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 length of the
string if you omit it. It is also convenient for certain functions to
accept an indefinite number of arguments, as the functions list
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 actual arguments beyond
that unless the lambda list uses &rest. 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. There is no way for the
function to distinguish between an explicit argument of nil and
an omitted argument. 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 to
substring means to use the length of the string supplied.
Common Lisp note: Common Lisp allows the function to specify what default value to use when an optional argument is omitted; Emacs Lisp always uses
nil. Emacs Lisp does not support "supplied-p" variables that tell you whether an argument was explicitly passed.
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.
Suppose three actual arguments are given; 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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