3.6 Symbols

► |  | Reference: Racket

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

▼ | 3 | Datatypes

3.1 | Booleans and Equality
3.2 | Numbers
3.3 | Strings
3.4 | Byte Strings
3.5 | Characters
3.6 | Symbols
3.7 | Regular Expressions
3.8 | Keywords
3.9 | Pairs and Lists
3.10 | Mutable Pairs and Lists
3.11 | Vectors
3.12 | Boxes
3.13 | Hash Tables
3.14 | Sequences
3.15 | Dictionaries
3.16 | Sets
3.17 | Procedures
3.18 | Void and Undefined

On this page:

symbol?
symbol-interned?
symbol-unreadable?
symbol->string
string->symbol
string->uninterned-symbol
string->unreadable-symbol
gensym

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#### 3.6`` Symbols

> > > Symbols in Guide: Racket introduces symbols.

A symbol is like an immutable string, but symbols are normally interned, so that two symbols with the same character content are normally eq?. All symbols produced by the default reader (see Reading Symbols) are interned.

The two procedures string->uninterned-symbol and gensym generate uninterned symbols, i.e., symbols that are not eq?, eqv?, or equal? to any other symbol, although they may print the same as other symbols.

The procedure string->unreadable-symbol returns an unreadable symbol that is partially interned. The default reader (see Reading Symbols) never produces a unreadable symbol, but two calls to string->unreadable-symbol with equal? strings produce eq? results. An unreadable symbol can print the same as an interned or uninterned symbol. Unreadable symbols are useful in expansion and compilation to avoid collisions with symbols that appear in the source; they are usually not generated directly, but they can appear in the result of functions like identifier-binding.

Interned and unreadable symbols are only weakly held by the internal symbol table. This weakness can never affect the result of an eq?, eqv?, or equal? test, but a symbol may disappear when placed into a weak box (see Weak Boxes) used as the key in a weak hash table (see Hash Tables), or used as an ephemeron key (see Ephemerons).

( symbol? v) → boolean?
v : any/c

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

Examples:
> ( symbol? 'Apple)
#t
> ( symbol? 10)
#f

( symbol-interned? sym) → boolean?
sym : symbol?

Returns #t if sym is interned, #f otherwise.

Examples:
> ( symbol-interned? 'Apple)
#t
> ( symbol-interned? ( gensym))
#f
> ( symbol-interned? ( string->unreadable-symbol "Apple"))
#f

( symbol-unreadable? sym) → boolean?
sym : symbol?

Returns #t if sym is an unreadable symbol, #f otherwise.

Examples:
> ( symbol-unreadable? 'Apple)
#f
> ( symbol-unreadable? ( gensym))
#f
> ( symbol-unreadable? ( string->unreadable-symbol "Apple"))
#t

( symbol->string sym) → string?
sym : symbol?

Returns a freshly allocated mutable string whose characters are the same as in sym.

Example:
> ( symbol->string 'Apple)
"Apple"

( string->symbol str) → symbol?
str : string?

Returns an interned symbol whose characters are the same as in str.

Examples:
> ( string->symbol "Apple")
'Apple
> ( string->symbol "1")
'|1|

( string->uninterned-symbol str) → symbol?
str : string?

Like ( string->symbol str), but the resulting symbol is a new uninterned symbol. Calling string->uninterned-symbol twice with the same str returns two distinct symbols.

Examples:
> ( string->uninterned-symbol "Apple")
'Apple
> ( eq? 'a ( string->uninterned-symbol "a"))
#f

> ( eq? ( string->uninterned-symbol "a")
( string->uninterned-symbol "a"))

#f

( string->unreadable-symbol str) → symbol?
str : string?

Like ( string->symbol str), but the resulting symbol is a new unreadable symbol. Calling string->unreadable-symbol twice with equivalent strs returns the same symbol, but read never produces the symbol.

Examples:
> ( string->unreadable-symbol "Apple")
'Apple
> ( eq? 'a ( string->unreadable-symbol "a"))
#f

> ( eq? ( string->unreadable-symbol "a")
( string->unreadable-symbol "a"))

#t

( gensym [base]) → symbol?
base : ( or/c string? symbol?) = "g"

Returns a new uninterned symbol with an automatically-generated name. The optional base argument is a prefix symbol or string.

Example:
> ( gensym "apple")
'apple1623

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