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type: keyword
location: built-in
source file: xleval.c
Common LISP compatible: similar
supported on: all machines


&optional [ <opt-arg> | (  [  [  ] ] ) ] ...
        <opt-arg>       -       optional argument
        <opt-value>     -       optional argument initialization
        <exist-symbol>  -       optional argument existence symbol


In XLISP, there are several times that you define a formal argument list for a body of code (like DEFUN, DEFMACRO, :ANSWER and LAMBDA). All of the formal arguments that are defined are required to appear in the invocation of the defined function or operation. If there are any &OPTIONAL arguments defined, they will be filled in order. If there are insufficient parameters for the &OPTIONAL arguments, they will contain NIL, unless the arguments have an <opt-value> initial value specified. The <exist-symbol>, if it is specified, will contain a T if the <opt-arg> was supplied by the function call and a NIL if it was not supplied by the function call. This <exist-symbol> allows the programmer to test for an arguments existence. At the end of the function or operation execution, these local symbols and their values are are removed.


(defun foo                              ; define function FOO
  (a &optional b (c 1) )                ;   with some optional args
  (print a) (print b) (print c))        ;
(foo)                                   ; error: too few arguments 
(foo 1)                                 ; prints 1 NIL 1
(foo 1 2)                               ; prints 1 2 1
(foo 1 2 3)                             ; prints 1 2 3 
(defun fee                              ; define function FEE
  (a &optional (b 9 b-passed) )         ;   with some optional args
  (print a) (print b)                   ;
  (if b-passed (print "b was passed")   ;
               (print "b not passed"))) ;
(fee 1)                                 ; prints 1 9 "b not passed"
(fee 1 2)                               ; prints 1 2 "b was passed"

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