adambard/learnxinyminutes-docs
1
1
Fork 0
mirror of https://github.com/adambard/learnxinyminutes-docs.git synced 2024-12-23 15:24:09 +03:00
Code Issues Projects Releases Wiki Activity

Merge pull request #1 from elibarzilay/master

Browse Source 
Lots of improvements
This commit is contained in:
voila 2013-07-16 02:13:05 -07:00
commit f6a72e179d
1 changed files with 300 additions and 170 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

470
racket.html.markdown
View File

@ -7,7 +7,7 @@ contributors:
---
Racket is a general purpose, multi-paradigm programming language in the Lisp/Scheme family.
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]
@ -17,18 +17,19 @@ Feedback is appreciated! You can reach me at [@th3rac25](http://twitter.com/th3r
;;; Comments
; Single line comments start with a semicolon
;; Single line comments start with a semicolon
#| Block comments
can span multiple lines and...
#|
they can be nested !
|#
they can be nested!
|#
|#
; S-expression comments discard the following expression
#; "this expression will be discarded" "2nd expression" ; => "2nd expression"
;; S-expression comments discard the following expression,
;; useful to comment expressions when debugging
#; (this expression is discarded)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 1. Primitive Datatypes and Operators
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
@ -40,15 +41,15 @@ Feedback is appreciated! You can reach me at [@th3rac25](http://twitter.com/th3r
#x111 ; hexadecimal => 273
3.14 ; reals
6.02e+23
1/2 ; rationals
1+2i ; complex numbers
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
;; 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
;; Now, some arithmetic operations
(+ 1 1) ; => 2
(- 8 1) ; => 7
(* 10 2) ; => 20
@ -60,189 +61,242 @@ Feedback is appreciated! You can reach me at [@th3rac25](http://twitter.com/th3r
(exact->inexact 1/3) ; => 0.3333333333333333
(+ 1+2i 2-3i) ; => 3-1i
;;; Booleans
#t ; for true
;;; 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
;;; Characters
#\A ; => #\A
#\λ ; => #\λ
#\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
"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!
;; Strings can be added too!
(string-append "Hello " "world!") ; => "Hello world!"
; A string can be treated like a list of characters
;; A string can be treated like a list of characters
(string-ref "Apple" 0) ; => #\A
; format can be used to format strings:
;; format can be used to format strings:
(format "~a can be ~a" "strings" "formatted")
; Printing is pretty easy
;; 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: ()[]{}",'`;#|\
;; 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
;; You can also use unicode characters
(define ⊆ subset?)
(⊆ (set 3 2) (set 1 2 3)); => #t
(⊆ (set 3 2) (set 1 2 3)) ; => #t
; Accessing a previously unassigned variable is an exception
;x ; => x: undefined ...
;; Accessing a previously unassigned variable is an exception
; x ; => x: undefined ...
; Local binding: me is bound to "Bob" only within (let ...)
;; Local binding: `me' is bound to "Bob" only within the (let ...)
(let ([me "Bob"])
"Alice"
me) ; => "Bob"
"Alice"
me) ; => "Bob"
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 3. Structs and Collections
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Structs
;; Structs
(struct dog (name breed age))
(define my-pet
(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' 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
;; 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)
; Use "cons" to add an item to the beginning of a list
(cons 4 '(1 2 3)) ; => (4 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)
;; 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
;; Vectors are fixed-length arrays
#(1 2 3) ; => '#(1 2 3)
; Use "vector-append" to add vectors together
;; 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
;; 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)
;; 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"
;; Remove one with `set-remove'
(set-remove (set 1 2 3) 1) ; => (set 2 3)
; Test for existence with "set-member?"
;; 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)
;; Create an immutable hash table (mutable example below)
(define m (hash 'a 1 'b 2 'c 3))
; Retrieve a value
;; Retrieve a value
(hash-ref m 'a) ; => 1
; Retrieving a non-present value is an exception
;; Retrieving a non-present value is an exception
; (hash-ref m 'd) => no value found
; You can provide a default value for missing keys
;; 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))
;; 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))
;; Remember, these hashes are immutable!
m ; => '#hash((b . 2) (a . 1) (c . 3)) <-- no `d'
; Use "hash-remove" to remove keys
;; Use `hash-remove' to remove keys (functional too)
(hash-remove m 'a) ; => '#hash((b . 2) (c . 3))
;; 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)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 3. Functions
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Use lambda to create new functions.
; A function always returns its last statement.
;; 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 () "Hellow World") ; => same function
; (You need extra parens to call it)
;; 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
;; Assign a function to a var
(define hello-world (lambda () "Hello World"))
(hello-world) ; => "Hello World"
; You can shorten this to:
;; You can shorten this using the function definition syntatcic sugae:
(define (hello-world2) "Hello World")
; The () is the list of arguments for the function.
(define hello
;; 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
(define hello2
(case-lambda
;; You can have multi-variadic functions too, using `case-lambda'
(define hello3
(case-lambda
[() "Hello World"]
[(name) (string-append "Hello " name)]))
(hello2 "Jake") ; => "Hello Jake"
(hello2) ; => "Hello World"
(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
;; 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
;; 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 "="
;; for numbers use `='
(= 3 3.0) ; => #t
(= 2 1) ; => #f
; for object identity use "eq?"
;; for object identity use `eq?'
(eq? 3 3) ; => #t
(eq? 3 3.0) ; => #f
(eq? (list 3) (list 3)) ; => #f
; for collections use "equal?"
;; for collections use `equal?'
(equal? (list 'a 'b) (list 'a 'b)) ; => #t
(equal? (list 'a 'b) (list 'b 'a)) ; => #f
@ -254,20 +308,20 @@ m ; => '#hash((b . 2) (a . 1) (c . 3))
(if #t ; test expression
"this is true" ; then expression
"this is false" ; else expression
) ; => "this is true"
"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
;; 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
;; `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
@ -277,45 +331,36 @@ m ; => '#hash((b . 2) (a . 1) (c . 3))
[(list 0 _) 'fizz]
[(list _ 0) 'buzz]
[_ #f]))
(fizzbuzz? 15) ; => 'fizzbuzz
(fizzbuzz? 37) ; => #f
(fizzbuzz? 37) ; => #f
;;; Loops
; looping can be done through recursion
;; Looping can be done through (tail-) recursion
(define (loop i)
(when (< i 10)
(printf "i:~a~n" i)
(loop (add1 i))))
(printf "i=~a\n" i)
(loop (add1 i))))
(loop 5) ; => i=5, i=6, ...
(loop 5) ; => i:5 i:6 ...
; similarly, with a named let
;; Similarly, with a named let
(let loop ((i 0))
(when (< i 10)
(printf "i:~a~n" i)
(loop (add1 i)))) ; => i:0 i:1 ...
(printf "i=~a\n" i)
(loop (add1 i)))) ; => i=0, i=1, ...
;;; Comprehensions
;; 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, ...
(for/list ([i '(1 2 3)])
(add1 i)) ; => '(2 3 4)
;;; Iteration Over Other Sequences
;; `for' allows iteration over many other kinds of sequences:
;; lists, vectors, strings, sets, hash tables, etc...
(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))
@ -329,91 +374,170 @@ m ; => '#hash((b . 2) (a . 1) (c . 3))
(displayln i))
(for ([(k v) (in-hash (hash 'a 1 'b 2 'c 3 ))])
(printf "key:~a value:~a ~n" k v))
(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 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"))
;; 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
;; Use `set!' to assign a new value to an existing variable
(define n 5)
(set! n 6)
(set! n (add1 n))
n ; => 6
; Many Racket datatypes can be immutable or mutable
; (Pairs, Lists, Strings, Vectors, Hash Tables, etc...)
;; 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
; Use "vector" to create a mutable vector
;; 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))
; Use vector-set! to update a slot
(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)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; 7. Modules
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Modules let you organize code into multiple files and reusable libraries
;; 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
(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
(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
;; 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%
;; Create a class fish% (-% is idomatic for class bindings)
(define fish%
(class object%
(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)))))
(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
;; Create an instance of fish%
(define charlie
(new fish% [size 10]))
; Use "send" to call an object's methods
;; 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
;; Macros let you extend the syntax of the language
; Let's add a while loop
;; Let's add a while loop
(define-syntax-rule (while condition body ...)
(let loop ()
(when condition
@ -425,48 +549,54 @@ vec ; => #(1 2 3 4)
(displayln i)
(set! i (add1 i))))
; Macros are hygienic, you cannot clobber existing variables!
(define-syntax-rule (swap x y)
(begin
(define tmp x)
;; 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 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
(swap! a b)
(printf "tmp = ~a; a = ~a; b = ~a\n" tmp a b) ; tmp is unaffected
;; But the 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
;; 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
[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
; Any client that attempt to deposit a non-positive amount, will be blamed
; (deposit -5) ; => deposit: contract violation
; expected: positive?
; given: -5
; more details....
;; 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 [Quick: An Introduction to Racket with Pictures](http://docs.racket-lang.org/quick/)
Still up for more? Try [Getting Started with Racket](http://docs.racket-lang.org/getting-started/)