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---
language: commonlisp
filename: commonlisp.lisp
contributors:
- ["Paul Nathan", "https://github.com/pnathan"]
---
ANSI Common Lisp is a general purpose, multi-paradigm programming
language suited for a wide variety of industry applications. It is
frequently referred to a programmable programming language.
The classic starting point is [Practical Common Lisp and freely available.](http://www.gigamonkeys.com/book/)
Another popular and recent book is
[Land of Lisp](http://landoflisp.com/).
```commonlisp
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; 0. Syntax
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; General form.
;; Lisp has two fundamental pieces of syntax: the ATOM and the
;; S-expression. Typically, grouped S-expressions are called `forms`.
10 ; an atom; it evaluates to itself
:THING ;Another atom; evaluating to the symbol :thing.
t ; another atom, denoting true.
(+ 1 2 3 4) ; an s-expression
'(4 :foo t) ;another one
;;; Comments
;; Single line comments start with a semicolon; use two for normal
;; comments, three for section comments, and four for file-level
;; comments.
#| Block comments
can span multiple lines and...
#|
they can be nested!
|#
|#
;;; Environment.
;; A variety of implementations exist; most are
;; standard-conformant. CLISP is a good starting one.
;; Libraries are managed through Quicklisp.org's Quicklisp system.
;; Common Lisp is usually developed with a text editor and a REPL
;; (Read Evaluate Print Loop) running at the same time. The REPL
;; allows for interactive exploration of the program as it is "live"
;; in the system.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; 1. Primitive Datatypes and Operators
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Symbols
'foo ; => FOO
(intern "AAAA") ; => AAAA
;;; Numbers
9999999999999999999999 ; integers
#b111 ; binary => 7
#o111 ; octal => 73
#x111 ; hexadecimal => 273
3.14159 ; floating point
1/2 ; ratios
#C(1 2) ; complex numbers
;; Function application is written (f x y z ...)
;; where f is a function and x, y, z, ... are operands
;; If you want to create a literal list of data, use ' to stop it from
;; being evaluated - literally, "quote" the data.
'(+ 1 2) ; => (+ 1 2)
;; You can also call a function manually:
(funcall #'+ 1 2 3) ; => 6
;; Some arithmetic operations
(+ 1 1) ; => 2
(- 8 1) ; => 7
(* 10 2) ; => 20
(expt 2 3) ; => 8
(mod 5 2) ; => 1
(/ 35 5) ; => 7
(/ 1 3) ; => 1/3
(+ #C(1 2) #C(6 -4)) ; => #C(7 -2)
;;; Booleans
t ; for true (any not-nil value is true)
nil ; for false
(not nil) ; => t
(and 0 t) ; => t
(or 0 nil) ; => 0
;;; Characters
#\A ; => #\A
#\λ ; => #\GREEK_SMALL_LETTER_LAMDA
#\u03BB ; => #\GREEK_SMALL_LETTER_LAMDA
;;; Strings are fixed-length simple-arrays of characters.
"Hello, world!"
"Benjamin \"Bugsy\" Siegel" ; backslash is an escaping character
;; Strings can be concatenated too!
(concatenate 'string "Hello " "world!") ; => "Hello world!"
;; A string can be treated like a list of characters
(elt "Apple" 0) ; => #\A
;; format can be used to format strings:
(format nil "~a can be ~a" "strings" "formatted")
;; Printing is pretty easy
(format t "Common Lisp is groovy. Dude.\n")
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 2. Variables
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; You can create a global (dynamically scoped) using defparameter
;; a variable name can use any character except: ()[]{}",'`;#|\
(defparameter *some-var* 5)
*some-var* ; => 5
;; You can also use unicode characters. Not very easy to use though...
(defparameter *foo#\u03BBooo* nil)
;; Accessing a previously unassigned variable is an undefined
;; behavior (but possible). Don't do it.
;; Local binding: `me` is bound to "dance with you" only within the
;; (let ...). Let always returns the value of the last `form` in the
;; let form.
(let ((me "dance with you"))
me)
;; => "dance with you"
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 3. Structs and Collections
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Structs
(defstruct dog name breed age)
(defparameter *rover*
(make-dog :name "rover"
:breed "collie"
:age 5))
*rover* ; => #S(DOG :NAME "rover" :BREED "collie" :AGE 5)
(dog-p *rover*) ; => t ;; ewww)
(dog-name *rover*) ; => "rover"
;;; Pairs
;; `cons' constructs pairs, `car' and `cdr' extract the first
;; and second elements
(cons 'SUBJECT 'VERB) ; => '(SUBJECT . VERB)
(car (cons 'SUBJECT 'VERB)) ; => SUBJECT
(cdr (cons 'SUBJECT 'VERB)) ; => VERB
;;; Lists
;; Lists are linked-list data structures, made of `cons' pairs and end
;; with a `nil' (or '()) to mark the end of the list
(cons 1 (cons 2 (cons 3 nil))) ; => '(1 2 3)
;; `list' is a convenience variadic constructor for lists
(list 1 2 3) ; => '(1 2 3)
;; and a quote can also be used for a literal list value
'(1 2 3) ; => '(1 2 3)
;; Can still use `cons' to add an item to the beginning of a list
(cons 4 '(1 2 3)) ; => '(4 1 2 3)
;; Use `append' to add lists together
(append '(1 2) '(3 4)) ; => '(1 2 3 4)
;; Lists are a very basic type, so there is a wide variety of functionality for
;; them, a few examples:
(mapcar #1+ '(1 2 3)) ; => '(2 3 4)
(mapcar #'+ '(1 2 3) '(10 20 30)) ; => '(11 22 33)
(remove-if-not #'evenp '(1 2 3 4)) ; => '(2 4)
(every #'evenp '(1 2 3 4)) ; => nil
(some #'oddp '(1 2 3 4)) ; => T
(butlast '(subject verb object)) ; => (SUBJECT VERB)
;;; Vectors
;; Vectors are fixed-length arrays
#(1 2 3) ; => #(1 2 3)
;; Use concatenate to add vectors together
(concatenate 'vector #(1 2 3) #(4 5 6)) ; => #(1 2 3 4 5 6)
;;; Arrays
;; Both vectors and strings are special-cases of arrays.
;; 2D arrays
(make-array (list 2 2))
; => #2A((0 0) (0 0))
(make-array (list 2 2 2))
; => #3A(((0 0) (0 0)) ((0 0) (0 0)))
; access the element at 1,1,1,
(aref (make-array (list 2 2 2)) 1 1 1)
; => 0
;;; Sets are just lists:
(set-difference '(1 2 3 4) '(4 5 6 7)) ; => (3 2 1)
(intersection '(1 2 3 4) '(4 5 6 7)) ; => 4
(union '(1 2 3 4) '(4 5 6 7)) ; => (3 2 1 4 5 6 7)
(adjoin 4 '(1 2 3 4)) ; => (1 2 3 4)
;;; Dictionaries are implemented as hash tables.
;; Create a hash table
(defparameter m (hash-table))
;; set a value
(setf (gethash 'a hash-table 1))
;; Retrieve a value
(gethash 'a m) ; => 1
;; Retrieving a non-present value returns a nil
(gethash m 'd) ;=> nil
;; You can provide a default value for missing keys
(gethash m 'd :not-found) ; => :NOT-FOUND
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 3. Functions
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Use `lambda' to create anonymous functions.
;; A function always returns the value of its last expression
(lambda () "Hello World") ; => #<function>
;; Use funcall to call lambda functions
(funcall (lambda () "Hello World")) ; => "Hello World"
;; De-anonymize the function
(defun hello-world () "Hello World")
(hello-world) ; => "Hello World"
;; The () in the above is the list of arguments for the function
(defun hello (name)
(format nil "Hello, ~a " name))
(hello "Steve") ; => "Hello, Steve"
;; Functions can have optional arguments; they default to nil
(defun hello (name &optional from)
(if from
(format t "Hello, ~a, from ~a" name from)
(format t "Hello, ~a" name)))
(hello "Jim" "Alpacas") ;; => Hello, Jim, from Alpacas
;; And the defaults can be set...
(defun hello (name &optional (from "The world"))
(format t "Hello, ~a, from ~a" name from))
;; And of course, keywords are allowed as well... usually more
;; flexible than &optional.
(defun generalized-greeter (name &key (from "the world") (honorific "Mx"))
(format t "Hello, ~a ~a, from ~a" honorific name from))
(generalized-greeter "Jim") ; => Hello, Mx Jim, from the world
(generalized-greeter "Jim" :from "the alpacas you met last summer" :honorific "Mr")
; => Hello, Mr Jim, from the alpacas you met last summer
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 4. Equality
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Common Lisp has a sophisticated equality system.
;; for numbers use `='
(= 3 3.0) ; => t
(= 2 1) ; => nil
;; for object identity (approximately) use `eq?'
(eql 3 3) ; => t
(eql 3 3.0) ; => nil
(eql (list 3) (list 3)) ; => nil
;; for collections use `equal'
(equal (list 'a 'b) (list 'a 'b)) ; => t
(equal (list 'a 'b) (list 'b 'a)) ; => nil
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 5. Control Flow
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Conditionals
(if t ; test expression
"this is true" ; then expression
"this is false") ; else expression
; => "this is true"
;; In conditionals, all non-nil values are treated as true
(member 'Groucho '(Harpo Groucho Zeppo)) ; => '(GROUCHO ZEPPO)
(if (member 'Groucho '(Harpo Groucho Zeppo))
'yep
'nope)
; => 'YEP
;; `cond' chains a series of tests to select a result
(cond ((> 2 2) (error "wrong!"))
((< 2 2) (error "wrong again!"))
(t 'ok)) ; => 'OK
;; Typecase switches on the type of the value
(typecase 1
(string :string)
(integer :int))
; => :int
;;; Iteration
;; Of course recursion is supported:
(defun walker (n)
(if (= n 0)
:walked
(walker (1- n))))
(walker) ; => :walked
;; Most of the time, we use DOLIST or LOOP
(dolist (i '(1 2 3 4))
(format t "~a" i))
; => 1234
(loop for i from 0 below 10
collect i)
; => (0 1 2 3 4 5 6 7 8 9)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 6. Mutation
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Use `setf' to assign a new value to an existing variable. This was
;; demonstrated earlier in the hash table example.
(let ((variable 10))
(setf variable 10))
; => 10
;; Good Lisp style is to minimize destructive functions and to avoid
;; mutation when reasonable.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 7. Classes and Objects
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; No more Animal classes, let's have Human-Powered Mechanical
;; Conveyances.
(defclass human-powered-conveyance ()
((velocity
:accessor velocity
:initarg :velocity)
(average-efficiency
:accessor average-efficiency)
:initarg :average-efficiency)
(:documentation "A human powered conveyance"))
(defclass bicycle (human-powered-conveyance)
((wheel-size
:accessor wheel-size
:initarg :wheel-size
:documentation "Diameter of the wheel.")
(height
:accessor height
:initarg :height)))
(defclass recumbent (bicycle)
((chain-type
:accessor chain-type
:initarg :chain-type)))
(defclass unicycle (human-powered-conveyance) nil)
(defclass canoe (human-powered-conveyance)
((number-of-rowers
:accessor number-of-rowers
:initarg :number-of-rowers)))
;; Calling DESCRIBE on the human-powered-conveyance class in the REPL gives:
(describe 'human-powered-conveyance)
; COMMON-LISP-USER::HUMAN-POWERED-CONVEYANCE
; [symbol]
;
; HUMAN-POWERED-CONVEYANCE names the standard-class #<STANDARD-CLASS
; HUMAN-POWERED-CONVEYANCE>:
; Documentation:
; A human powered conveyance
; Direct superclasses: STANDARD-OBJECT
; Direct subclasses: UNICYCLE, BICYCLE, CANOE
; Not yet finalized.
; Direct slots:
; VELOCITY
; Readers: VELOCITY
; Writers: (SETF VELOCITY)
; AVERAGE-EFFICIENCY
; Readers: AVERAGE-EFFICIENCY
; Writers: (SETF AVERAGE-EFFICIENCY)
;; Note the reflective behavior available to you! Common Lisp is
;; designed to be an interactive system
;; To define a method, let's find out what our circumference of the
;; bike turns out to be using the equation: C = d * pi
(defmethod circumference ((object bicycle))
(* 3.14159 (wheel-size object)))
;; Let's suppose we find out that the efficiency value of the number
;; of rowers in a canoe is roughly logarithmic. This should probably be set
;; in the constructor/initializer.
;; Here's how to initialize your instance after Common Lisp gets done
;; constructing it:
(defmethod initialize-instance :after ((object canoe) &rest args)
(setf (average-efficiency object) (log (1+ (number-of-rowers object)))))
;; Then to construct an instance and check the average efficiency...
(average-efficiency (make-instance 'canoe :number-of-rowers 15))
; => 2.7725887
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 8. Macros
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Macros let you extend the syntax of the language
;; Common Lisp doesn't come with a WHILE loop- let's add one.
;; If we obey our assembler instincts, we wind up with:
(defmacro while (condition &body body)
"While `condition` is true, `body` is executed.
`condition` is tested prior to each execution of `body`"
(let ((block-name (gensym)))
`(tagbody
(when (not ,condition)
(go ,block-name))
(progn
,@body)
,block-name)))
;; Let's look at the high-level version of this:
(defmacro while (condition &body body)
"While `condition` is true, `body` is executed.
`condition` is tested prior to each execution of `body`"
`(loop while ,condition
do
,@body))
;; However, with a modern compiler, this is not required; the LOOP
;; form compiles equally well and is easier to read.
;; Note that ` is used, as well as , and @. ` is a quote-type operator
;; known as quasiquote; it allows the use of ,. , allows "unquoting"
;; variables. @ interpolates lists.
;; Gensym creates a unique symbol guaranteed to not exist elsewhere in
;; the system. This is because macros are expanded at compile time and
;; variables declared in the macro can collide with variables used in
;; regular code.
;; See Practical Common Lisp for more information on macros.
## Further Reading
[Keep moving on to the Practical Common Lisp book.](http://www.gigamonkeys.com/book/)
## Credits.
Lots of thanks to the Scheme people for rolling up a great starting
point which could be easily moved to Common Lisp.