Pair

List of symbols:

car

Usage: (car pair)

Return the car (first item) from a pair. If used on a proper list this will be the first element.

Example:

(def tst-pairs-two (list 'x 'y 'z))
(test::assert-equal 'x (car tst-pairs-two))
(test::assert-equal 10 (car '(10)))
(test::assert-equal 9 (car '(9 11 13)))

cdr

Usage: (cdr pair)

Return the cdr (second item) from a pair. If used on a proper list this will be the list minus the first element.

Example:

(def tst-pairs-three (list 'x 'y 'z))
(test::assert-equal '(y z) (cdr tst-pairs-three))
(test::assert-equal nil (cdr '(10)))
(test::assert-equal '(13) (cdr '(9 13)))
(test::assert-equal '(11 13) (cdr '(9 11 13)))

list

Usage: (list item0 item1 .. itemN)

Create a proper list from pairs with items 0 - N.

Example:

(test::assert-equal '(1 2 3) (list 1 2 3))

xar!

Usage: (xar! pair expression)

Destructive form that replaces the car (first item) in a pair with a new expression.

If used on a proper list will replace the first item. Can be used on nil to create a pair (expression . nil).

Example:

(def tst-pairs-three (list 'x 'y 'z))
(test::assert-equal '(x y z) tst-pairs-three)
(test::assert-equal '(s y z) (xar! tst-pairs-three 's))
(test::assert-equal '(s y z) tst-pairs-three)
(def tst-pairs-four (list 't))
(test::assert-equal '(y) (xar! tst-pairs-four 'y))
(test::assert-equal '(y) tst-pairs-four)

xdr!

Usage: (xdr! pair expression)

Destructive form that replaces the cdr (second item) in a pair with a new expression.

If used on a proper list will replace everything after the first item. Can be used on nil to create a pair (nil . expression).

Example:

(def tst-pairs-five (list 'a 'b 'c))
(test::assert-equal '(a b c) tst-pairs-five)
(test::assert-equal '(a y z) (xdr! tst-pairs-five '(y z)))
(test::assert-equal '(a y z) tst-pairs-five)
(def tst-pairs-six (list 'v))
(test::assert-equal (list 'v) tst-pairs-six)
(test::assert-equal '(v . v) (xdr! tst-pairs-six 'v))
(test::assert-equal '(v . v) tst-pairs-six)