chickadee » srfi-14 » char-set-unfold

This is a fundamental constructor for char-sets.

• G is used to generate a series of "seed" values from the initial seed: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...
• P tells us when to stop -- when it returns true when applied to one of these seed values.
• F maps each seed value to a character. These characters are added to the base character set BASE-CS to form the result; BASE-CS defaults to the empty set. char-set-unfold! adds the characters to BASE-CS in a linear-update -- it is allowed, but not required, to side-effect and use BASE-CS's storage to construct the result.

More precisely, the following definitions hold, ignoring the optional-argument issues:

(define (char-set-unfold p f g seed base-cs) 
  (char-set-unfold! p f g seed (char-set-copy base-cs)))
  
(define (char-set-unfold! p f g seed base-cs)
  (let lp ((seed seed) (cs base-cs))
        (if (p seed) cs                                 ; P says we are done.
            (lp (g seed)                                ; Loop on (G SEED).
                (char-set-adjoin! cs (f seed))))))      ; Add (F SEED) to set.

(Note that the actual implementation may be more efficient.)

Examples:

 (port->char-set p) = (char-set-unfold eof-object? values
                                       (lambda (x) (read-char p))
                                       (read-char p))
 
 (list->char-set lis) = (char-set-unfold null? car cdr lis)