A symbol in
computer programming is a
primitive data type whose
instances have a unique human-readable form. Symbols can be used as
identifier
An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique ''class'' of objects, where the "object" or class may be an idea, physical countable object (or class thereof), or physical noncountable ...
s. In some
programming languages, they are called atoms.
Uniqueness is enforced by holding them in a
symbol table. The most common use of symbols by programmers is for performing language
reflection (particularly for
callbacks
In computer programming, a callback or callback function is any reference to executable code that is passed as an argument to another piece of code; that code is expected to ''call back'' (execute) the callback function as part of its job. Thi ...
), and most common indirectly is their use to create object
linkages.
In the most trivial
implementation, they are essentially named
integers (e.g. the
enumerated type in C).
Support
The following
programming languages provide
runtime support for symbols:
Julia
Symbols in 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).
Lisp
A symbol in
Lisp
A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech.
Types
* A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lisping ...
is unique in a
namespace (or ''package'' in
Common Lisp
Common Lisp (CL) is a dialect of the Lisp programming language, published in ANSI standard document ''ANSI INCITS 226-1994 (S20018)'' (formerly ''X3.226-1994 (R1999)''). The Common Lisp HyperSpec, a hyperlinked HTML version, has been derived fro ...
). 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.
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't 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. Keyword symbols are self-evaluating and interned in the package named KEYWORD.
Examples
The following is a simple external representation of a
Common Lisp
Common Lisp (CL) is a dialect of the Lisp programming language, published in ANSI standard document ''ANSI INCITS 226-1994 (S20018)'' (formerly ''X3.226-1994 (R1999)''). The Common Lisp HyperSpec, a hyperlinked HTML version, has been derived fro ...
symbol:
this-is-a-symbol
Symbols can contain whitespace (and all other characters):
, This is a symbol with whitespace,
In Common Lisp symbols with a leading colon in their printed representations are
keyword symbols. These are interned in the keyword package.
:keyword-symbol
A printed representation of a symbol may include a package name. Two colons are written between the name of the package and the name of the symbol.
package-name::symbol-name
Packages can export symbols. Then only one colon is written between the name of the package and the name of the symbol.
package:exported-symbol
Symbols, which are not interned in a package, can also be created and have a notation:
#:uninterned-symbol
PostScript
In
PostScript
PostScript (PS) is a page description language in the electronic publishing and desktop publishing realm. It is a dynamically typed, concatenative programming language. It was created at Adobe Systems by John Warnock, Charles Geschke, Doug Br ...
, 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 primary primitive data types, similar to numbers.
The exact notation may differ in different Prolog's dialects. However, it is always quite simple (no quotations or special beginning characters are necessary).
Contrary to other languages, it is possible to give symbols some ''meaning'' by creating some Prolog's facts and/or rules.
Examples
The following example demonstrates two facts (describing what ''father'' is) and one rule (describing the ''meaning'' of ''sibling''). These three sentences use symbols (father, zeus, hermes, perseus and sibling) and some abstract variables (X, Y and Z). The ''mother'' relationship has been omitted for clarity.
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.
They can be used as an identifier or an interned string.
Two symbols with the same contents will always refer to the same object.
It is considered a
best practice to use symbols as keys to an
associative array in Ruby.
Examples
The following is a simple example of a symbol literal in Ruby:
my_symbol = :a
my_symbol = :"an identifier"
Strings can be coerced into symbols, vice versa:
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"
Symbols are objects of the
Symbol
class in Ruby:
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
Symbols are commonly used to dynamically send messages to (call methods on) objects:
irb(main):007:0> "aoboc".split("o")
=> a", "b", "c"irb(main):008:0> "aoboc".send(:split, "o") # same result
=> a", "b", "c"
Symbols as keys of an associative array:
irb(main):009:0> my_hash =
=> {:a=>"apple", :b=>"banana"}
irb(main):010:0> my_hash a
=> "apple"
irb(main):011:0> my_hash b
=> "banana"
Smalltalk
In
Smalltalk
Smalltalk is an object-oriented, dynamically typed reflective programming language. It was designed and created in part for educational use, specifically for constructionist learning, at the Learning Research Group (LRG) of Xerox PARC by Alan Ka ...
, 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.
[http://wiki.squeak.org/squeak/uploads/172/standard_v1_9-indexed.pdf ANSI Smalltalk standard.] In most Smalltalk implementations, selectors (method names) are implemented as symbols.
Examples
The following is a simple example of a symbol literal in Smalltalk:
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 "
Strings can be coerced into symbols, vice versa:
my_symbol := 'Hello, world!' asSymbol " => #'Hello, world!' "
my_string := #hello: asString " => 'hello:' "
Symbols conform to the
symbol
protocol, and their class is called
Symbol
in most implementations:
my_symbol := #hello_world
my_symbol class " => Symbol "
Symbols are commonly used to dynamically send messages to (call methods on) objects:
" same as 'foo' at: 2 "
'foo' perform: #at: with: 2 " => $o "
References
Articles with example Ruby code
Programming constructs