Editing Lithe

{{Short description|Experimental programming language}}
{{for|the Australian singer|Lithe (singer)}}
{{For|the fictional holiday|Shire (Middle-earth)#Calendar}}
{{no footnotes|date=July 2019}}

'''Lithe''' is an experimental [[programming language]] created in 1982 by David Sandberg at the [[University of Washington]] which allows the programmer to freely choose their own syntax. Lithe combines the ideas of [[syntax-directed translation]] and [[class (computer science)|classes]] in a novel manner that results in a remarkably simple yet powerful language.

==Overview==
The standard [[class-based programming]] model does not specify a mechanism by which to manipulate objects: where [[Smalltalk]] uses message passing, Lithe uses [[syntax-directed translation]] (SDT). SDT is a method of translating a string into a sequence of ''actions'' by attaching one such action to each rule of a grammar. Thus, parsing a string of the grammar produces a sequence of rule applications. Lithe merges SDT with the class model by using classes as the non-terminal alphabet of the grammar. Since the grammar class used by Lithe properly contains all context-free grammars, a wide variety of syntax can be described, and SDT provides a simple way to attach semantics to any such syntax.

The ''package'' is the unit of [[modularity (programming)|program modularity]] in Lithe. A package is divided into two parts: one that defines classes and another that defines rules. [[Information hiding]] is achieved by requiring both ''export'' keywords on those rules and classes that are to be seen outside a package ''and'' that the source package names be included in the ''with'' clause of the consumer package.

==Examples==
Programming in Lithe consists of defining rule-action pairs and classes. Take the task of computing the [[absolute value]] of a number, for example:

 '''rule''' "|" &lt;i:'''int'''> "|" '''return int''';
   {'''if''' i<0 '''then return''' -i '''else return''' i '''end'''}

After this rule-action pair has been defined, it can be freely used within Lithe source code, for example, the following expression will thereafter be valid:

 |x - y| + 3

The equivalent rule in [[Backus–Naur form|BNF]] would be:

 &lt;'''int'''>  ::=  "|" <'''int'''> "|"

Note that the Lithe description swaps the left and right sides of the BNF one, then adds a name to each nonterminal in the BNF right-hand side. These names are used in the action part, which is expressed as a string that is translated into a sequence of actions by using other, previously defined, rule-action pairs (eventually, some of those rule-action pairs will invoke primitive actions). 

==External links==
* [http://portal.acm.org/citation.cfm?id=582169 "Lithe: a language combining a flexible syntax and classes"]

[[Category:Declarative programming languages]]
[[Category:Experimental programming languages]]
[[Category:Extensible syntax programming languages]]