Warning: This is the manual of the legacy Guile 2.0 series. You may want to read the manual of the current stable series instead.

6.7.2.2 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? x

C Function: scm_list_p (x)

Return #t if x 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? x

C Function: scm_null_p (x)

Return #t if x is the empty list, else #f.

C Function: int scm_is_null (SCM x)

Return 1 when x is the empty list; otherwise return 0.