Racket

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Modern dialect of Lisp

Language Features

Booleans

#t true or #f false

#lang racket

#t

#t

Numbers

#lang racket
(+ 1 1)

Strings

#lang racket
"Damien"

Damien

#lang racket
(list->string (reverse (string->list "Damien")))

neimaD

Characters

#lang racket

#\A

#\A

Variables

#lang racket
(define x 9)
x

Pairs

Can use cons to construct a pair:

#lang racket

(car (cons 1 2))
(cdr (cons 1 2))
(pair? (cons 1 2))

1
2
#t

Or the quote with . notation:

#lang racket

(car '(1 . 2))
(cdr '(1 . 2))
(pair? '(1 . 2))

1
2
#t

Lists

https://docs.racket-lang.org/guide/Lists__Iteration__and_Recursion.html

(list i j k) is similar to '(i j k)

#lang racket
(list 1 2 3)

'(1 2 3)

#lang racket
'(1 2 3)

'(1 2 3)

Length

#lang racket
(length '(1 2 3))

Sort

#lang racket
(sort '(1 2 3) >)

'(3 2 1)

Take

#lang racket
(take '(1 2 3) 2)

'(1 2)

Map

Can insert lambda in Emacs with C-M-y or racket-insert-lambda

#lang racket
(map (λ (n) (* 2 n)) '(1 2 3))

'(2 4 6)

Foldl

#lang racket
(foldl (λ (acc n) (+ acc n)) 0 '(1 2 3))

Actually a way simpler way of writing it:

#lang racket
(foldl + 0 '(1 2 3))

Hash Tables

#lang racket

(hash "apple" "red" "banana" "yellow")

'#hash(("apple" . "red") ("banana" . "yellow"))

Get value by key

#lang racket

(define fruit-colours (hash "apple" "red" "banana" "yellow"))
(hash-ref fruit-colours "apple")

red

Set value by key

hash-set returns a new list

#lang racket

(define fruit-colours (hash "apple" "red" "banana" "yellow"))
(hash-set fruit-colours "apple" "red-ish")

'#hash(("apple" . "red-ish") ("banana" . "yellow"))

Functions

https://docs.racket-lang.org/guide/syntax-overview.html

#lang racket
(define (double x) (* x 2))

(double 5)

Anonymous Functions

Either lambda or λ can be used

#lang racket
((lambda (n) (* 2 n)) 2)

#lang racket
((λ (n) (* 2 n)) 2)

Thread Macros

#lang racket
(require threading)
(~> "Damien" string->list reverse list->string)

neimaD

Pattern Matching

#lang racket

(match 9
  [7 'seven]
  [8 'eight]
  [9 'nine])

nine

Eval

racket -e '(+ 1 1)'

Typed Racket

Annotations

#lang typed/racket

(: x Number)
(define x 9)
x

#lang typed/racket

(define x : Number 9)
x

Basic Types

Name	Example
String	"Hello, world"
Char	#\|̣
True	#t
False	#f
Symbol	'foo
Integer	1
Flonum	2.1

Example

#lang typed/racket

(struct pt ([x : Real] [y : Real]))

(: distance (-> pt pt Real))
(define (distance p1 p2)
  (sqrt (+ (sqr (- (pt-x p2) (pt-x p1)))
           (sqr (- (pt-y p2) (pt-y p1))))))

(distance (pt 0 0) (pt 2 2))

2.8284271247461903

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