1
1
mirror of https://github.com/adambard/learnxinyminutes-docs.git synced 2024-12-22 06:41:35 +03:00
learnxinyminutes-docs/racket.html.markdown
sorawee ed198750d4
[racket/en] fix indentation and spacing ()
* Fix indentation and spacing

* One more fix
2021-10-23 18:46:48 +02:00

724 lines
21 KiB
Markdown
Raw Blame History

This file contains ambiguous Unicode characters

This file contains Unicode characters that might be confused with other characters. If you think that this is intentional, you can safely ignore this warning. Use the Escape button to reveal them.

---
language: racket
filename: learnracket.rkt
contributors:
- ["th3rac25", "https://github.com/voila"]
- ["Eli Barzilay", "https://github.com/elibarzilay"]
- ["Gustavo Schmidt", "https://github.com/gustavoschmidt"]
- ["Duong H. Nguyen", "https://github.com/cmpitg"]
- ["Keyan Zhang", "https://github.com/keyanzhang"]
---
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"
;; let* is like let, but allows you to use previous bindings in creating later bindings
(let* ([x 1]
[y (+ x 1)])
(* x y))
;; finally, letrec allows you to define recursive and mutually recursive functions
(letrec ([is-even? (lambda (n)
(or (zero? n)
(is-odd? (sub1 n))))]
[is-odd? (lambda (n)
(and (not (zero? n))
(is-even? (sub1 n))))])
(is-odd? 11))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 3. Structs and Collections
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Structs
; By default, structs are immutable
(struct dog (name breed age))
(define my-pet
(dog "lassie" "collie" 5))
my-pet ; => #<dog>
; returns whether the variable was constructed with the dog constructor
(dog? my-pet) ; => #t
; accesses the name field of the variable constructed with the dog constructor
(dog-name my-pet) ; => "lassie"
; You can explicitly declare a struct to be mutable with the #:mutable option
(struct rgba-color (red green blue alpha) #:mutable)
(define burgundy
(rgba-color 144 0 32 1.0))
(set-rgba-color-green! burgundy 10)
(rgba-color-green burgundy) ; => 10
;;; 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)
;; a quote can also be used for a literal list value
'(1 2 3) ; => '(1 2 3)
;; a quasiquote (represented by the backtick character) with commas
;; can be used to evaluate functions
`(1 ,(+ 1 1) 3) ; => '(1 2 3)
;; With lists, car/cdr work slightly differently
(car '(1 2 3)) ; => 1
(cdr '(1 2 3)) ; => '(2 3)
;; Racket also has predefined functions on top of car and cdr, to extract parts of a list
(cadr (list 1 2 3)) ; => 2
(car (cdr (list 1 2 3))) ; => 2
(cddr (list 1 2 3)) ; => '(3)
(cdr (cdr (list 1 2 3))) ; => '(3)
(caddr (list 1 2 3)) ; => 3
(car (cdr (cdr (list 1 2 3)))) ; => 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 instead 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))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 4. 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 syntactic sugar:
(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"
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 5. Equality
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; for numbers use `='
(= 3 3.0) ; => #t
(= 2 1) ; => #f
;; `eq?' returns #t if 2 arguments refer to the same object (in memory),
;; #f otherwise.
;; In other words, it's a simple pointer comparison.
(eq? '() '()) ; => #t, since there exists only one empty list in memory
(let ([x '()] [y '()])
(eq? x y)) ; => #t, same as above
(eq? (list 3) (list 3)) ; => #f
(let ([x (list 3)] [y (list 3)])
(eq? x y)) ; => #f — not the same list in memory!
(let* ([x (list 3)] [y x])
(eq? x y)) ; => #t, since x and y now point to the same stuff
(eq? 'yes 'yes) ; => #t
(eq? 'yes 'no) ; => #f
(eq? 3 3) ; => #t — be careful here
; Its better to use `=' for number comparisons.
(eq? 3 3.0) ; => #f
(eq? (expt 2 100) (expt 2 100)) ; => #f
(eq? (integer->char 955) (integer->char 955)) ; => #f
(eq? (string-append "foo" "bar") (string-append "foo" "bar")) ; => #f
;; `eqv?' supports the comparison of number and character datatypes.
;; for other datatypes, `eqv?' and `eq?' return the same result.
(eqv? 3 3.0) ; => #f
(eqv? (expt 2 100) (expt 2 100)) ; => #t
(eqv? (integer->char 955) (integer->char 955)) ; => #t
(eqv? (string-append "foo" "bar") (string-append "foo" "bar")) ; => #f
;; `equal?' supports the comparison of the following datatypes:
;; strings, byte strings, pairs, mutable pairs, vectors, boxes,
;; hash tables, and inspectable structures.
;; for other datatypes, `equal?' and `eqv?' return the same result.
(equal? 3 3.0) ; => #f
(equal? (string-append "foo" "bar") (string-append "foo" "bar")) ; => #t
(equal? (list 3) (list 3)) ; => #t
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 6. 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
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 7. 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)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 8. 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
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 9. Classes and Objects
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Create a class fish% (-% is idiomatic 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)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 10. 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 idiomatic 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))
;; 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
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 11. 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....
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 12. Input & output
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Racket has this concept of "port", which is very similar to file
;; descriptors 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"
#:exists 'append))
(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))))
```
## Further Reading
Still up for more? Try [Getting Started with Racket](http://docs.racket-lang.org/getting-started/)