Next: , Previous: List Syntax, Up: Lists

22.2.2 List Predicates

Often it is useful to test whether a given Scheme object is a list or not. List-processing procedures could use this information to test whether their input is valid, or they could do different things depending on the datatype of their arguments.

— Scheme Procedure: list? obj
— C Function: scm_list_p (obj)

Return #t iff obj is a proper list, else #f.

The predicate null? is often used in list-processing code to tell whether a given list has run out of elements. That is, a loop somehow deals with the elements of a list until the list satisfies null?. Then, the algorithm terminates.

— Scheme Procedure: null? obj
— C Function: scm_null_p (obj)

Return #t iff obj is the empty list, else #f.