chickadee » srfi-14 » char-set-ref

Cursors are a low-level facility for iterating over the characters in a set. A cursor is a value that indexes a character in a char set. char-set-cursor produces a new cursor for a given char set. The set element indexed by the cursor is fetched with char-set-ref. A cursor index is incremented with char-set-cursor-next; in this way, code can step through every character in a char set. Stepping a cursor "past the end" of a char set produces a cursor that answers true to end-of-char-set?. It is an error to pass such a cursor to char-set-ref or to char-set-cursor-next.

A cursor value may not be used in conjunction with a different character set; if it is passed to char-set-ref or char-set-cursor-next with a character set other than the one used to create it, the results and effects are undefined.

Cursor values are not necessarily distinct from other types. They may be integers, linked lists, records, procedures or other values. This license is granted to allow cursors to be very "lightweight" values suitable for tight iteration, even in fairly simple implementations.

Note that these primitives are necessary to export an iteration facility for char sets to loop macros.

Example:

(define cs (char-set #\G #\a #\T #\e #\c #\h))
 
;; Collect elts of CS into a list.
(let lp ((cur (char-set-cursor cs)) (ans '()))
  (if (end-of-char-set? cur) ans
      (lp (char-set-cursor-next cs cur)
          (cons (char-set-ref cs cur) ans))))
  => (#\G #\T #\a #\c #\e #\h)
 
;; Equivalently, using a list unfold (from SRFI 1):
(unfold-right end-of-char-set? 
              (curry char-set-ref cs)
	      (curry char-set-cursor-next cs)
	      (char-set-cursor cs))
  => (#\G #\T #\a #\c #\e #\h)

Rationale: Note that the cursor API's four functions "fit" the functional protocol used by the unfolders provided by the list, string and char-set SRFIs (see the example above). By way of contrast, here is a simpler, two-function API that was rejected for failing this criterion. Besides char-set-cursor, it provided a single function that mapped a cursor and a character set to two values, the indexed character and the next cursor. If the cursor had exhausted the character set, then this function returned false instead of the character value, and another end-of-char-set cursor. In this way, the other three functions of the current API were combined together.