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Macro

pop

pop place ⇒ element

Arguments and Values

placea place, the value of which is a list (possibly, but necessarily, a dotted list or circular list).
elementan object (the car of the contents of place).

Description

pop reads the value of place, remembers the car of the list which was retrieved, writes the cdr of the list back into the place, and finally yields the car of the originally retrieved list.

For information about the evaluation of subforms of place, see Evaluation of Subforms to Places.

Examples

 (setq stack '(a b c)) ⇒  (A B C)
 (pop stack) ⇒  A  
 stack ⇒  (B C)
 (setq llst '((1 2 3 4))) ⇒  ((1 2 3 4))
 (pop (car llst)) ⇒  1
 llst ⇒  ((2 3 4))

Side Effects

The contents of place are modified.

See Also

push, pushnew, Generalized Reference

Notes

The effect of (pop place) is roughly equivalent to 

 (prog1 (car place) (setf place (cdr place)))

except that the latter would evaluate any subforms of place three times, while pop evaluates them only once.

Backlinks

Conses as Lists, Conses Dictionary, Evaluation of Subforms to Places, Overview of Places and Generalized Reference, pprint-logical-block, push, Symbols in the COMMON-LISP Package, Treatment of Other Macros Based on SETF