] knil) start+1 end)

The string-fold-right operator maps the kons procedure across the string from right to left

(kons s[0] (... (kons s[end-3] (kons s[end-2] (kons s[end-1] knil)))))

obeying the (tail) recursion

(string-fold-right kons knil s start end) =
    (string-fold-right kons (kons s[end-1] knil) start end-1)

Examples:

;;; Convert a string to a list of chars.
(string-fold-right cons '() s)

;;; Count the number of lower-case characters in a string.
(string-fold (lambda (c count)
               (if (char-lower-case? c)
                   (+ count 1)
                   count))
             0
             s)

;;; Double every backslash character in S.
(let* ((ans-len (string-fold (lambda (c sum)
                               (+ sum (if (char=? c #\\) 2 1)))
                             0 s))
       (ans (make-string ans-len)))
  (string-fold (lambda (c i)
                 (let ((i (if (char=? c #\\)
                              (begin (string-set! ans i #\\) (+ i 1))
                              i)))
                   (string-set! ans i c)
                   (+ i 1)))
               0 s)
  ans)

The right-fold combinator is sometimes called a "catamorphism."