25.5 Predicates

Function: string-null? obj

Return #t if obj is the empty string, otherwise return #f.

Function: %string-every char/char-set/pred str start past

Function: %string-any char/char-set/pred str start past

Macro: string-every char/char-set/pred S

Macro: string-any char/char-set/pred S

Check to see if the given criteria is true on every/any character in str, proceeding from left (index start) to right (index past).

If the selected substring is empty, the return value is #f.

If char/char-set/pred is a character, it is tested for equality with the elements of S.

If char/char-set/pred is a character set, the elements of S are tested for membership in the set.

If char/char-set/pred is a predicate procedure, it is applied to the elements of S. The predicate is “witness–generating”:

• If string-any returns true, the returned true value is the one produced by the application of the predicate.
• If string-every returns true, the returned true value is the one produced by the application of the predicate to the last code point in the substring.

If the predicate is applied to the final element of the selected substring, that final application is a tail call.

The names of these procedures do not end with a question mark; this is to indicate that, in the predicate case, they do not return a simple boolean (#t or #f), but a general value.