--- 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") ; => # ;;; 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 #: ; 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.