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Where X=Common Lisp

ANSI 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 and freely available.

Another popular and recent book is Land of Lisp.

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;;; 0. Syntax
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;;; 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.


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;;; 1. Primitive Datatypes and Operators
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;;; 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 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 - and the empty list
(not nil)            ; => t
(and 0 t)            ; => t
(or 0 nil)           ; => 0

                     ;;; Characters
#\A                  ; => #\A
#                  ; => #\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 too!
(concatenate 'string "Hello " "world!") ; => "Hello world!"

;; A string can be treated like a sequence 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; ~% is the format specifier for newline.
(format t "Common Lisp is groovy. Dude.~%")


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 2. Variables
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;; You can create a global (dynamically scoped) using defparameter
;; a variable name can use any character except: ()",'`;#|\

;; Dynamically scoped variables should 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 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
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;; 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"

;; Dog-p, make-dog, and dog-name are all created by defstruct!

;;; 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 - surprisingly - append lists together
(append '(1 2) '(3 4)) ; => '(1 2 3 4)

;; Or use concatenate -

(concatenate 'list '(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))

;; (make-array '(2 2)) works as well.

; => #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 are
;; implementation-defined. Here's how to define them:

(make-array '(2) :initial-element 'unset)

; => #(UNSET UNSET)

;; And, to access the element at 1,1,1 -
(aref (make-array (list 2 2 2)) 1 1 1)

; => 0

;;; 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 element:
(vector-push-extend 4 *adjvec*) ; => 3

*adjvec* ; => #(1 2 3 4)



;;; Naively, 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)

;; But you'll want to use a better data structure than a linked list
;; for performant work!

;;; Dictionaries are implemented as hash tables.

;; Create a hash table
(defparameter *m* (make-hash-table))

;; set a value
(setf (gethash 'a *m*) 1)

;; Retrieve a value
(gethash 'a *m*) ; => 1, t

;; Detail - Common Lisp has multiple return values possible. gethash
;; returns t in the second value if anything was found, and nil if
;; not.

;; 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
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;; Use `lambda' to create anonymous functions.
;; A function always returns the value of its last expression.
;; The exact printable representation of a function will vary...

(lambda () "Hello World") ; => #<FUNCTION (LAMBDA ()) {1004E7818B}>

;; Use funcall to call lambda functions
(funcall (lambda () "Hello World")) ; => "Hello World"

;; Or Apply
(apply (lambda () "Hello World") nil) ; => "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))

(hello "Steve")
; => Hello, Steve, from The world

(hello "Steve" "the alpacas")
; => Hello, Steve, from the alpacas


;; 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
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;; Common Lisp has a sophisticated equality system. A couple 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

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;; 5. Control Flow
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;;; 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 (zerop n)
      :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
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;; 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 destructive functions and to avoid
;; mutation when reasonable.

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 7. Classes and Objects
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;; 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, followed by name, followed by the superclass list,
;; followed by slot list, followed by optional qualities such as
;; :documentation.

;; 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 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 wheel turns out to be using the equation: C = d * pi

(defmethod circumference ((object bicycle))
  (* pi (wheel-size object)))

;; pi is defined in Lisp already for us!

;; 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
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;; 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
           (unless ,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
         (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 for more information on macros.

Further Reading

Keep moving on to the Practical Common Lisp book.

Credits.

Lots of thanks to the Scheme people for rolling up a great starting point which could be easily moved to Common Lisp.


Got a suggestion? A correction, perhaps? Open an Issue on the Github Repo, or make a pull request yourself!

Originally contributed by Paul Nathan, and updated by 5 contributors