- (textual-map-index proc textual [start end]) → textprocedure
Calls proc on each valid index of the specified subtext or substring, converts the results of those calls into texts, and returns the concatenation of those texts. It is an error for proc to return anything other than a character, string, or text. The dynamic order in which proc is called on the indexes is unspecified, as is the dynamic order in which the coercions are performed. If any strings returned by proc are mutated after they have been returned and before the call to textual-map-index has returned, then textual-map-index returns a text with unspecified contents; the textual-map-index procedure itself does not mutate those strings.