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Our simple sample function, (lambda (a b c) (+ a b c)),
specifies three argument variables, so it must be called with three
arguments: if you try to call it with only two arguments or four
arguments, you get a wrong-number-of-arguments error.
It is often convenient to write a function that allows certain arguments
to be omitted. For example, the function substring accepts three
arguments--a string, the start index and the end index--but the third
argument defaults to the end of the string if you omit it. It is also
convenient for certain functions to accept an indefinite number of
arguments, as the functions and and + do.
To specify optional arguments that may be omitted when a function
is called, simply include the keyword &optional before the optional
arguments. To specify a list of zero or more extra arguments, include the
keyword &rest before one final argument.
Thus, the complete syntax for an argument list is as follows:
(required-vars... [&optional optional-vars...] [&rest rest-var])
The square brackets indicate that the &optional and &rest
clauses, and the variables that follow them, are optional.
A call to the function requires one actual argument for each of the
required-vars. There may be actual arguments for zero or more of the
optional-vars, and there cannot be any more actual arguments than
these unless &rest exists. In that case, there may be any number of
extra actual arguments.
If actual arguments for the optional and rest variables are omitted,
then they always default to nil. However, the body of the function
is free to consider nil an abbreviation for some other meaningful
value. This is what substring does; nil as the third argument
means to use the length of the string supplied. There is no way for the
function to distinguish between an explicit argument of nil and
an omitted argument.
Common Lisp note: Common Lisp allows the function to specify what
default value to use when an optional argument is omitted; GNU Emacs
Lisp always uses nil.
For example, an argument list that looks like this:
(a b &optional c d &rest e)
binds a and b to the first two actual arguments, which are
required. If one or two more arguments are provided, c and
d are bound to them respectively; any arguments after the first
four are collected into a list and e is bound to that list. If
there are only two arguments, c is nil; if two or three
arguments, d is nil; if four arguments or fewer, e
is nil.
There is no way to have required arguments following optional
ones--it would not make sense. To see why this must be so, suppose
that c in the example were optional and d were required.
If three actual arguments are given; then which variable would the third
argument be for? Similarly, it makes no sense to have any more
arguments (either required or optional) after a &rest argument.
Here are some examples of argument lists and proper calls:
((lambda (n) (1+ n)) ; One required:
1) ; requires exactly one argument.
=> 2
((lambda (n &optional n1) ; One required and one optional:
(if n1 (+ n n1) (1+ n))) ; 1 or 2 arguments.
1 2)
=> 3
((lambda (n &rest ns) ; One required and one rest:
(+ n (apply '+ ns))) ; 1 or more arguments.
1 2 3 4 5)
=> 15
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