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7.5.3.2 Predicates

The procedures in this section test specific properties of lists.

Scheme Procedure: proper-list? obj

Return #t if obj is a proper list, or #f otherwise. This is the same as the core list? (see List Predicates).

A proper list is a list which ends with the empty list () in the usual way. The empty list () itself is a proper list too.

(proper-list? '(1 2 3))  ⇒ #t
(proper-list? '())       ⇒ #t
Scheme Procedure: circular-list? obj

Return #t if obj is a circular list, or #f otherwise.

A circular list is a list where at some point the cdr refers back to a previous pair in the list (either the start or some later point), so that following the cdrs takes you around in a circle, with no end.

(define x (list 1 2 3 4))
(set-cdr! (last-pair x) (cddr x))
x ⇒ (1 2 3 4 3 4 3 4 ...)
(circular-list? x)  ⇒ #t
Scheme Procedure: dotted-list? obj

Return #t if obj is a dotted list, or #f otherwise.

A dotted list is a list where the cdr of the last pair is not the empty list (). Any non-pair obj is also considered a dotted list, with length zero.

(dotted-list? '(1 2 . 3))  ⇒ #t
(dotted-list? 99)          ⇒ #t

It will be noted that any Scheme object passes exactly one of the above three tests proper-list?, circular-list? and dotted-list?. Non-lists are dotted-list?, finite lists are either proper-list? or dotted-list?, and infinite lists are circular-list?.

Scheme Procedure: null-list? lst

Return #t if lst is the empty list (), #f otherwise. If something else than a proper or circular list is passed as lst, an error is signalled. This procedure is recommended for checking for the end of a list in contexts where dotted lists are not allowed.

Scheme Procedure: not-pair? obj

Return #t is obj is not a pair, #f otherwise. This is shorthand notation (not (pair? obj)) and is supposed to be used for end-of-list checking in contexts where dotted lists are allowed.

Scheme Procedure: list= elt= list1 …

Return #t if all argument lists are equal, #f otherwise. List equality is determined by testing whether all lists have the same length and the corresponding elements are equal in the sense of the equality predicate elt=. If no or only one list is given, #t is returned.

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