list?  check if an object is a list
LIBRARY
(import (rnrs)) ;R6RS
(import (rnrs base)) ;R6RS
(import (scheme r5rs)) ;R7RS
(import (scheme base)) ;R7RS
SYNOPSIS
(list? obj)
DESCRIPTION
Returns
#t
if
obj
is a list,
#f
otherwise. By definition, all lists are chains of pairs that have
finite length and are terminated by the empty list.
IMPLEMENTATION NOTES
This procedure is expected to run in time linear to the number of
elements in the list. Implementations can be expected to use an
algorithm like Floyd's cyclefinding algorithm ("tortoise and hare"),
which handles the case of circular lists created with
setcdr!(3scm).
RETURN VALUES
Returns a boolean object.
EXAMPLES
(list? '(a b c)) => #t
(list? '()) => #t
(list? '(a . b)) => #f
(let ((x (list 'a)))
(setcdr! x x)
(list? x)) => #f
COMPATIBILITY
The
list?
procedure is identical in all revisions of the Scheme reports. IEEE
Scheme explicitly mentions the case of circular structures.
ERRORS
This procedure can raise exceptions with the following condition types:
 &assertion (R6RS)

The wrong number of arguments was passed.
 R7RS

The assertions described above are errors.
Implementations may signal an error, extend the procedure's
domain of definition to include such arguments,
or fail catastrophically.
SEE ALSO
pair?(3scm)
STANDARDS
R4RS,
IEEE Scheme,
R5RS,
R6RS,
R7RS
HISTORY
The
list?
procedure first appeared in R4RS.
AUTHORS
This page is part of the
schememanpages
project.
It includes materials from the RnRS documents.
More information can be found at
https://github.com/schemedoc/manpages/
.
BUGS
A fairly common mistake is to call
list?
when destructuring an object, e.g. immediately before using
car(3scm)
or
cdr(3scm).
Use
pair?(3scm)
instead.
Markup created by unroff 1.0sc, March 04, 2023.