► |  | The Racket Reference

1 | Language Model
2 | Notation for Documentation
3 | Syntactic Forms
4 | Datatypes
5 | Structures
6 | Classes and Objects
7 | Units
8 | Contracts
9 | Pattern Matching
10 | Control Flow
11 | Concurrency and Parallelism
12 | Macros
13 | Input and Output
14 | Reflection and Security
15 | Operating System
16 | Memory Management
17 | Unsafe Operations
18 | Running Racket
| Bibliography
| Index

▼ | 4 | Datatypes

4.1 | Booleans and Equality
4.2 | Numbers
4.3 | Strings
4.4 | Byte Strings
4.5 | Characters
4.6 | Symbols
4.7 | Regular Expressions
4.8 | Keywords
4.9 | Pairs and Lists
4.10 | Mutable Pairs and Lists
4.11 | Vectors
4.12 | Boxes
4.13 | Hash Tables
4.14 | Sequences and Streams
4.15 | Dictionaries
4.16 | Sets
4.17 | Procedures
4.18 | Void and Undefined

On this page:

box?
box
box-immutable
unbox
set-box!
box-cas!

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#### 4.12`` Boxes

> > > Boxes in The Racket Guide introduces boxes.

A box is like a single-element vector, normally used as minimal mutable storage.

A literal or printed box starts with #&. See Reading Boxes for information on reading boxes and Printing Boxes for information on printing boxes.

> procedure (box? v) → boolean?
> v : any/c

Returns #t if v is a box, #f otherwise.

> procedure (box v) → box?
> v : any/c

Returns a new mutable box that contains v.

> procedure (box-immutable v) → (and/c box? immutable?)
> v : any/c

Returns a new immutable box that contains v.

> procedure (unbox box) → any/c
> box : box?

Returns the content of box.

For any v, (unbox (box v)) returns v.

> procedure (set-box! box v) → void?
> box : (and/c box? (not/c immutable?))
> v : any/c

Sets the content of box to v.

> procedure (box-cas! box old new) → boolean?
> box : (and/c box? (not/c immutable?) (not/c impersonator?))
> old : any/c
> new : any/c

Atomically updates the contents of box to new, provided that box currently contains a value that is eq? to old, and returns #t in that case. If box does not contain old, then the result is #f.

If no other threads or futures attempt to access box, the operation is equivalent to

> (and (eq? old (unbox loc)) (set-box! loc new) #t)

When Racket is compiled with support for futures, box-cas! uses a hardware compare and set operation. Uses of box-cas! be performed safely in a future (i.e., allowing the future thunk to continue in parallel).

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