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Adam Bard
commit
40c839654a
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---
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language: racket
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author: th3rac25
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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
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#lang racket ; defines the language we are using
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;;; Comments
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; Single line comments start with a semicolon
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#| Block comments
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can span multiple lines and...
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#|
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they can be nested !
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|#
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|#
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; S-expression comments discard the following expression
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#; "this expression will be discarded" "2nd expression" ; => "2nd expression"
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 1. Primitive Datatypes and Operators
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;;; Numbers
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9999999999999999999999 ; integers
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3.14 ; reals
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6.02e+23
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1/2 ; rationals
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1+2i ; complex numbers
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; Function application is written (f x y z ...)
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; where f is a function and x, y, z, ... are operands
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; If you want to create a literal list of data, use ' to stop it from
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; being evaluated
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'(+ 1 2) ; => (+ 1 2)
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; Now, some arithmetic operations
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(+ 1 1) ; => 2
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(- 8 1) ; => 7
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(* 10 2) ; => 20
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(quotient 5 2) ; => 2
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(remainder 5 2) ; => 1
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(/ 35 5) ; => 7
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(/ 1 3) ; => 1/3
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(exact->inexact 1/3) ; => 0.3333333333333333
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(+ 1+2i 2-3i) ; => 3-1i
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;;; Booleans
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#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
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(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!"
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"Benjamin \"Bugsy\" Siegel" ; backslash is an escaping character
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"λx:(μα.α→α).xx" ; any Unicode character can appear in a string constant
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; Strings can be added too!
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(string-append "Hello " "world!") ; => "Hello world!"
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; A string can be treated like a list of characters
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(string-ref "Apple" 0) ; => #\A
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; format can be used to format strings:
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(format "~a can be ~a" "strings" "formatted")
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; Printing is pretty easy
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(printf "I'm Racket. Nice to meet you!\n")
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 2. Variables
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; You can create a variable using define
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; a variable name can use any character except: ()[]{}",'`;#|\
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(define some-var 5)
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some-var ; => 5
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; You can also use unicode characters
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(define ⊆ subset?)
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(⊆ (set 3 2) (set 1 2 3)); => #t
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; Accessing a previously unassigned variable is an exception
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;x ; => x: undefined ...
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; Local binding: me is bound to "Bob" only within (let ...)
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(let ([me "Bob"])
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"Alice"
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me) ; => "Bob"
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 3. Structs and Collections
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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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))
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my-pet ; => #<dog>
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(dog? my-pet) ; => #t
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(dog-name my-pet) ; => "lassie"
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;;; Pairs (immutable)
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; "cons" constructs pairs, "car" and "cdr" extract the first
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; and second elements
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(cons 1 2) ; => '(1 . 2)
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(car (cons 1 2)) ; => 1
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(cdr (cons 1 2)) ; => 2
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;;; Lists
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; Lists are linked-list data structures
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(list 1 2 3) ; => '(1 2 3)
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; Use "cons" to add an item to the beginning of a list
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(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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;;; Vectors
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; 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
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; 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"
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(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
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(set-member? (set 1 2 3) 4) ; => #f
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;;; Hashes
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; Create an immutable hash table (There are also mutables ones)
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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
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; You can provide a default value for missing keys
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(hash-ref m 'd 0) ; => 0
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; Use "hash-set" to extend a hash table
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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))
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; Remember, these hashes are immutable!
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m ; => '#hash((b . 2) (a . 1) (c . 3))
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; Use "hash-remove" to remove keys
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(hash-remove m 'a) ; => '#hash((b . 2) (c . 3))
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 3. Functions
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Use lambda to create new functions.
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; A function always returns its last statement.
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(lambda () "Hello World") ; => #<procedure>
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; (You need extra parens to call it)
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((lambda () "Hello World")) ; => "Hello World"
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; Assign a function to a var
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(define hello-world (lambda () "Hello World"))
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(hello-world) ; => "Hello World"
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; You can shorten this to:
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(define (hello-world2) "Hello World")
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; The () is the list of arguments for the function.
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(define hello
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(lambda (name)
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(string-append "Hello " name)))
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(hello "Steve") ; => "Hello Steve"
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; You can have multi-variadic functions, too
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(define hello2
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(case-lambda
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[() "Hello World"]
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[(name) (string-append "Hello " name)]))
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(hello2 "Jake") ; => "Hello Jake"
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(hello2) ; => "Hello World"
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; Functions can pack extra arguments up in a list
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(define (count-args . args)
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(format "You passed ~a args: ~a" (length args) args))
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(count-args 1 2 3) ; => "You passed 3 args: (1 2 3)"
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; You can mix regular and packed arguments
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(define (hello-count name . args)
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(format "Hello ~a, you passed ~a extra args" name (length args)))
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(hello-count "Finn" 1 2 3)
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; => "Hello Finn, you passed 3 extra args"
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 4. Equality
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; for numbers use "="
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(= 3 3.0) ; => #t
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(= 2 1) ; => #f
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; for object identity use "eq?"
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(eq? 3 3) ; => #t
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(eq? 3 3.0) ; => #f
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(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
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(equal? (list 'a 'b) (list 'b 'a)) ; => #f
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 5. Control Flow
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;;; Conditionals
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(if #t ; test expression
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"this is true" ; then expression
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"this is false" ; else expression
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) ; => "this is true"
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; In conditionals, all non-#f values are treated as true
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(member "Groucho" '("Harpo" "Groucho" "Zeppo")) ; => '("Groucho" "Zeppo")
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(if (member "Groucho" '("Harpo" "Groucho" "Zeppo"))
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'yep
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'nope) ; => 'yep
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; "cond" chains a series of tests to select a result
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(cond
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[(> 2 2) (error "wrong!")]
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[(< 2 2) (error "wrong again!")]
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[else 'ok]) ; => 'ok
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;;; Pattern Matching
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(define (fizzbuzz? n)
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(match (list (remainder n 3) (remainder n 5))
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[(list 0 0) 'fizzbuzz]
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[(list 0 _) 'fizz]
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[(list _ 0) 'buzz]
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[_ #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 recursion
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(define (loop i)
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(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))
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(when (< i 10)
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(printf "i:~a~n" i)
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(loop (add1 i)))) ; => i:0 i:1 ...
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;;; Comprehensions
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(for/list ([i '(1 2 3)])
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(add1 i)) ; => '(2 3 4)
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(for/list ([i '(1 2 3)] #:when (even? i))
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i) ; => '(2)
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(for/hash ([i '(1 2 3)])
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(values i (number->string i))) ; => '#hash((1 . "1") (2 . "2") (3 . "3"))
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; To combine iteration results, use "for/fold"
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(for/fold ([sum 0]) ([i '(1 2 3 4)])
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(+ sum i)) ; => 10
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;;; Sequences
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; "for" allows iteration over sequences:
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; lists, vectors, strings, sets, hash tables, etc...
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(for ([i (in-list '(l i s t))])
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(displayln i))
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(for ([i (in-vector #(v e c t o r))])
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(displayln i))
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(for ([i (in-string "string")])
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(displayln i))
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(for ([i (in-set (set 'x 'y 'z))])
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(displayln i))
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(for ([(k v) (in-hash (hash 'a 1 'b 2 'c 3 ))])
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(printf "key:~a value:~a ~n" k v))
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;;; Exceptions
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; To catch an exception, use the "with-handlers" form
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; To throw an exception use "raise"
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(with-handlers
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([(lambda (v) (equal? v "infinity"))
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(lambda (exn) +inf.0)])
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(raise "infinity"))
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 6. Mutation
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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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 6)
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n ; => 6
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; Many Racket datatypes can be immutable or mutable
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; (Pairs, Lists, Strings, Vectors, Hash Tables, etc...)
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; Use "vector" to create a mutable vector
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(define vec (vector 2 2 3 4))
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; Use vector-set! to update a slot
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(vector-set! vec 0 1)
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vec ; => #(1 2 3 4)
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 7. Modules
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Modules let you organize code into multiple files and reusable libraries
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(module cake racket/base ; define a new module 'cake' based on racket/base
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(provide print-cake) ; function exported by the module
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(define (print-cake n)
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(show " ~a " n #\.)
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(show " .-~a-. " n #\|)
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(show " | ~a | " n #\space)
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(show "---~a---" n #\-))
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(define (show fmt n ch) ;; internal function
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(printf fmt (make-string n ch))
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(newline)))
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; Use "require" to import all functions from the module
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(require 'cake)
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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%
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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
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(define/public (get-size) current-size)
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(define/public (grow amt) (set! current-size (+ amt current-size)))
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(define/public (eat other-fish) (grow (send other-fish get-size)))))
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|
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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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|
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; Use "send" to call an object's methods
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(send charlie grow 6)
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(send charlie get-size) ; => 16
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 9. Macros
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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|
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; Macros let you extend the syntax of the language
|
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(define-syntax-rule (unless test then else)
|
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(if test else then))
|
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|
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(unless (even? 10) "odd" "even") ; => "even"
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|
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; Macros are hygienic, you cannot clobber existing variables!
|
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(define-syntax-rule (swap x y)
|
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(begin
|
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(define tmp x)
|
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(set! x y)
|
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(set! y tmp)))
|
||||
|
||||
(define tmp 1)
|
||||
(define a 2)
|
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(define b 3)
|
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(swap a b)
|
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(printf "tmp = ~a; a = ~a; b = ~a~n" tmp a b) ; tmp is unaffected by swap
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|
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
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;; 10. Contracts
|
||||
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
||||
|
||||
; Contracts impose constraints on values exported from modules
|
||||
|
||||
(module bank-account racket
|
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(provide (contract-out
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[deposit (-> positive? any)] ; amount will always be a positive number
|
||||
[balance (-> positive?)]))
|
||||
|
||||
(define amount 0)
|
||||
(define (deposit a) (set! amount (+ amount a)))
|
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(define (balance) amount)
|
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)
|
||||
|
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(require 'bank-account)
|
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(deposit 5)
|
||||
|
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(balance) ; => 5
|
||||
|
||||
; Any client that attempt to deposit a non-positive amount, will be blamed
|
||||
; (deposit -5) ; => deposit: contract violation
|
||||
; expected: positive?
|
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; given: -5
|
||||
; more details....
|
||||
```
|
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|
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## Further Reading
|
||||
|
||||
Still up for more? Try [Quick: An Introduction to Racket with Pictures](http://docs.racket-lang.org/quick/)
|
||||
|
||||
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