— Scheme Procedure: string-map proc s [start [end]]

proc is a char->char procedure, it is mapped over s. The order in which the procedure is applied to the string elements is not specified.

— Scheme Procedure: string-map! proc s [start [end]]

proc is a char->char procedure, it is mapped over s. The order in which the procedure is applied to the string elements is not specified. The string s is modified in-place, the return value is not specified.

— Scheme Procedure: string-fold kons knil s [start [end]]

Fold kons over the characters of s, with knil as the terminating element, from left to right. kons must expect two arguments: The actual character and the last result of kons' application.

— Scheme Procedure: string-unfold p f g seed [base [make_final]]

• 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 the corresponding character in the result string. These chars are assembled into the string in a left-to-right order.
• base is the optional initial/leftmost portion of the constructed string; it default to the empty string.
• make_final is applied to the terminal seed value (on which p returns true) to produce the final/rightmost portion of the constructed string. It defaults to (lambda (x) ).

— Scheme Procedure: string-for-each proc s [start [end]]

proc is mapped over s in left-to-right order. The return value is not specified.