Symbol (programming): Difference between revisions

Revision as of 16:59, 26 July 2023

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{{Short description|Datatype in programming}}

A '''symbol''' in [[computer programming]] is a [[Primitive data type|primitive]] [[data type]] whosewhich [[Instance (computer science)|instances]] have a unique human-readable form. Symbols can be used as [[Identifier (computer languages)|identifier]]s. In some [[programming languageslanguage]]s, they are called '''atoms'''.<ref name=pickaxe>{{cite book |lastlast1=HuntThomas |firstfirst1=Dave |author1-link=Dave Thomas ;(programmer) |last2=Fowler |first2=Chad Fowler|last3=Hunt ;|first3=Andy |author3-link=Andy Hunt (author) |year=2001 |title=Programming Ruby the pragmatic programmers' guide ; [includes Ruby 1.8]|year=2001 |publisher=The Pragmatic Bookshelf |___location=Raleigh, NCNorth Carolina |isbn=978-0-9745140-5-5 |url=https://archive.org/details/programmingrubyp00thom |edition=2. ed.2nd, 10. print. |url-access=registration}}</ref> Uniqueness is enforced by holding them in a [[symbol table]]. The most common use of symbols by programmers is forto performingperform language [[reflectionReflective (programming) | reflection]] (particularly for [[callbackCallback (computer programming)|callbacks]]), and most common indirectly is their use to create object [[linkageLinkage (software)|linkages]].

In the most trivial [[implementation]], they are essentially named [[integer]]s; (e.g., the [[enumerated type]] in [[C (programming language)|C]] language.

==Support==

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| [[Julia (programming language)|Julia]] || Symbol || <code>:sym</code>

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| [[K (programming language)|K]] || symbol || `sym

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| [[Objective-C]] || SEL || <code>@selector(sym)</code>

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| [[PostScript]] || name || <code>/sym</code> or <code>sym</code>

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| [[Prolog (programming language)|Prolog]] || atom, symbol || <code>sym</code> or <code>'sym'</code>

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| [[Ruby (programming language)|Ruby]] || Symbol || <code>:sym</code> or <code>:'sym'</code>

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===Prolog===

In [[Prolog (programming language)|Prolog]], symbols (or atoms) are the primarymain primitive data types, similar to numbers.<ref name=Bratko2001>{{Cite book | last1 last= Bratko | first1 first= Ivan |year=2001 |title = Prolog programming for artificial intelligence | year = 2001 | publisher = Addison Wesley | ___location = Harlow, England ; New York | isbn = 978-0-201-40375-6 }}</ref> 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 many other languages, it is possible to give symbols somea ''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 beenis omitted for clarity.

<syntaxhighlight lang="prolog">

father(zeus, hermes).

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===Ruby===

In [[Ruby (programming language)|Ruby]], symbols can be created with a literal form, or by converting a string.<ref name=pickaxe />

They can be used as an identifier or an interned string.<ref name="rubysymbol">{{cite web |last=Kidd |first=Eric |date=20 January 2007 |title=13 Ways of Looking at a Ruby Symbol |url=http://www.randomhacks.net/articles/2007/01/20/13-ways-of-looking-at-a-ruby-symbol#9 |work=Random Hacks |access-date=10 July 2011}}</ref> Two symbols with the same contents will always refer to the same object.<ref>{{Cite web | url=http://www.ruby-doc.org/docs/ProgrammingRuby/html/language.html#UI | title=Programming Ruby: The Pragmatic Programmer's Guide}}</ref>

It is considered a [[best practice]] to use symbols as keys to an [[associative array]] in Ruby.<ref name=rubysymbol /><ref name="wrongreason">{{cite web|title=Using Symbols for the Wrong Reason|url=http://microjet.ath.cx/WebWiki/2005.12.27_UsingSymbolsForTheWrongReason.html|work=Gnomic Notes}}</ref>

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===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.<ref>http://wiki.squeak.org/squeak/uploads/172/standard_v1_9-indexed.pdf ANSI Smalltalk standard.</ref> In most Smalltalk implementations, selectors (method names) are implemented as symbols.

They can be used as an identifier or an interned string. Two symbols with the same contents will always refer to the same object.<ref>http://wiki.squeak.org/squeak/uploads/172/standard_v1_9-indexed.pdf ANSI Smalltalk standard.</ref> In most Smalltalk implementations, selectors (method names) are implemented as symbols.

====Examples====

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==References==

{{Reflist}}

<references/>

[[Category:Articles with example Ruby code]]