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
funcallcalls function with arguments, and returns whatever function returns.
funcallis a function, all of its arguments, including function, are evaluated before
funcallis called. This means that you can use any expression to obtain the function to be called. It also means that
funcalldoes 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
funcallis 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 expressions.
funcallcannot 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
applycalls function with arguments, just like
funcallbut 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
applyspreads this list so that each individual element becomes an argument.
applyreturns 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
apply.(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 function1. 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
<n arguments will produce a new function of n
Here's how we could define the built-in function
1+, if it didn't exist, using
+, another built-in function:(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:
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.