4.21 Undefined

package: base

The bindings documented in this section are provided by the racket/undefined library, not racket/base or racket.

The constant undefined can be used as a placeholder value for a value to be installed later, especially for cases where premature access of the value is either difficult or impossible to detect or prevent.

The undefined value is always eq? to itself.

Added in version 6.0.0.6 of package base.

value
undefined : any/c

The “undefined” constant.

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

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