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---
language: "Common Lisp"
filename: commonlisp.lisp
contributors:
- ["Paul Nathan", "https://github.com/pnathan"]
- ["Rommel Martinez", "https://ebzzry.io"]
---
Common Lisp is a general-purpose, multi-paradigm programming language suited for a wide variety of
industry applications. It is frequently referred to as a programmable programming language.
The classic starting point is [Practical Common Lisp](http://www.gigamonkeys.com/book/). Another
popular and recent book is [Land of Lisp](http://landoflisp.com/). A new book about best practices,
[Common Lisp Recipes](http://weitz.de/cl-recipes/), was recently published.
```lisp
;;;-----------------------------------------------------------------------------
;;; 0. Syntax
;;;-----------------------------------------------------------------------------
;;; General form
;;; CL has two fundamental pieces of syntax: ATOM and 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 s-expression
;;; Comments
;;; Single-line comments start with a semicolon; use four for file-level
;;; comments, three for section descriptions, two inside definitions, and one
;;; for single lines. For example,
;;;; life.lisp
;;; Foo bar baz, because quu quux. Optimized for maximum krakaboom and umph.
;;; Needed by the function LINULUKO.
(defun meaning (life)
"Return the computed meaning of LIFE"
(let ((meh "abc"))
;; Invoke krakaboom
(loop :for x :across meh
:collect x))) ; store values into x, then return it
;;; Block comments, on the other hand, allow for free-form comments. They are
;;; delimited with #| and |#
#| This is a block comment which
can span multiple lines and
#|
they can be nested!
|#
|#
;;; Environment
;;; A variety of implementations exist; most are standards-conformant. SBCL
;;; is a good starting point. Third party libraries can be easily installed with
;;; Quicklisp
;;; CL is usually developed with a text editor and a Read Eval Print
;;; Loop (REPL) running at the same time. The REPL allows for interactive
;;; exploration of the program while it is running "live".
;;;-----------------------------------------------------------------------------
;;; 1. Primitive datatypes and operators
;;;-----------------------------------------------------------------------------
;;; Symbols
'foo ; => FOO Notice that the symbol is upper-cased automatically.
;;; INTERN manually creates a symbol from a string.
(intern "AAAA") ; => AAAA
(intern "aaa") ; => |aaa|
;;; Numbers
9999999999999999999999 ; integers
#b111 ; binary => 7
#o111 ; octal => 73
#x111 ; hexadecimal => 273
3.14159s0 ; single
3.14159d0 ; double
1/2 ; ratios
#C(1 2) ; complex numbers
;;; Function application are written as (f x y z ...) where f is a function and
;;; x, y, z, ... are the arguments.
(+ 1 2) ; => 3
;;; If you want to create literal data, use QUOTE to prevent it from being
;;; evaluated
(quote (+ 1 2)) ; => (+ 1 2)
(quote a) ; => A
;;; The shorthand for QUOTE is '
'(+ 1 2) ; => (+ 1 2)
'a ; => A
;;; Basic 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 ; true; any non-NIL value is true
nil ; false; also, the empty list: ()
(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 arrays of characters
"Hello, world!"
"Benjamin \"Bugsy\" Siegel" ; backslash is an escaping character
;;; Strings can be concatenated
(concatenate 'string "Hello, " "world!") ; => "Hello, world!"
;;; A string can be treated like a sequence of characters
(elt "Apple" 0) ; => #\A
;;; FORMAT is used to create formatted output, which ranges from simple string
;;; interpolation to loops and conditionals. The first argument to FORMAT
;;; determines where will the formatted string go. If it is NIL, FORMAT
;;; simply returns the formatted string as a value; if it is T, FORMAT outputs
;;; to the standard output, usually the screen, then it returns NIL.
(format nil "~A, ~A!" "Hello" "world") ; => "Hello, world!"
(format t "~A, ~A!" "Hello" "world") ; => NIL
;;;-----------------------------------------------------------------------------
;;; 2. Variables
;;;-----------------------------------------------------------------------------
;;; You can create a global (dynamically scoped) variable using DEFVAR and
;;; DEFPARAMETER. The variable name can use any character except: ()",'`;#|\
;;; The difference between DEFVAR and DEFPARAMETER is that re-evaluating a
;;; DEFVAR expression doesn't change the value of the variable. DEFPARAMETER,
;;; on the other hand, does.
;;; By convention, dynamically scoped variables have earmuffs in their name.
(defparameter *some-var* 5)
*some-var* ; => 5
;;; You can also use unicode characters.
(defparameter *AΛB* nil)
;;; Accessing a previously unbound variable results in an UNBOUND-VARIABLE
;;; error, however it is defined behavior. Don't do it.
;;; You can create local bindings with LET. In the following snippet, `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
(dog-name *rover*) ; => "rover"
;;; DOG-P, MAKE-DOG, and DOG-NAME are all automatically created by DEFSTRUCT
;;; Pairs
;;; CONS constructs pairs. CAR and CDR return the head and tail of a CONS-pair.
(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)
;;; When the first argument to CONS is an atom and the second argument is a
;;; list, CONS returns a new CONS-pair with the first argument as the first
;;; item and the second argument as the rest of the CONS-pair
(cons 4 '(1 2 3)) ; => '(4 1 2 3)
;;; Use APPEND to join lists
(append '(1 2) '(3 4)) ; => '(1 2 3 4)
;;; Or CONCATENATE
(concatenate 'list '(1 2) '(3 4)) ; => '(1 2 3 4)
;;; Lists are a very central 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
;;; Vector's literals 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 '(2 2)) ; => #2A((0 0) (0 0))
(make-array (list 2 2 2)) ; => #3A(((0 0) (0 0)) ((0 0) (0 0)))
;;; Caution: the default initial values of MAKE-ARRAY are implementation-defined.
;;; To explicitly specify them:
(make-array '(2) :initial-element 'unset) ; => #(UNSET UNSET)
;;; To access the element at 1, 1, 1:
(aref (make-array (list 2 2 2)) 1 1 1) ; => 0
;;; This value is implementation-defined:
;;; NIL on ECL, 0 on SBCL and CCL.
;;; Adjustable vectors
;;; Adjustable vectors have the same printed representation as
;;; fixed-length vector's literals.
(defparameter *adjvec* (make-array '(3) :initial-contents '(1 2 3)
:adjustable t :fill-pointer t))
*adjvec* ; => #(1 2 3)
;;; Adding new elements
(vector-push-extend 4 *adjvec*) ; => 3
*adjvec* ; => #(1 2 3 4)
;;; Sets, naively, 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)
;;; However, you'll need a better data structure than linked lists when working
;;; with larger data sets
;;; Dictionaries are implemented as hash tables.
;;; Create a hash table
(defparameter *m* (make-hash-table))
;;; Set value
(setf (gethash 'a *m*) 1)
;;; Retrieve value
(gethash 'a *m*) ; => 1, T
;;; CL expressions have the ability to return multiple values.
(values 1 2) ; => 1, 2
;;; which can be bound with MULTIPLE-VALUE-BIND
(multiple-value-bind (x y)
(values 1 2)
(list y x))
; => '(2 1)
;;; GETHASH is an example of a function that returns multiple values. The first
;;; value it return is the value of the key in the hash table; if the key is
;;; not found it returns NIL.
;;; The second value determines if that key is indeed present in the hash
;;; table. If a key is not found in the table it returns NIL. This behavior
;;; allows us to check if the value of a key is actually NIL.
;;; Retrieving a non-present value returns nil
(gethash 'd *m*) ;=> NIL, NIL
;;; You can provide a default value for missing keys
(gethash 'd *m* :not-found) ; => :NOT-FOUND
;;; Let's handle the multiple return values here in code.
(multiple-value-bind (a b)
(gethash 'd *m*)
(list a b))
; => (NIL NIL)
(multiple-value-bind (a b)
(gethash 'a *m*)
(list a b))
; => (1 T)
;;;-----------------------------------------------------------------------------
;;; 3. Functions
;;;-----------------------------------------------------------------------------
;;; Use LAMBDA to create anonymous functions. Functions always returns the
;;; value of the last expression. The exact printable representation of a
;;; function varies between implementations.
(lambda () "Hello World") ; => #<FUNCTION (LAMBDA ()) {1004E7818B}>
;;; Use FUNCALL to call anonymous functions
(funcall (lambda () "Hello World")) ; => "Hello World"
(funcall #'+ 1 2 3) ; => 6
;;; A call to FUNCALL is also implied when the lambda expression is the CAR of
;;; an unquoted list
((lambda () "Hello World")) ; => "Hello World"
((lambda (val) val) "Hello World") ; => "Hello World"
;;; FUNCALL is used when the arguments are known beforehand. Otherwise, use APPLY
(apply #'+ '(1 2 3)) ; => 6
(apply (lambda () "Hello World") nil) ; => "Hello World"
;;; To name a function, use DEFUN
(defun hello-world () "Hello World")
(hello-world) ; => "Hello World"
;;; The () in the definition above is the list of arguments
(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
;;; The default values can also be specified
(defun hello (name &optional (from "The world"))
(format nil "Hello, ~A, from ~A" name from))
(hello "Steve") ; => Hello, Steve, from The world
(hello "Steve" "the alpacas") ; => Hello, Steve, from the alpacas
;;; Functions also have keyword arguments to allow non-positional arguments
(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
;;;-----------------------------------------------------------------------------
;;; CL has a sophisticated equality system. Some are covered here.
;;; For numbers, use `='
(= 3 3.0) ; => T
(= 2 1) ; => NIL
;;; For object identity (approximately) use EQL
(eql 3 3) ; => T
(eql 3 3.0) ; => NIL
(eql (list 3) (list 3)) ; => NIL
;;; for lists, strings, and bit-vectors 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
;;; Looping
;;; Recursion
(defun fact (n)
(if (< n 2)
1
(* n (fact(- n 1)))))
(fact 5) ; => 120
;;; Iteration
(defun fact (n)
(loop :for result = 1 :then (* result i)
:for i :from 2 :to n
:finally (return result)))
(fact 5) ; => 120
(loop :for x :across "abcd" :collect x)
; => (#\a #\b #\c #\d)
(dolist (i '(1 2 3 4))
(format t "~A" i))
; => 1234
;;;-----------------------------------------------------------------------------
;;; 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 2))
; => 2
;;; Good Lisp style is to minimize the use of 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"))
;;; The arguments to DEFCLASS, in order are:
;;; 1. class name
;;; 2. superclass list
;;; 3. slot list
;;; 4. optional specifiers
;;; When no superclass list is set, the empty list defaults to the
;;; standard-object class. This *can* be changed, but not until you
;;; know what you're doing. Look up the Art of the Metaobject Protocol
;;; for more information.
(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. CL was designed to be an
;;; interactive system
;;; To define a method, let's find out what our circumference of the
;;; bike wheel turns out to be using the equation: C = d * pi
(defmethod circumference ((object bicycle))
(* pi (wheel-size object)))
;;; PI is defined as a built-in in CL
;;; 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.
;;; To initialize your instance after CL 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. CL doesn't come
;;; with a WHILE loop, however, it's trivial to write 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)) (done (gensym)))
`(tagbody
,block-name
(unless ,condition
(go ,done))
(progn
,@body)
(go ,block-name)
,done)))
;;; 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
(progn
,@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 and On Lisp for more information on macros.
```
## Further reading
- [Practical Common Lisp](http://www.gigamonkeys.com/book/)
- [Common Lisp: A Gentle Introduction to Symbolic Computation](https://www.cs.cmu.edu/~dst/LispBook/book.pdf)
## Extra information
- [CLiki](http://www.cliki.net/)
- [common-lisp.net](https://common-lisp.net/)
- [Awesome Common Lisp](https://github.com/CodyReichert/awesome-cl)
- [Lisp Lang](http://lisp-lang.org/)
## Credits
Lots of thanks to the Scheme people for rolling up a great starting
point which could be easily moved to Common Lisp.
- [Paul Khuong](https://github.com/pkhuong) for some great reviewing.
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