Julia

Symbols in Julia are interned strings used to represent identifiers in parsed Julia code(ASTs) and as names or labels to identify entities (for example as keys in a dictionary).^[5]

Lisp

A symbol in Lisp is unique in a namespace (or package in Common Lisp). Symbols can be tested for equality with the function EQ. Lisp programs can generate new symbols at runtime. When Lisp reads data that contains textual represented symbols, existing symbols are referenced. If a symbol is unknown, the Lisp reader creates a new symbol.

In Common Lisp, symbols have the following attributes: a name, a value, a function, a list of properties and a package.^[6]

In Common Lisp it is also possible that a symbol is not interned in a package. Such symbols can be printed, but when read back, a new symbol needs to be created. Since it is not interned, the original symbol can not be retrieved from a package.

In Common Lisp symbols may use any characters, including whitespace, such as spaces and newlines. If a symbol contains a whitespace character, it needs to be written as |this is a symbol|. Symbols can be used as identifiers for any kind of named programming constructs: variables, functions, macros, classes, types, goto tags and more. Symbols can be interned in a package.^[7] Keyword symbols are self-evaluating,^[8] and interned in the package named KEYWORD.

this-is-a-symbol

|This is a symbol with whitespace|

:keyword-symbol

package-name::symbol-name

package:exported-symbol

#:uninterned-symbol

PostScript

In PostScript, references to name objects can be either literal or executable, influencing the behaviour of the interpreter when encountering them. The cvx and cvl operators can be used to convert between the two forms. When names are constructed from strings by means of the cvn operator, the set of allowed characters is unrestricted.

Prolog

In Prolog, symbols (or atoms) are the main primitive data types, similar to numbers.^[9] The exact notation may differ in different Prolog dialects. However, it is always quite simple (no quotations or special beginning characters are necessary).

Contrary to many other languages, it is possible to give symbols a meaning by creating some Prolog facts and/or rules.

father(zeus, hermes).
father(zeus, perseus).

sibling(X, Y) :- father(Z, X), father(Z, Y).

Ruby

In Ruby, symbols can be created with a literal form, or by converting a string.^[1] They can be used as an identifier or an interned string.^[10] Two symbols with the same contents will always refer to the same object.^[11] It is considered a best practice to use symbols as keys to an associative array in Ruby.^[10][12]

my_symbol = :a
my_symbol = :"an identifier"

irb(main):001:0> my_symbol = "Hello, world!".intern 
=> :"Hello, world!"
irb(main):002:0> my_symbol = "Hello, world!".to_sym 
=> :"Hello, world!"
irb(main):003:0> my_string = :hello.to_s
=> "hello"

irb(main):004:0> my_symbol = :hello_world
=> :hello_world
irb(main):005:0> my_symbol.length 
=> 11
irb(main):006:0> my_symbol.class 
=> Symbol

irb(main):007:0> "aoboc".split("o")
=> ["a", "b", "c"]
irb(main):008:0> "aoboc".send(:split, "o") # same result
=> ["a", "b", "c"]

irb(main):009:0> my_hash = { a: "apple", b: "banana" }
=> {:a=>"apple", :b=>"banana"}
irb(main):010:0> my_hash[:a] 
=> "apple"
irb(main):011:0> my_hash[:b] 
=> "banana"

Smalltalk

In Smalltalk, symbols can be created with a literal form, or by converting a string. They can be used as an identifier or an interned string. Two symbols with the same contents will always refer to the same object.^[14] In most Smalltalk implementations, selectors (method names) are implemented as symbols.

my_symbol := #'an identifier' " Symbol literal "
my_symbol := #a               " Technically, this is a selector literal. In most implementations, "
                              " selectors are symbols, so this is also a symbol literal "

my_symbol := 'Hello, world!' asSymbol " => #'Hello, world!' "
my_string := #hello: asString         " => 'hello:' "

my_symbol := #hello_world
my_symbol class            " => Symbol "

" same as 'foo' at: 2 "
'foo' perform: #at: with: 2 " => $o "