LAMBDA LISTS

The lambda list starts with required arguments. Required arguments must be specified in every call to the function.

The required arguments are followed by the &optional arguments. Optional arguments may be provided or omitted in a call. An initialization expression may be specified to provide a default value for an &optional argument if it is omitted from a call. If no initialization expression is specified, an omitted argument is initialized to NIL. It is also possible to provide the name of a 'supplied-p' variable that can be used to determine if a call provided a value for the argument or if the initialization expression was used. If specified, the supplied-p variable will be bound to T if a value was specified in the call and NIL if the default value was used.

The &optional arguments are followed by the &rest argument. The &rest argument gets bound to the remainder of the argument list after the required and &optional arguments have been removed.

The &rest argument is followed by the &key arguments. When a keyword argument is passed to a function, a pair of values appears in the argument list. The first expression in the pair should evaluate to a keyword symbol (a symbol that begins with a ':'). The value of the second expression is the value of the keyword argument. Like &optional arguments, &key arguments can have initialization expressions and supplied-p variables. It is possible to specify the keyword to be used in a function call. If no keyword is specified, the keyword obtained by adding a ':' to the beginning of the keyword argument symbol is used. In other words, if the keyword argument symbol is 'foo', the keyword will be ':foo'.

If identical keywords occur, those after the first are ignored. Extra keywords will signal an error unless &allow-other-keys is present, in which case the extra keywords are ignored. Also, if the keyword :allow-other-keys is used in the function/macro call, and has a non-nil value, then additional keys will be ignored.

The &key arguments are followed by the &aux variables. These are local variables that are bound during the evaluation of the function body. It is possible to have initialization expressions for the &aux variables.

Here is the complete syntax for lambda lists:

(<rarg>...
	[&optional [<oarg> | (<oarg> [<init> [<svar>]])]...]
	[&rest <rarg>]
	[&key
	[<karg> | ([<karg> | (<key> <karg>)] [<init> [<svar>]])]
	 ... [&allow-other-keys]]
	[&aux [<aux> | (<aux> [<init>])]...])