Defining functions is only half the battle. Functions don’t do anything until you call them, i.e., tell them to run. Calling a function is also known as invocation.
The most common way of invoking a function is by evaluating a list.
For example, evaluating the list
(concat "a" "b") calls the
concat with arguments
See Evaluation, for a description of evaluation.
When you write a list as an expression in your program, you specify
which function to call, and how many arguments to give it, in the text
of the program. Usually that’s just what you want. Occasionally you
need to compute at run time which function to call. To do that, use
funcall. When you also need to determine at run
time how many arguments to pass, use
funcall calls function with arguments, and returns
whatever function returns.
funcall is a function, all of its arguments, including
function, are evaluated before
funcall is called. This
means that you can use any expression to obtain the function to be
called. It also means that
funcall does not see the
expressions you write for the arguments, only their values.
These values are not evaluated a second time in the act of
calling function; the operation of
funcall is like the
normal procedure for calling a function, once its arguments have
already been evaluated.
The argument function must be either a Lisp function or a
primitive function. Special forms and macros are not allowed, because
they make sense only when given the “unevaluated” argument
funcall cannot provide these because, as we saw
above, it never knows them in the first place.
(setq f 'list) ⇒ list
(funcall f 'x 'y 'z) ⇒ (x y z)
(funcall f 'x 'y '(z)) ⇒ (x y (z))
(funcall 'and t nil) error→ Invalid function: #<subr and>
Compare these examples with the examples of
apply calls function with arguments, just like
funcall but with one difference: the last of arguments is a
list of objects, which are passed to function as separate
arguments, rather than a single list. We say that
spreads this list so that each individual element becomes an
apply returns the result of calling function. As with
funcall, function must either be a Lisp function or a
primitive function; special forms and macros do not make sense in
(setq f 'list) ⇒ list
(apply f 'x 'y 'z) error→ Wrong type argument: listp, z
(apply '+ 1 2 '(3 4)) ⇒ 10
(apply '+ '(1 2 3 4)) ⇒ 10
(apply 'append '((a b c) nil (x y z) nil)) ⇒ (a b c x y z)
For an interesting example of using
apply, see Definition of mapcar.
Sometimes it is useful to fix some of the function’s arguments at certain values, and leave the rest of arguments for when the function is actually called. The act of fixing some of the function’s arguments is called partial application of the function9. The result is a new function that accepts the rest of arguments and calls the original function with all the arguments combined.
Here’s how to do partial application in Emacs Lisp:
This function returns a new function which, when called, will call
func with the list of arguments composed from args and
additional arguments specified at the time of the call. If func
accepts n arguments, then a call to
m < n arguments will produce a new function of
n - m arguments.
Here’s how we could define the built-in function
1+, if it
didn’t exist, using
(defalias '1+ (apply-partially '+ 1) "Increment argument by one.")
(1+ 10) ⇒ 11
It is common for Lisp functions to accept functions as arguments or
find them in data structures (especially in hook variables and property
lists) and call them using
that accept function arguments are often called functionals.
Sometimes, when you call a functional, it is useful to supply a no-op function as the argument. Here are two different kinds of no-op function:
This function returns arg and has no side effects.
This function ignores any arguments and returns
Some functions are user-visible commands, which can be called
interactively (usually by a key sequence). It is possible to invoke
such a command exactly as though it was called interactively, by using
call-interactively function. See Interactive Call.
This is related to, but different from currying, which transforms a function that takes multiple arguments in such a way that it can be called as a chain of functions, each one with a single argument.