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+---
+
+language: racket
+filename: learnracket.rkt
+contributors:
+ - ["th3rac25", "https://github.com/voila"]
+ - ["Eli Barzilay", "https://github.com/elibarzilay"]
+---
+
+Racket is a general purpose, multi-paradigm programming language in the Lisp/Scheme family.
+
+Feedback is appreciated! You can reach me at [@th3rac25](http://twitter.com/th3rac25) or th3rac25 [at] [google's email service]
+
+
+```racket
+#lang racket ; defines the language we are using
+
+;;; Comments
+
+;; Single line comments start with a semicolon
+
+#| Block comments
+ can span multiple lines and...
+ #|
+ they can be nested!
+ |#
+|#
+
+;; S-expression comments discard the following expression,
+;; useful to comment expressions when debugging
+#; (this expression is discarded)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 1. Primitive Datatypes and Operators
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;;; Numbers
+9999999999999999999999 ; integers
+#b111 ; binary => 7
+#o111 ; octal => 73
+#x111 ; hexadecimal => 273
+3.14 ; reals
+6.02e+23
+1/2 ; rationals
+1+2i ; 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
+'(+ 1 2) ; => (+ 1 2)
+;; Now, some arithmetic operations
+(+ 1 1) ; => 2
+(- 8 1) ; => 7
+(* 10 2) ; => 20
+(expt 2 3) ; => 8
+(quotient 5 2) ; => 2
+(remainder 5 2) ; => 1
+(/ 35 5) ; => 7
+(/ 1 3) ; => 1/3
+(exact->inexact 1/3) ; => 0.3333333333333333
+(+ 1+2i 2-3i) ; => 3-1i
+
+;;; Booleans
+#t ; for true
+#f ; for false -- any value other than #f is true
+(not #t) ; => #f
+(and 0 #f (error "doesn't get here")) ; => #f
+(or #f 0 (error "doesn't get here")) ; => 0
+
+;;; Characters
+#\A ; => #\A
+#\λ ; => #\λ
+#\u03BB ; => #\λ
+
+;;; Strings are fixed-length array of characters.
+"Hello, world!"
+"Benjamin \"Bugsy\" Siegel" ; backslash is an escaping character
+"Foo\tbar\41\x21\u0021\a\r\n" ; includes C escapes, Unicode
+"λx:(μα.α→α).xx" ; can include Unicode characters
+
+;; Strings can be added too!
+(string-append "Hello " "world!") ; => "Hello world!"
+
+;; A string can be treated like a list of characters
+(string-ref "Apple" 0) ; => #\A
+
+;; format can be used to format strings:
+(format "~a can be ~a" "strings" "formatted")
+
+;; Printing is pretty easy
+(printf "I'm Racket. Nice to meet you!\n")
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 2. Variables
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; You can create a variable using define
+;; a variable name can use any character except: ()[]{}",'`;#|\
+(define some-var 5)
+some-var ; => 5
+
+;; You can also use unicode characters
+(define ⊆ subset?)
+(⊆ (set 3 2) (set 1 2 3)) ; => #t
+
+;; Accessing a previously unassigned variable is an exception
+; x ; => x: undefined ...
+
+;; Local binding: `me' is bound to "Bob" only within the (let ...)
+(let ([me "Bob"])
+ "Alice"
+ me) ; => "Bob"
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 3. Structs and Collections
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Structs
+(struct dog (name breed age))
+(define my-pet
+ (dog "lassie" "collie" 5))
+my-pet ; => #<dog>
+(dog? my-pet) ; => #t
+(dog-name my-pet) ; => "lassie"
+
+;;; Pairs (immutable)
+;; `cons' constructs pairs, `car' and `cdr' extract the first
+;; and second elements
+(cons 1 2) ; => '(1 . 2)
+(car (cons 1 2)) ; => 1
+(cdr (cons 1 2)) ; => 2
+
+;;; Lists
+
+;; Lists are linked-list data structures, made of `cons' pairs and end
+;; with a `null' (or '()) to mark the end of the list
+(cons 1 (cons 2 (cons 3 null))) ; => '(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 *lot* of functionality for
+;; them, a few examples:
+(map add1 '(1 2 3)) ; => '(2 3 4)
+(map + '(1 2 3) '(10 20 30)) ; => '(11 22 33)
+(filter even? '(1 2 3 4)) ; => '(2 4)
+(count even? '(1 2 3 4)) ; => 2
+(take '(1 2 3 4) 2) ; => '(1 2)
+(drop '(1 2 3 4) 2) ; => '(3 4)
+
+;;; Vectors
+
+;; Vectors are fixed-length arrays
+#(1 2 3) ; => '#(1 2 3)
+
+;; Use `vector-append' to add vectors together
+(vector-append #(1 2 3) #(4 5 6)) ; => #(1 2 3 4 5 6)
+
+;;; Sets
+
+;; Create a set from a list
+(list->set '(1 2 3 1 2 3 3 2 1 3 2 1)) ; => (set 1 2 3)
+
+;; Add a member with `set-add'
+;; (Functional: returns the extended set rather than mutate the input)
+(set-add (set 1 2 3) 4) ; => (set 1 2 3 4)
+
+;; Remove one with `set-remove'
+(set-remove (set 1 2 3) 1) ; => (set 2 3)
+
+;; Test for existence with `set-member?'
+(set-member? (set 1 2 3) 1) ; => #t
+(set-member? (set 1 2 3) 4) ; => #f
+
+;;; Hashes
+
+;; Create an immutable hash table (mutable example below)
+(define m (hash 'a 1 'b 2 'c 3))
+
+;; Retrieve a value
+(hash-ref m 'a) ; => 1
+
+;; Retrieving a non-present value is an exception
+; (hash-ref m 'd) => no value found
+
+;; You can provide a default value for missing keys
+(hash-ref m 'd 0) ; => 0
+
+;; Use `hash-set' to extend an immutable hash table
+;; (Returns the extended hash instdead of mutating it)
+(define m2 (hash-set m 'd 4))
+m2 ; => '#hash((b . 2) (a . 1) (d . 4) (c . 3))
+
+;; Remember, these hashes are immutable!
+m ; => '#hash((b . 2) (a . 1) (c . 3)) <-- no `d'
+
+;; Use `hash-remove' to remove keys (functional too)
+(hash-remove m 'a) ; => '#hash((b . 2) (c . 3))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 3. Functions
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Use `lambda' to create functions.
+;; A function always returns the value of its last expression
+(lambda () "Hello World") ; => #<procedure>
+;; Can also use a unicode `λ'
+(λ () "Hello World") ; => same function
+
+;; Use parens to call all functions, including a lambda expression
+((lambda () "Hello World")) ; => "Hello World"
+((λ () "Hello World")) ; => "Hello World"
+
+;; Assign a function to a var
+(define hello-world (lambda () "Hello World"))
+(hello-world) ; => "Hello World"
+
+;; You can shorten this using the function definition syntatcic sugae:
+(define (hello-world2) "Hello World")
+
+;; The () in the above is the list of arguments for the function
+(define hello
+ (lambda (name)
+ (string-append "Hello " name)))
+(hello "Steve") ; => "Hello Steve"
+;; ... or equivalently, using a sugared definition:
+(define (hello2 name)
+ (string-append "Hello " name))
+
+;; You can have multi-variadic functions too, using `case-lambda'
+(define hello3
+ (case-lambda
+ [() "Hello World"]
+ [(name) (string-append "Hello " name)]))
+(hello3 "Jake") ; => "Hello Jake"
+(hello3) ; => "Hello World"
+;; ... or specify optional arguments with a default value expression
+(define (hello4 [name "World"])
+ (string-append "Hello " name))
+
+;; Functions can pack extra arguments up in a list
+(define (count-args . args)
+ (format "You passed ~a args: ~a" (length args) args))
+(count-args 1 2 3) ; => "You passed 3 args: (1 2 3)"
+;; ... or with the unsugared `lambda' form:
+(define count-args2
+ (lambda args
+ (format "You passed ~a args: ~a" (length args) args)))
+
+;; You can mix regular and packed arguments
+(define (hello-count name . args)
+ (format "Hello ~a, you passed ~a extra args" name (length args)))
+(hello-count "Finn" 1 2 3)
+; => "Hello Finn, you passed 3 extra args"
+;; ... unsugared:
+(define hello-count2
+ (lambda (name . args)
+ (format "Hello ~a, you passed ~a extra args" name (length args))))
+
+;; And with keywords
+(define (hello-k #:name [name "World"] #:greeting [g "Hello"] . args)
+ (format "~a ~a, ~a extra args" g name (length args)))
+(hello-k) ; => "Hello World, 0 extra args"
+(hello-k 1 2 3) ; => "Hello World, 3 extra args"
+(hello-k #:greeting "Hi") ; => "Hi World, 0 extra args"
+(hello-k #:name "Finn" #:greeting "Hey") ; => "Hey Finn, 0 extra args"
+(hello-k 1 2 3 #:greeting "Hi" #:name "Finn" 4 5 6)
+ ; => "Hi Finn, 6 extra args"
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 4. Equality
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; for numbers use `='
+(= 3 3.0) ; => #t
+(= 2 1) ; => #f
+
+;; for object identity use `eq?'
+(eq? 3 3) ; => #t
+(eq? 3 3.0) ; => #f
+(eq? (list 3) (list 3)) ; => #f
+
+;; for collections use `equal?'
+(equal? (list 'a 'b) (list 'a 'b)) ; => #t
+(equal? (list 'a 'b) (list 'b 'a)) ; => #f
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 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-#f 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!")]
+ [else 'ok]) ; => 'ok
+
+;;; Pattern Matching
+
+(define (fizzbuzz? n)
+ (match (list (remainder n 3) (remainder n 5))
+ [(list 0 0) 'fizzbuzz]
+ [(list 0 _) 'fizz]
+ [(list _ 0) 'buzz]
+ [_ #f]))
+
+(fizzbuzz? 15) ; => 'fizzbuzz
+(fizzbuzz? 37) ; => #f
+
+;;; Loops
+
+;; Looping can be done through (tail-) recursion
+(define (loop i)
+ (when (< i 10)
+ (printf "i=~a\n" i)
+ (loop (add1 i))))
+(loop 5) ; => i=5, i=6, ...
+
+;; Similarly, with a named let
+(let loop ((i 0))
+ (when (< i 10)
+ (printf "i=~a\n" i)
+ (loop (add1 i)))) ; => i=0, i=1, ...
+
+;; See below how to add a new `loop' form, but Racket already has a very
+;; flexible `for' form for loops:
+(for ([i 10])
+ (printf "i=~a\n" i)) ; => i=0, i=1, ...
+(for ([i (in-range 5 10)])
+ (printf "i=~a\n" i)) ; => i=5, i=6, ...
+
+;;; Iteration Over Other Sequences
+;; `for' allows iteration over many other kinds of sequences:
+;; lists, vectors, strings, sets, hash tables, etc...
+
+(for ([i (in-list '(l i s t))])
+ (displayln i))
+
+(for ([i (in-vector #(v e c t o r))])
+ (displayln i))
+
+(for ([i (in-string "string")])
+ (displayln i))
+
+(for ([i (in-set (set 'x 'y 'z))])
+ (displayln i))
+
+(for ([(k v) (in-hash (hash 'a 1 'b 2 'c 3 ))])
+ (printf "key:~a value:~a\n" k v))
+
+;;; More Complex Iterations
+
+;; Parallel scan of multiple sequences (stops on shortest)
+(for ([i 10] [j '(x y z)]) (printf "~a:~a\n" i j))
+; => 0:x 1:y 2:z
+
+;; Nested loops
+(for* ([i 2] [j '(x y z)]) (printf "~a:~a\n" i j))
+; => 0:x, 0:y, 0:z, 1:x, 1:y, 1:z
+
+;; Conditions
+(for ([i 1000]
+ #:when (> i 5)
+ #:unless (odd? i)
+ #:break (> i 10))
+ (printf "i=~a\n" i))
+; => i=6, i=8, i=10
+
+;;; Comprehensions
+;; Very similar to `for' loops -- just collect the results
+
+(for/list ([i '(1 2 3)])
+ (add1 i)) ; => '(2 3 4)
+
+(for/list ([i '(1 2 3)] #:when (even? i))
+ i) ; => '(2)
+
+(for/list ([i 10] [j '(x y z)])
+ (list i j)) ; => '((0 x) (1 y) (2 z))
+
+(for/list ([i 1000] #:when (> i 5) #:unless (odd? i) #:break (> i 10))
+ i) ; => '(6 8 10)
+
+(for/hash ([i '(1 2 3)])
+ (values i (number->string i)))
+; => '#hash((1 . "1") (2 . "2") (3 . "3"))
+
+;; There are many kinds of other built-in ways to collect loop values:
+(for/sum ([i 10]) (* i i)) ; => 285
+(for/product ([i (in-range 1 11)]) (* i i)) ; => 13168189440000
+(for/and ([i 10] [j (in-range 10 20)]) (< i j)) ; => #t
+(for/or ([i 10] [j (in-range 0 20 2)]) (= i j)) ; => #t
+;; And to use any arbitrary combination, use `for/fold'
+(for/fold ([sum 0]) ([i '(1 2 3 4)]) (+ sum i)) ; => 10
+;; (This can often replace common imperative loops)
+
+;;; Exceptions
+
+;; To catch exceptions, use the `with-handlers' form
+(with-handlers ([exn:fail? (lambda (exn) 999)])
+ (+ 1 "2")) ; => 999
+(with-handlers ([exn:break? (lambda (exn) "no time")])
+ (sleep 3)
+ "phew") ; => "phew", but if you break it => "no time"
+
+;; Use `raise' to throw exceptions or any other value
+(with-handlers ([number? ; catch numeric values raised
+ identity]) ; return them as plain values
+ (+ 1 (raise 2))) ; => 2
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 6. Mutation
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Use `set!' to assign a new value to an existing variable
+(define n 5)
+(set! n (add1 n))
+n ; => 6
+
+;; Use boxes for explicitly mutable values (similar to pointers or
+;; references in other languages)
+(define n* (box 5))
+(set-box! n* (add1 (unbox n*)))
+(unbox n*) ; => 6
+
+;; Many Racket datatypes are immutable (pairs, lists, etc), some come in
+;; both mutable and immutable flavors (strings, vectors, hash tables,
+;; etc...)
+
+;; Use `vector' or `make-vector' to create mutable vectors
+(define vec (vector 2 2 3 4))
+(define wall (make-vector 100 'bottle-of-beer))
+;; Use vector-set! to update a slot
+(vector-set! vec 0 1)
+(vector-set! wall 99 'down)
+vec ; => #(1 2 3 4)
+
+;; Create an empty mutable hash table and manipulate it
+(define m3 (make-hash))
+(hash-set! m3 'a 1)
+(hash-set! m3 'b 2)
+(hash-set! m3 'c 3)
+(hash-ref m3 'a) ; => 1
+(hash-ref m3 'd 0) ; => 0
+(hash-remove! m3 'a)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 7. Modules
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Modules let you organize code into multiple files and reusable
+;; libraries; here we use sub-modules, nested in the whole module that
+;; this text makes (starting from the "#lang" line)
+
+(module cake racket/base ; define a `cake' module based on racket/base
+
+ (provide print-cake) ; function exported by the module
+
+ (define (print-cake n)
+ (show " ~a " n #\.)
+ (show " .-~a-. " n #\|)
+ (show " | ~a | " n #\space)
+ (show "---~a---" n #\-))
+
+ (define (show fmt n ch) ; internal function
+ (printf fmt (make-string n ch))
+ (newline)))
+
+;; Use `require' to get all `provide'd names from a module
+(require 'cake) ; the ' is for a local submodule
+(print-cake 3)
+; (show "~a" 1 #\A) ; => error, `show' was not exported
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 8. Classes and Objects
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Create a class fish% (-% is idomatic for class bindings)
+(define fish%
+ (class object%
+ (init size) ; initialization argument
+ (super-new) ; superclass initialization
+ ;; Field
+ (define current-size size)
+ ;; Public methods
+ (define/public (get-size)
+ current-size)
+ (define/public (grow amt)
+ (set! current-size (+ amt current-size)))
+ (define/public (eat other-fish)
+ (grow (send other-fish get-size)))))
+
+;; Create an instance of fish%
+(define charlie
+ (new fish% [size 10]))
+
+;; Use `send' to call an object's methods
+(send charlie get-size) ; => 10
+(send charlie grow 6)
+(send charlie get-size) ; => 16
+
+;; `fish%' is a plain "first class" value, which can get us mixins
+(define (add-color c%)
+ (class c%
+ (init color)
+ (super-new)
+ (define my-color color)
+ (define/public (get-color) my-color)))
+(define colored-fish% (add-color fish%))
+(define charlie2 (new colored-fish% [size 10] [color 'red]))
+(send charlie2 get-color)
+;; or, with no names:
+(send (new (add-color fish%) [size 10] [color 'red]) get-color)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 9. Macros
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Macros let you extend the syntax of the language
+
+;; Let's add a while loop
+(define-syntax-rule (while condition body ...)
+ (let loop ()
+ (when condition
+ body ...
+ (loop))))
+
+(let ([i 0])
+ (while (< i 10)
+ (displayln i)
+ (set! i (add1 i))))
+
+;; Macros are hygienic, you cannot clobber existing variables!
+(define-syntax-rule (swap! x y) ; -! is idomatic for mutation
+ (let ([tmp x])
+ (set! x y)
+ (set! y tmp)))
+
+(define tmp 1)
+(define a 2)
+(define b 3)
+(swap! a b)
+(printf "tmp = ~a; a = ~a; b = ~a\n" tmp a b) ; tmp is unaffected
+
+;; But they are still code transformations, for example:
+(define-syntax-rule (bad-while condition body ...)
+ (when condition
+ body ...
+ (bad-while condition body ...)))
+;; this macro is broken: it generates infinite code, if you try to use
+;; it, the compiler will get in an infinite loop
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 10. Contracts
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Contracts impose constraints on values exported from modules
+
+(module bank-account racket
+ (provide (contract-out
+ [deposit (-> positive? any)] ; amounts are always positive
+ [balance (-> positive?)]))
+
+ (define amount 0)
+ (define (deposit a) (set! amount (+ amount a)))
+ (define (balance) amount)
+ )
+
+(require 'bank-account)
+(deposit 5)
+
+(balance) ; => 5
+
+;; Clients that attempt to deposit a non-positive amount are blamed
+;; (deposit -5) ; => deposit: contract violation
+;; expected: positive?
+;; given: -5
+;; more details....
+```
+
+## Further Reading
+
+Still up for more? Try [Getting Started with Racket](http://docs.racket-lang.org/getting-started/)