learnxinyminutes-docs/racket.html.markdown

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---
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language: racket
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filename: learnracket.rkt
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contributors:
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- ["th3rac25", "https://github.com/voila"]
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- ["Eli Barzilay", "https://github.com/elibarzilay"]
- ["Gustavo Schmidt", "https://github.com/gustavoschmidt"]
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- ["Duong H. Nguyen", "https://github.com/cmpitg"]
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---
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Racket is a general purpose, multi-paradigm programming language in the Lisp/Scheme family.
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Feedback is appreciated! You can reach me at [@th3rac25](http://twitter.com/th3rac25) or th3rac25 [at] [google's email service]
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```racket
#lang racket ; defines the language we are using
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;;; Comments
;; Single line comments start with a semicolon
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#| Block comments
can span multiple lines and...
#|
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they can be nested!
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|#
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|#
;; S-expression comments discard the following expression,
;; useful to comment expressions when debugging
#; (this expression is discarded)
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 1. Primitive Datatypes and Operators
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;;; Numbers
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9999999999999999999999 ; integers
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#b111 ; binary => 7
#o111 ; octal => 73
#x111 ; hexadecimal => 273
3.14 ; reals
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6.02e+23
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1/2 ; rationals
1+2i ; complex numbers
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;; 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
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'(+ 1 2) ; => (+ 1 2)
;; Now, some arithmetic operations
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(+ 1 1) ; => 2
(- 8 1) ; => 7
(* 10 2) ; => 20
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(expt 2 3) ; => 8
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(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
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;;; Booleans
#t ; for true
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#f ; for false -- any value other than #f is true
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(not #t) ; => #f
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(and 0 #f (error "doesn't get here")) ; => #f
(or #f 0 (error "doesn't get here")) ; => 0
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;;; Characters
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#\A ; => #\A
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#\λ ; => #\λ
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#\u03BB ; => #\λ
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;;; Strings are fixed-length array of characters.
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"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
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;; Strings can be added too!
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(string-append "Hello " "world!") ; => "Hello world!"
;; A string can be treated like a list of characters
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(string-ref "Apple" 0) ; => #\A
;; format can be used to format strings:
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(format "~a can be ~a" "strings" "formatted")
;; Printing is pretty easy
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(printf "I'm Racket. Nice to meet you!\n")
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 2. Variables
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; You can create a variable using define
;; a variable name can use any character except: ()[]{}",'`;#|\
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(define some-var 5)
some-var ; => 5
;; You can also use unicode characters
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(define ⊆ subset?)
(⊆ (set 3 2) (set 1 2 3)) ; => #t
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;; Accessing a previously unassigned variable is an exception
; x ; => x: undefined ...
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;; Local binding: `me' is bound to "Bob" only within the (let ...)
(let ([me "Bob"])
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"Alice"
me) ; => "Bob"
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 3. Structs and Collections
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Structs
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(struct dog (name breed age))
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(define my-pet
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(dog "lassie" "collie" 5))
my-pet ; => #<dog>
(dog? my-pet) ; => #t
(dog-name my-pet) ; => "lassie"
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;;; Pairs (immutable)
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;; `cons' constructs pairs, `car' and `cdr' extract the first
;; and second elements
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(cons 1 2) ; => '(1 . 2)
(car (cons 1 2)) ; => 1
(cdr (cons 1 2)) ; => 2
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;;; Lists
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;; 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
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(list 1 2 3) ; => '(1 2 3)
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;; and a quote can also be used for a literal list value
'(1 2 3) ; => '(1 2 3)
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;; Can still use `cons' to add an item to the beginning of a list
(cons 4 '(1 2 3)) ; => '(4 1 2 3)
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;; Use `append' to add lists together
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(append '(1 2) '(3 4)) ; => '(1 2 3 4)
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;; 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)
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;;; Vectors
;; Vectors are fixed-length arrays
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#(1 2 3) ; => '#(1 2 3)
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;; Use `vector-append' to add vectors together
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(vector-append #(1 2 3) #(4 5 6)) ; => #(1 2 3 4 5 6)
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;;; Sets
;; Create a set from a list
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(list->set '(1 2 3 1 2 3 3 2 1 3 2 1)) ; => (set 1 2 3)
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;; 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)
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;; Remove one with `set-remove'
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(set-remove (set 1 2 3) 1) ; => (set 2 3)
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;; Test for existence with `set-member?'
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(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)
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(define m (hash 'a 1 'b 2 'c 3))
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;; Retrieve a value
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(hash-ref m 'a) ; => 1
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;; Retrieving a non-present value is an exception
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; (hash-ref m 'd) => no value found
;; You can provide a default value for missing keys
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(hash-ref m 'd 0) ; => 0
;; Use `hash-set' to extend an immutable hash table
;; (Returns the extended hash instdead of mutating it)
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(define m2 (hash-set m 'd 4))
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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'
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;; Use `hash-remove' to remove keys (functional too)
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(hash-remove m 'a) ; => '#hash((b . 2) (c . 3))
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 3. Functions
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Use `lambda' to create functions.
;; A function always returns the value of its last expression
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(lambda () "Hello World") ; => #<procedure>
;; Can also use a unicode `λ'
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(λ () "Hello World") ; => same function
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;; Use parens to call all functions, including a lambda expression
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((lambda () "Hello World")) ; => "Hello World"
((λ () "Hello World")) ; => "Hello World"
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;; Assign a function to a var
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(define hello-world (lambda () "Hello World"))
(hello-world) ; => "Hello World"
;; You can shorten this using the function definition syntatcic sugae:
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(define (hello-world2) "Hello World")
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;; The () in the above is the list of arguments for the function
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(define hello
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(lambda (name)
(string-append "Hello " name)))
(hello "Steve") ; => "Hello Steve"
;; ... or equivalently, using a sugared definition:
(define (hello2 name)
(string-append "Hello " name))
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;; You can have multi-variadic functions too, using `case-lambda'
(define hello3
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(case-lambda
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[() "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))
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;; Functions can pack extra arguments up in a list
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(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)))
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;; You can mix regular and packed arguments
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(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"
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 4. Equality
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; for numbers use `='
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(= 3 3.0) ; => #t
(= 2 1) ; => #f
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;; for object identity use `eq?'
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(eq? 3 3) ; => #t
(eq? 3 3.0) ; => #f
(eq? (list 3) (list 3)) ; => #f
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;; for collections use `equal?'
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(equal? (list 'a 'b) (list 'a 'b)) ; => #t
(equal? (list 'a 'b) (list 'b 'a)) ; => #f
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 5. Control Flow
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;;; Conditionals
(if #t ; test expression
"this is true" ; then expression
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"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))
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'yep
'nope)
; => 'yep
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;; `cond' chains a series of tests to select a result
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(cond [(> 2 2) (error "wrong!")]
[(< 2 2) (error "wrong again!")]
[else 'ok]) ; => 'ok
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;;; Pattern Matching
(define (fizzbuzz? n)
(match (list (remainder n 3) (remainder n 5))
[(list 0 0) 'fizzbuzz]
[(list 0 _) 'fizz]
[(list _ 0) 'buzz]
[_ #f]))
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(fizzbuzz? 15) ; => 'fizzbuzz
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(fizzbuzz? 37) ; => #f
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;;; Loops
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;; Looping can be done through (tail-) recursion
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(define (loop i)
(when (< i 10)
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(printf "i=~a\n" i)
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(loop (add1 i))))
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(loop 5) ; => i=5, i=6, ...
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;; Similarly, with a named let
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(let loop ((i 0))
(when (< i 10)
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(printf "i=~a\n" i)
(loop (add1 i)))) ; => i=0, i=1, ...
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;; 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, ...
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;;; Iteration Over Other Sequences
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;; `for' allows iteration over many other kinds of sequences:
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;; lists, vectors, strings, sets, hash tables, etc...
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(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))
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;;; 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)
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;;; 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
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 6. Mutation
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Use `set!' to assign a new value to an existing variable
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(define n 5)
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(set! n (add1 n))
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n ; => 6
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;; 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
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;; Many Racket datatypes are immutable (pairs, lists, etc), some come in
;; both mutable and immutable flavors (strings, vectors, hash tables,
;; etc...)
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;; Use `vector' or `make-vector' to create mutable vectors
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(define vec (vector 2 2 3 4))
(define wall (make-vector 100 'bottle-of-beer))
;; Use vector-set! to update a slot
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(vector-set! vec 0 1)
(vector-set! wall 99 'down)
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vec ; => #(1 2 3 4)
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;; 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)
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 7. Modules
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 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)
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(module cake racket/base ; define a `cake' module based on racket/base
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(provide print-cake) ; function exported by the module
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(define (print-cake n)
(show " ~a " n #\.)
(show " .-~a-. " n #\|)
(show " | ~a | " n #\space)
(show "---~a---" n #\-))
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(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
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(print-cake 3)
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; (show "~a" 1 #\A) ; => error, `show' was not exported
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 8. Classes and Objects
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Create a class fish% (-% is idomatic for class bindings)
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(define fish%
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(class object%
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(init size) ; initialization argument
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(super-new) ; superclass initialization
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;; Field
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(define current-size size)
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;; 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)))))
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;; Create an instance of fish%
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(define charlie
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(new fish% [size 10]))
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;; Use `send' to call an object's methods
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(send charlie get-size) ; => 10
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(send charlie grow 6)
(send charlie get-size) ; => 16
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;; `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)
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 9. Macros
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Macros let you extend the syntax of the language
;; Let's add a while loop
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(define-syntax-rule (while condition body ...)
(let loop ()
(when condition
body ...
(loop))))
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(let ([i 0])
(while (< i 10)
(displayln i)
(set! i (add1 i))))
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;; Macros are hygienic, you cannot clobber existing variables!
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(define-syntax-rule (swap! x y) ; -! is idomatic for mutation
(let ([tmp x])
(set! x y)
(set! y tmp)))
(define tmp 2)
(define other 3)
(swap! tmp other)
(printf "tmp = ~a; other = ~a\n" tmp other)
;; The variable `tmp` is renamed to `tmp_1`
;; in order to avoid name conflict
;; (let ([tmp_1 tmp])
;; (set! tmp other)
;; (set! other tmp_1))
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;; But they are still code transformations, for example:
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(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
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 10. Contracts
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Contracts impose constraints on values exported from modules
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(module bank-account racket
(provide (contract-out
[deposit (-> positive? any)] ; amounts are always positive
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[balance (-> positive?)]))
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(define amount 0)
(define (deposit a) (set! amount (+ amount a)))
(define (balance) amount)
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)
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(require 'bank-account)
(deposit 5)
(balance) ; => 5
;; Clients that attempt to deposit a non-positive amount are blamed
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;; (deposit -5) ; => deposit: contract violation
;; expected: positive?
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;; given: -5
;; more details....
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 11. Input & output
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Racket has this concept of "port", which is very similar to file descriptor
;; in other languages
;; Open "/tmp/tmp.txt" and write "Hello World"
;; This would trigger an error if the file's already existed
(define out-port (open-output-file "/tmp/tmp.txt"))
(displayln "Hello World" out-port)
(close-output-port out-port)
;; Append to "/tmp/tmp.txt"
(define out-port (open-output-file "/tmp/tmp.txt"
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#:exists 'append))
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(displayln "Hola mundo" out-port)
(close-output-port out-port)
;; Read from the file again
(define in-port (open-input-file "/tmp/tmp.txt"))
(displayln (read-line in-port))
; => "Hello World"
(displayln (read-line in-port))
; => "Hola mundo"
(close-input-port in-port)
;; Alternatively, with call-with-output-file you don't need to explicitly
;; close the file
(call-with-output-file "/tmp/tmp.txt"
#:exists 'update ; Rewrite the content
(λ (out-port)
(displayln "World Hello!" out-port)))
;; And call-with-input-file does the same thing for input
(call-with-input-file "/tmp/tmp.txt"
(λ (in-port)
(displayln (read-line in-port))))
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```
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## Further Reading
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Still up for more? Try [Getting Started with Racket](http://docs.racket-lang.org/getting-started/)