mirror of
https://github.com/adambard/learnxinyminutes-docs.git
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723 lines
21 KiB
Markdown
723 lines
21 KiB
Markdown
---
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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"]
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- ["Gustavo Schmidt", "https://github.com/gustavoschmidt"]
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- ["Duong H. Nguyen", "https://github.com/cmpitg"]
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- ["Keyan Zhang", "https://github.com/keyanzhang"]
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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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;; useful to comment expressions when debugging
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#; (this expression is discarded)
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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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#b111 ; binary => 7
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#o111 ; octal => 73
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#x111 ; hexadecimal => 273
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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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(expt 2 3) ; => 8
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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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"Foo\tbar\41\x21\u0021\a\r\n" ; includes C escapes, Unicode
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"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!"
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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 the (let ...)
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(let ([me "Bob"])
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"Alice"
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me) ; => "Bob"
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;; let* is like let, but allows you to use previous bindings in creating later bindings
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(let* ([x 1]
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[y (+ x 1)])
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(* x y))
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;; finally, letrec allows you to define recursive and mutually recursive functions
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(letrec ([is-even? (lambda (n)
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(or (zero? n)
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(is-odd? (sub1 n))))]
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[is-odd? (lambda (n)
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(and (not (zero? n))
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(is-even? (sub1 n))))])
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(is-odd? 11))
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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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; By default, structs are immutable
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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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; returns whether the variable was constructed with the dog constructor
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(dog? my-pet) ; => #t
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; accesses the name field of the variable constructed with the dog constructor
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(dog-name my-pet) ; => "lassie"
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; You can explicitly declare a struct to be mutable with the #:mutable option
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(struct rgba-color (red green blue alpha) #:mutable)
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(define burgundy
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(rgba-color 144 0 32 1.0))
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(set-color-green! burgundy 10)
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(color-green burgundy) ; => 10
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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, made of `cons' pairs and end
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;; with a `null' (or '()) to mark the end of the list
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(cons 1 (cons 2 (cons 3 null))) ; => '(1 2 3)
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;; `list' is a convenience variadic constructor for lists
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(list 1 2 3) ; => '(1 2 3)
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;; a quote can also be used for a literal list value
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'(1 2 3) ; => '(1 2 3)
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;; a quasiquote (represented by the backtick character) with commas
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;; can be used to evaluate functions
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`(1 ,(+ 1 1) 3) ; => '(1 2 3)
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;; With lists, car/cdr work slightly differently
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(car '(1 2 3)) ; => 1
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(cdr '(1 2 3)) ; => '(2 3)
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;; Racket also has predefined functions on top of car and cdr, to extract parts of a list
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(cadr (list 1 2 3)) ; => 2
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(car (cdr (list 1 2 3))) ; => 2
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(cddr (list 1 2 3)) ; => '(3)
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(cdr (cdr (list 1 2 3))) ; => '(3)
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(caddr (list 1 2 3)) ; => 3
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(car (cdr (cdr (list 1 2 3)))) ; => 3
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;; Can still 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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;; Lists are a very basic type, so there is a *lot* of functionality for
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;; them, a few examples:
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(map add1 '(1 2 3)) ; => '(2 3 4)
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(map + '(1 2 3) '(10 20 30)) ; => '(11 22 33)
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(filter even? '(1 2 3 4)) ; => '(2 4)
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(count even? '(1 2 3 4)) ; => 2
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(take '(1 2 3 4) 2) ; => '(1 2)
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(drop '(1 2 3 4) 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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;; (Functional: returns the extended set rather than mutate the input)
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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 (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
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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 an immutable hash table
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;; (Returns the extended hash instead 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))
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;; Remember, these hashes are immutable!
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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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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 3. Functions
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Use `lambda' to create functions.
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;; A function always returns the value of its last expression
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(lambda () "Hello World") ; => #<procedure>
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;; 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"
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((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"))
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(hello-world) ; => "Hello World"
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;; You can shorten this using the function definition syntactic sugar:
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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)
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(string-append "Hello " name)))
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(hello "Steve") ; => "Hello Steve"
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;; ... or equivalently, using a sugared definition:
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(define (hello2 name)
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(string-append "Hello " name))
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;; You can have multi-variadic functions too, using `case-lambda'
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(define hello3
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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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(hello3 "Jake") ; => "Hello Jake"
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(hello3) ; => "Hello World"
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;; ... or specify optional arguments with a default value expression
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(define (hello4 [name "World"])
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(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)
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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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;; ... or with the unsugared `lambda' form:
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(define count-args2
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(lambda args
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(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)
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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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;; ... unsugared:
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(define hello-count2
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(lambda (name . args)
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(format "Hello ~a, you passed ~a extra args" name (length args))))
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;; And with keywords
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(define (hello-k #:name [name "World"] #:greeting [g "Hello"] . args)
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(format "~a ~a, ~a extra args" g name (length args)))
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(hello-k) ; => "Hello World, 0 extra args"
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(hello-k 1 2 3) ; => "Hello World, 3 extra args"
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(hello-k #:greeting "Hi") ; => "Hi World, 0 extra args"
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(hello-k #:name "Finn" #:greeting "Hey") ; => "Hey Finn, 0 extra args"
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(hello-k 1 2 3 #:greeting "Hi" #:name "Finn" 4 5 6)
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; => "Hi Finn, 6 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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;; `eq?' returns #t if 2 arguments refer to the same object (in memory),
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;; #f otherwise.
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;; In other words, it's a simple pointer comparison.
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(eq? '() '()) ; => #t, since there exists only one empty list in memory
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(let ([x '()] [y '()])
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(eq? x y)) ; => #t, same as above
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(eq? (list 3) (list 3)) ; => #f
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(let ([x (list 3)] [y (list 3)])
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(eq? x y)) ; => #f — not the same list in memory!
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(let* ([x (list 3)] [y x])
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(eq? x y)) ; => #t, since x and y now point to the same stuff
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(eq? 'yes 'yes) ; => #t
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(eq? 'yes 'no) ; => #f
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(eq? 3 3) ; => #t — be careful here
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; It’s better to use `=' for number comparisons.
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(eq? 3 3.0) ; => #f
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(eq? (expt 2 100) (expt 2 100)) ; => #f
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(eq? (integer->char 955) (integer->char 955)) ; => #f
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(eq? (string-append "foo" "bar") (string-append "foo" "bar")) ; => #f
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;; `eqv?' supports the comparison of number and character datatypes.
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;; for other datatypes, `eqv?' and `eq?' return the same result.
|
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(eqv? 3 3.0) ; => #f
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(eqv? (expt 2 100) (expt 2 100)) ; => #t
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(eqv? (integer->char 955) (integer->char 955)) ; => #t
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|
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(eqv? (string-append "foo" "bar") (string-append "foo" "bar")) ; => #f
|
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|
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;; `equal?' supports the comparison of the following datatypes:
|
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;; strings, byte strings, pairs, mutable pairs, vectors, boxes,
|
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;; hash tables, and inspectable structures.
|
||
;; for other datatypes, `equal?' and `eqv?' return the same result.
|
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(equal? 3 3.0) ; => #f
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(equal? (string-append "foo" "bar") (string-append "foo" "bar")) ; => #t
|
||
(equal? (list 3) (list 3)) ; => #t
|
||
|
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 5. Control Flow
|
||
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
||
|
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;;; Conditionals
|
||
|
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(if #t ; test expression
|
||
"this is true" ; then expression
|
||
"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))
|
||
'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
|
||
|
||
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
||
;; 6. 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)
|
||
|
||
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
||
;; 7. 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
|
||
|
||
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
||
;; 8. 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)
|
||
|
||
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
||
;; 9. 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
|
||
|
||
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
||
;; 10. 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....
|
||
|
||
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
||
;; 11. 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/)
|