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12 KiB
language | author |
---|---|
racket | th3rac25 |
Racket is a general purpose, multi-paradigm programming language in the Lisp/Scheme family.
Feedback is appreciated! You can reach me at @th3rac25 or th3rac25 [at] [google's email service]
#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
#; "this expression will be discarded" "2nd expression" ; => "2nd expression"
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 1. Primitive Datatypes and Operators
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Numbers
9999999999999999999999 ; integers
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
(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
"λx:(μα.α→α).xx" ; any Unicode character can appear in a string constant
; 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 (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
(list 1 2 3) ; => '(1 2 3)
; 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)
;;; 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"
(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 (There are also mutables ones)
(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 a hash table
(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))
; Use "hash-remove" to remove keys
(hash-remove m 'a) ; => '#hash((b . 2) (c . 3))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 3. Functions
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Use lambda to create new functions.
; A function always returns its last statement.
(lambda () "Hello World") ; => #<procedure>
; (You need extra parens to call it)
((lambda () "Hello World")) ; => "Hello World"
; Assign a function to a var
(define hello-world (lambda () "Hello World"))
(hello-world) ; => "Hello World"
; You can shorten this to:
(define (hello-world2) "Hello World")
; The () is the list of arguments for the function.
(define hello
(lambda (name)
(string-append "Hello " name)))
(hello "Steve") ; => "Hello Steve"
; You can have multi-variadic functions, too
(define hello2
(case-lambda
[() "Hello World"]
[(name) (string-append "Hello " name)]))
(hello2 "Jake") ; => "Hello Jake"
(hello2) ; => "Hello World"
; 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)"
; 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"
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 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 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 ...
;;; Comprehensions
(for/list ([i '(1 2 3)])
(add1 i)) ; => '(2 3 4)
(for/list ([i '(1 2 3)] #:when (even? i))
i) ; => '(2)
(for/hash ([i '(1 2 3)])
(values i (number->string i))) ; => '#hash((1 . "1") (2 . "2") (3 . "3"))
; To combine iteration results, use "for/fold"
(for/fold ([sum 0]) ([i '(1 2 3 4)])
(+ sum i)) ; => 10
;;; Sequences
; "for" allows iteration over 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))
;;; Exceptions
; To catch an exception, use the "with-handlers" form
; To throw an exception use "raise"
(with-handlers
([(lambda (v) (equal? v "infinity"))
(lambda (exn) +inf.0)])
(raise "infinity"))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 6. Mutation
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Use set! to assign a new value to an existing variable
(define n 5)
(set! n 6)
n ; => 6
; Many Racket datatypes can be immutable or mutable
; (Pairs, Lists, Strings, Vectors, Hash Tables, etc...)
; Use "vector" to create a mutable vector
(define vec (vector 2 2 3 4))
; Use vector-set! to update a slot
(vector-set! vec 0 1)
vec ; => #(1 2 3 4)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 7. Modules
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Modules let you organize code into multiple files and reusable libraries
(module cake racket/base ; define a new module 'cake' 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 import all functions from the module
(require 'cake)
(print-cake 3)
;(show "~a" 1 #\A) ; => error, "show" was not exported
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 8. Classes and Objects
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Create a class fish%
(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 grow 6)
(send charlie get-size) ; => 16
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 9. Macros
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Macros let you extend the syntax of the language
(define-syntax-rule (unless test then else)
(if test else then))
(unless (even? 10) "odd" "even") ; => "even"
; Macros are hygienic, you cannot clobber existing variables!
(define-syntax-rule (swap x y)
(begin
(define 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 by swap
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 10. Contracts
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Contracts impose constraints on values exported from modules
(module bank-account racket
(provide (contract-out
[deposit (-> positive? any)] ; amount will always be a positive number
[balance (-> positive?)]))
(define amount 0)
(define (deposit a) (set! amount (+ amount a)))
(define (balance) amount)
)
(require 'bank-account)
(deposit 5)
(balance) ; => 5
; Any client that attempt to deposit a non-positive amount, will be blamed
; (deposit -5) ; => deposit: contract violation
; expected: positive?
; given: -5
; more details....
Further Reading
Still up for more? Try Quick: An Introduction to Racket with Pictures