The Common Lisp Cookbook – Functions

Table of Contents

The Common Lisp Cookbook – Functions

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Named functions: defun

Creating named functions is done with the defun keyword. It follows this model:

(defun <name> (list of arguments)
  (function body))

The return value is the value returned by the last expression of the body (see below for more). There is no “return xx” statement.

So, for example:

(defun hello-world ()
  ;;               ^^ no arguments
  (print "hello world!"))

Call it:

;; "hello world!"  <-- output
;; "hello world!"  <-- a string is returned.


Base case: required arguments

Add in arguments like this:

(defun hello (name)
  "Say hello to `name'."
  (format t "hello ~a !~&" name))

(where ~a is the most used format directive to print a variable aesthetically and ~& prints a newline)

Call the function:

(hello "me")
;; hello me !  <-- this is printed by `format`
;; NIL         <-- return value: `format t` prints a string
;;                 to standard output and returns nil.

If you don’t specify the right amount of arguments, you’ll be trapped into the interactive debugger with an explicit error message:


invalid number of arguments: 0

Optional arguments: &optional

Optional arguments are declared after the &optional keyword in the lambda list. They are ordered, they must appear one after another.

This function:

(defun hello (name &optional age gender) …)

must be called like this:

(hello "me") ;; a value for the required argument,
             ;; zero optional arguments
(hello "me" "7")  ;; a value for age
(hello "me" 7 :h) ;; a value for age and gender

Named parameters: &key

It is not always convenient to remember the order of the arguments. It is thus possible to supply arguments by name: we declare them using &key <name>, we set them with :name <value> in the function call, and we use name as a regular variable in the function body. They are nil by default.

(defun hello (name &key happy)
  "If `happy' is `t', print a smiley"
  (format t "hello ~a " name)
  (when happy
    (format t ":)~&")))

The following calls are possible:

(hello "me")
(hello "me" :happy t)
(hello "me" :happy nil) ;; useless, equivalent to (hello "me")

and this is not valid: (hello "me" :happy):

odd number of &KEY arguments

A similar example of a function declaration, with several key parameters:

(defun hello (name &key happy lisper cookbook-contributor-p) …)

it can be called with zero or more key parameters, in any order:

(hello "me" :lisper t)
(hello "me" :lisper t :happy t)
(hello "me" :cookbook-contributor-p t :happy t)

Last but not least, you would quickly realize it, but we can choose the keys programmatically (they can be variables):

(let ((key :happy)
      (val t))
  (hello "me" key val))
;; hello me :)
;; NIL

Mixing optional and key parameters

It is generally a style warning, but it is possible.

(defun hello (&optional name &key happy)
  (format t "hello ~a " name)
  (when happy
    (format t ":)~&")))

In SBCL, this yields:

;       (BLOCK HELLO (FORMAT T "hello ~a " NAME) (WHEN HAPPY (FORMAT T ":)~&"))))
;   &OPTIONAL and &KEY found in the same lambda list: (&OPTIONAL (NAME "John") &KEY
;                                                      HAPPY)
; compilation unit finished
;   caught 1 STYLE-WARNING condition

We can call it:

(hello "me" :happy t)
;; hello me :)
;; NIL

Default values to key parameters

In the lambda list, use pairs to give a default value to an optional or a key argument, like (happy t) below:

(defun hello (name &key (happy t))

Now happy is true by default.

Was a key parameter specified?

You can skip this tip for now if you want, but come back later to it as it can turn handy.

We saw that a default key parameter is nil by default ((defun hello (name &key happy) …)). But how can be distinguish between “the value is NIL by default” and “the user wants it to be NIL”?

We saw how to use a tuple to set its default value:

&key (:happy t)

To answer our question, use a triple like this:

&key (happy t happy-p)

where happy-p serves as a predicate variable (using -p is only a convention, give it the name you want) to know if the key was supplied. If it was, then it will be T.

So now, we will print a sad face if :happy was explicitely set to NIL. We don’t print it by default.

(defun hello (name &key (happy nil happy-p))
  (format t "Key supplied? ~a~&" happy-p)
  (format t "hello ~a " name)
  (when happy-p
    (if happy
      (format t ":)")
      (format t ":("))))

Variable number of arguments: &rest

Sometimes you want a function to accept a variable number of arguments. Use &rest <variable>, where <variable> will be a list.

(defun mean (x &rest numbers)
    (/ (apply #'+ x numbers)
       (1+ (length numbers))))
(mean 1)
(mean 1 2)  ;; => 3/2 (yes, it is printed as a ratio)
(mean 1 2 3 4 5) ;;  => 3

Defining key arguments, and allowing more: &allow-other-keys


(defun hello (name &key happy)
  (format t "hello ~a~&" name))

(hello "me" :lisper t)
;; => Error: unknown keyword argument


(defun hello (name &key happy &allow-other-keys)
  (format t "hello ~a~&" name))

(hello "me" :lisper t)
;; hello me

We might need &allow-other-keys when passing around arguments or with higher level manipulation of functions.

Here’s a real example. We define a function to open a file that always uses :if-exists :supersede, but still passes any other keys to the open function.

(defun open-supersede (f &rest other-keys &key &allow-other-keys)
  (apply #'open f :if-exists :supersede other-keys))

In the case of a duplicated :if-exists argument, our first one takes precedence.

Return values

The return value of the function is the value returned by the last executed form of the body.

There are ways for non-local exits (return-from <function name> <value>), but they are usually not needed.

Common Lisp has also the concept of multiple return values.

Multiple return values: values, multiple-value-bind and nth-value

Returning multiple values is not like returning a tuple or a list of results ;) This is a common misconception.

Multiple values are specially useful and powerful because a change in them needs little to no refactoring.

(defun foo (a b c)

This function returns a.

(defvar *res* (foo :a :b :c))
;; :A

We use values to return multiple values:

(defun foo (a b c)
  (values a b c))
(setf *res* (foo :a :b :c))
;; :A

Observe here that *res* is still :A.

All functions that use the return value of foo need not to change, they still work. If we had returned a list or an array, this would be different.


We destructure multiple values with multiple-value-bind (or mvb+TAB in Slime for short) and we can get one given its position with nth-value:

(multiple-value-bind (res1 res2 res3)
    (foo :a :b :c)
  (format t "res1 is ~a, res2 is ~a, res2 is ~a~&"
     res1 res2 res3))
;; res1 is A, res2 is B, res2 is C
;; NIL

Its general form is

(multiple-value-bind (var-1 .. var-n) expr

The variables var-n are not available outside the scope of multiple-value-bind.

With nth-value:

(nth-value 0 (values :a :b :c))  ;; => :A
(nth-value 2 (values :a :b :c))  ;; => :C
(nth-value 9 (values :a :b :c))  ;; => NIL

Look here too that values is different from a list:

(nth-value 0 '(:a :b :c)) ;; => (:A :B :C)
(nth-value 1 '(:a :b :c)) ;; => NIL

Note that (values) with no values returns… no values at all.


While we are at it: multiple-value-list turns multiple values to a list:

(multiple-value-list (values 1 2 3))
;; (1 2 3)

The reverse is values-list, it turns a list to multiple values:

(values-list '(1 2 3))
;; 1
;; 2
;; 3

Anonymous functions: lambda

Anonymous functions are created with lambda:

(lambda (x) (print x))

We can call a lambda with funcall or apply (see below).

If the first element of an unquoted list is a lambda expression, the lambda is called:

((lambda (x) (print x)) "hello")
;; hello

Calling functions programmatically: funcall and apply

funcall is to be used with a known number of arguments, when apply can be used on a list, for example from &rest:

(funcall #'+ 1 2)
(apply #'+ '(1 2))

Referencing functions by name: single quote ' or sharpsign-quote #'?

In the example above, we used #', but a single quote also works, and we can encounter it in the wild. Which one to use?

It is generally safer to use #', because it respects lexical scope. Observe:

(defun foo (x)
  (* x 100))

(flet ((foo (x) (1+ x)))
  (funcall #'foo 1))
;; => 2, as expected

;; But:

(flet ((foo (x) (1+ x)))
  (funcall 'foo 1))
;; => 100

#' is actually the shorthand for (function …):

(function +)
;; #<FUNCTION +>

(flet ((foo (x) (1+ x)))
  (print (function foo))
  (funcall (function foo) 1))
;; 2

Using function or the #' shorthand allows us to refer to local functions. If we pass instead a symbol to funcall, what is called is always the function named by that symbol in the global environment.

In addition, #' catches the function by value. If the function is redefined, bindings that refered to this function by #' will still run its original behaviour.

Let’s assign a function to a parameter:

(defparameter *foo-caller* #'foo)
(funcall *foo-caller* 1)
;; => 100

Now, if we redefine foo, the behaviour of *foo-caller* will not change:

(defun foo (x) (1+ x))
;; WARNING: redefining CL-USER::FOO in DEFUN
;; FOO

(funcall *foo-caller* 1)
;; 100  ;; and not 2

If we bind the caller with 'foo, a single quote, the function will be resolved at runtime:

(defun foo (x) (* x 100))  ;; back to original behavior.
(defparameter *foo-caller-2* 'foo)
(funcall *foo-caller-2* 1)
;; 100

;; We change the definition:
(defun foo (x) (1+ x))
;; WARNING: redefining CL-USER::FOO in DEFUN
;; FOO

;; We try again:
(funcall *foo-caller-2* 1)
;; 2

The behaviour you want depends on your use case. Generally, using sharpsign-quote is less surprising. But if you are running a tight loop and you want live-update mechanisms (think a game or live visualisations), you might want to use a single quote so that your loop picks up the user’s new function definition.

Higher order functions: functions that return functions

Writing functions that return functions is simple enough:

(defun adder (n)
  (lambda (x) (+ x n)))

Here we have defined the function adder which returns an object of type function.

To call the resulting function, we must use funcall or apply:

(adder 5)
(funcall (adder 5) 3)
;; 8

Trying to call it right away leads to an illegal function call:

((adder 3) 5)
In: (ADDER 3) 5
    ((ADDER 3) 5)
Error: Illegal function call.

Indeed, CL has different namespaces for functions and variables, i.e. the same name can refer to different things depending on its position in a form that’s evaluated.

;; The symbol foo is bound to nothing:
CL-USER> (boundp 'foo)
CL-USER> (fboundp 'foo)
;; We create a variable:
CL-USER> (defparameter foo 42)
* foo
;; Now foo is "bound":
CL-USER> (boundp 'foo)
;; but still not as a function:
CL-USER> (fboundp 'foo)
;; So let's define a function:
CL-USER> (defun foo (x) (* x x))
;; Now the symbol foo is bound as a function too:
CL-USER> (fboundp 'foo)
;; Get the function:
CL-USER> (function foo)
;; and the shorthand notation:
* #'foo
;; We call it:
(funcall (function adder) 5)
#<CLOSURE (LAMBDA (X) :IN ADDER) {100991761B}>
;; and call the lambda:
(funcall (funcall (function adder) 5) 3)

To simplify a bit, you can think of each symbol in CL having (at least) two “cells” in which information is stored. One cell - sometimes referred to as its value cell - can hold a value that is bound to this symbol, and you can use boundp to test whether the symbol is bound to a value (in the global environment). You can access the value cell of a symbol with symbol-value.

The other cell - sometimes referred to as its function cell - can hold the definition of the symbol’s (global) function binding. In this case, the symbol is said to be fbound to this definition. You can use fboundp to test whether a symbol is fbound. You can access the function cell of a symbol (in the global environment) with symbol-function.

Now, if a symbol is evaluated, it is treated as a variable in that its value cell is returned (just foo). If a compound form, i.e. a cons, is evaluated and its car is a symbol, then the function cell of this symbol is used (as in (foo 3)).

In Common Lisp, as opposed to Scheme, it is not possible that the car of the compound form to be evaluated is an arbitrary form. If it is not a symbol, it must be a lambda expression, which looks like (lambda lambda-list form*).

This explains the error message we got above - (adder 3) is neither a symbol nor a lambda expression.

If we want to be able to use the symbol *my-fun* in the car of a compound form, we have to explicitly store something in its function cell (which is normally done for us by the macro defun):

;;; continued from above
CL-USER> (fboundp '*my-fun*)
CL-USER> (setf (symbol-function '*my-fun*) (adder 3))
CL-USER> (fboundp '*my-fun*)
CL-USER> (*my-fun* 5)

Read the CLHS section about form evaluation for more.


Closures allow to capture lexical bindings:

(let ((limit 3)
      (counter -1))
    (defun my-counter ()
      (if (< counter limit)
          (incf counter)
          (setf counter 0))))


Or similarly:

(defun repeater (n)
  (let ((counter -1))
     (lambda ()
       (if (< counter n)
         (incf counter)
         (setf counter 0)))))

(defparameter *my-repeater* (repeater 3))
(funcall *my-repeater*)
(funcall *my-repeater*)
(funcall *my-repeater*)
(funcall *my-repeater*)
(funcall *my-repeater*)

See more on Practical Common Lisp.

setf functions

A function name can also be a list of two symbols with setf as the first one, and where the first argument is the new value:

(defun (setf <name>) (new-value <other arguments>)

This mechanism is particularly used for CLOS methods.

A silly example:

(defparameter *current-name* ""
  "A global name.")

(defun hello (name)
  (format t "hello ~a~&" name))

(defun (setf hello) (new-value)
  (hello new-value)
  (setf *current-name* new-value)
  (format t "current name is now ~a~&" new-value))

(setf (hello) "Alice")
;; hello Alice
;; current name is now Alice
;; NIL



A related concept is that of currying which you might be familiar with if you’re coming from a functional language. After we’ve read the last section that’s rather easy to implement:

CL-USER> (defun curry (function &rest args)
           (lambda (&rest more-args)
	           (apply function (append args more-args))))
CL-USER> (funcall (curry #'+ 3) 5)
CL-USER> (funcall (curry #'+ 3) 6)
CL-USER> (setf (symbol-function 'power-of-ten) (curry #'expt 10))
#<Interpreted Function "LAMBDA (FUNCTION &REST ARGS)" {482DB969}>
CL-USER> (power-of-ten 3)

With the Alexandria library

Now that you know how to do it, you may appreciate using the implementation of the Alexandria library (in Quicklisp).

(ql:quickload "alexandria")

(defun adder (foo bar)
  "Add the two arguments."
  (+ foo bar))

(defvar add-one (alexandria:curry #'adder 1) "Add 1 to the argument.")

(funcall add-one 10)  ;; => 11

(setf (symbol-function 'add-one) add-one)
(add-one 10)  ;; => 11


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