chickadee » srfi-135 » text-unfold

This is a fundamental constructor for texts.

• successor is used to generate a series of "seed" values from the initial seed: seed, (successor seed), (successor^2 seed), (successor^3 seed), ...
• stop? tells us when to stop — when it returns true when applied to one of these seed values.
• mapper maps each seed value to the corresponding character(s) in the result text, which are assembled into that text in left-to-right order. It is an error for mapper to return anything other than a character, string, or text.
• base is the optional initial/leftmost portion of the constructed text, which defaults to the empty text (text). It is an error if base is anything other than a character, string, or text.
• make-final is applied to the terminal seed value (on which stop? returns true) to produce the final/rightmost portion of the constructed text. It defaults to (lambda (x) (text)). It is an error for make-final to return anything other than a character, string, or text.

text-unfold is a fairly powerful text constructor. You can use it to convert a list to a text, read a port into a text, reverse a text, copy a text, and so forth. Examples:

(port->text p) = (text-unfold eof-object?
                              values
                              (lambda (x) (read-char p))
                              (read-char p))

(list->text lis) = (text-unfold null? car cdr lis)

(text-tabulate f size) = (text-unfold (lambda (i) (= i size)) f add1 0)

;; To map f over a list lis, producing a text:
(text-unfold null? (compose f car) cdr lis)