### 13.5 Calling Functions

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 function `concat` with arguments `"a"` and `"b"`. 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 the function `funcall`. When you also need to determine at run time how many arguments to pass, use `apply`.

Function: funcall function &rest arguments

`funcall` calls function with arguments, and returns whatever function returns.

Since `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 expressions. `funcall` cannot provide these because, as we saw above, it never knows them in the first place.

If you need to use `funcall` to call a command and make it behave as if invoked interactively, use `funcall-interactively` (see Interactive Call).

```(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`.

Function: apply function &rest arguments

`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 `apply` spreads this list so that each individual element becomes an argument.

`apply` with a single argument is special: the first element of the argument, which must be a non-empty list, is called as a function with the remaining elements as individual arguments. Passing two or more arguments will be faster.

`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 `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)
```
```
```
```(apply '(+ 3 4))
⇒ 7
```

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 function13. 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:

Function: apply-partially func &rest args

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 `apply-partially` with `m <= n` arguments will produce a new function of `n - m` arguments14.

Here’s how we could define the built-in function `1+`, if it didn’t exist, using `apply-partially` and `+`, another built-in function15:

```(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 `funcall` or `apply`. Functions 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:

Function: identity argument

This function returns argument and has no side effects.

Function: ignore &rest arguments

This function ignores any arguments and returns `nil`.

Function: always &rest arguments

This function ignores any arguments and returns `t`.

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 the `call-interactively` function. See Interactive Call.

#### Footnotes

##### (13)

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.

##### (14)

If the number of arguments that func can accept is unlimited, then the new function will also accept an unlimited number of arguments, so in that case `apply-partially` doesn’t reduce the number of arguments that the new function could accept.

##### (15)

Note that unlike the built-in function, this version accepts any number of arguments.