Predicates for Strings - GNU Emacs Lisp Reference Manual

For more information about general sequence and array predicates, see Sequences Arrays Vectors, and Arrays.

— Function: stringp object

This function returns t if object is a string, nil otherwise.

— Function: string-or-null-p object

This function returns t if object is a string or nil. It returns nil otherwise.

— Function: char-or-string-p object

This function returns t if object is a string or a character (i.e., an integer), nil otherwise.