common-lisp Functions Auxiliary Variables
Example
The &AUX keyword can be used to define local variables for the function. They are not parameters; the user cannot supply them.
&AUX variables are seldomly used. You can always use LET instead, or some other way of defining local variables in the function body.
&AUX variables have the advantages that local variables of the whole function body move to the top and it makes one indentation level (for example introduced by a LET) unnecessary.
(defun foobar (x y &aux (z (+ x y)))
(format t "X (~d) and Y (~d) are required.~@
Their sum is ~d."
x y z))
(foobar 10 20)
; X (10) and Y (20) are required.
; Their sum is 30.
;=> NIL
One typical usage may be resolving "designator" parameters. Again, you need not do it this way; using let is just as idiomatic.
(defun foo (a b &aux (as (string a)))
"Combines A and B in a funny way. A is a string designator, B a string."
(concatenate 'string as " is funnier than " b))
