View source for Factor (programming language)

{{Short description|Stack-oriented programming language}}
{{Primary sources|date=July 2019}}
{{Infobox programming language
|name = Factor
|logo = [[File:NewFactorLogo.png|200px]]
|paradigm = [[multi-paradigm programming language|multi-paradigm]]: [[concatenative programming language|concatenative]] ([[stack-oriented programming|stack-based]]), [[functional programming|functional]], [[object-oriented programming|object-oriented]]
|year = 2003
|developer = Slava Pestov
|latest_release_version = 0.100
|latest_release_date = {{start date|2024|9|11}}
|typing = [[strong typing|strong]], [[dynamic typing|dynamic]]
|influenced_by = [[Joy (programming language)|Joy]], [[Forth (programming language)|Forth]], [[Lisp (programming language)|Lisp]], [[Self (programming language)|Self]]
|influenced =
|operating_system = [[Microsoft Windows|Windows]], [[macOS]], [[Linux]]
|license = [[BSD license]]
|website = [http://factorcode.org/ factorcode.org]
}}

'''Factor''' is a [[stack-oriented programming language]] created by Slava Pestov. Factor is [[dynamically typed]] and has [[automatic memory management]], as well as powerful metaprogramming features. The language has a single implementation featuring a self-hosted [[compiler optimization|optimizing compiler]] and an [[Integrated development environment|interactive development environment]]. The Factor distribution includes a large [[standard library]].

==History==
Slava Pestov created Factor in 2003 as a [[scripting language]] for a [[video game]].<ref>{{cite web | url = http://factorcode.org/slava/ | first = Slava | last = Pestov | title = Slava Pestov's corner of the web}}</ref> The initial implementation, now referred to as JFactor, was implemented in [[Java (programming language)|Java]] and ran on the [[Java Virtual Machine]]. Though the early language resembled modern Factor superficially in terms of [[syntax (programming languages)|syntax]], the modern language is very different in practical terms and the current implementation is much faster.

The language has changed significantly over time. Originally, Factor programs centered on manipulating Java objects with Java's [[reflection (computer science)|reflection]] capabilities. From the beginning, the design philosophy has been to modify the language to suit programs written in it. As the Factor implementation and standard libraries grew more detailed, the need for certain language features became clear, and they were added. JFactor did not have an [[object system]] where the programmer could define their own [[class (computer science)|class]]es, and early versions of native Factor were the same; the language was similar to [[Scheme (programming language)|Scheme]] in this way. Today, the object system is a central part of Factor. Other important language features such as [[tuple]] classes, combinator inlining, [[macro (computer science)|macro]]s, user-defined [[parsing]] words and the modern vocabulary system were only added in a piecemeal fashion as their utility became clear.

The [[foreign function interface]] was present from very early versions to Factor, and an analogous system existed in JFactor. This was chosen over creating a plugin to the [[C (programming language)|C]] part of the implementation for each external [[library (computing)|library]] that Factor should communicate with, and has the benefit of being more [[declarative programming|declarative]], faster to compile and easier to write.

The Java implementation initially consisted of just an [[interpreter (computing)|interpreter]], but a compiler to [[Java bytecode]] was later added. This compiler only worked on certain procedures. The Java version of Factor was replaced by a version written in C and Factor. Initially, this consisted of just an interpreter, but the interpreter was replaced by two compilers, used in different situations. Over time, the Factor implementation has grown significantly faster.<ref>{{cite web | url = http://concatenative.org/wiki/view/Factor/Implementation%20history | title = Concatenative.org wiki: Factor/Implementation History}}</ref>

==Description==
Factor is a [[dynamically typed]], [[functional programming|functional]] and [[object-oriented programming|object-oriented]] [[programming language]]. Code is structured around small procedures, called words. In typical code, these are 1–3 lines long, and a procedure more than 7 lines long is very rare. Something that would idiomatically be expressed with one procedure in another programming language would be written as several words in Factor.<ref name="dls">{{cite journal |last1=Pestov |first1=Sviatoslav |last2=Ehrenberg |first2=Daniel |year=2010 |title=Factor: a dynamic stack-based programming language |journal=ACM SIGPLAN Notices |volume=45 |issue=12 |pages=43–58 |publisher=ACM |doi=10.1145/1899661.1869637 }}</ref>

Each word takes a fixed number of arguments and has a fixed number of return values. Arguments to words are passed on a [[stack (data structure)|data stack]], using [[reverse Polish notation]]. The stack is used just to organize calls to words, and not as a data structure. The stack in Factor is used in a similar way to the stack in [[Forth (programming language)|Forth]]; for this, they are both considered [[stack-oriented programming language|stack languages]]. For example, below is a snippet of code that prints out "hello world" to the current output stream:
 "hello world" print
<code>print</code> is a word in the <code>io</code> vocabulary that takes a string from the stack and returns nothing.  It prints the string to the current output stream (by default, the terminal or the graphical listener).<ref name = "dls" />

The [[factorial|factorial function]] <math>n!</math> can be implemented in Factor in the following way:
<syntaxhighlight lang="factor">
: factorial ( n -- n! ) dup 1 > [ [1..b] product ] [ drop 1 ] if ;
</syntaxhighlight>

Not all data has to be passed around only with the stack. [[Lexical scoping|Lexically scoped]] local variables let one store and access [[temporary variable|temporaries]] used within a procedure. [[Dynamic scoping|Dynamically scoped]] variables are used to pass things between procedure calls without using the stack. For example, the current input and output streams are stored in dynamically scoped variables.<ref name="dls" />

Factor emphasizes flexibility and the ability to extend the language.<ref name = "dls" />  There is a system for macros, as well as for arbitrary extension of Factor syntax. Factor's syntax is often extended to allow for new types of word definitions and new types of [[literal (computer science)|literal]]s for data structures.  It is also used in the [[XML]] library to provide literal syntax for generating XML.  For example, the following word takes a string and produces an XML document object which is an HTML document emphasizing the string:
<syntaxhighlight lang="factor">
 : make-html ( string -- xml )
    dup
    <XML
        <html>
            <head><title><-></title></head>
            <body><h1><-></h1></body>
        </html>
    XML> ;
</syntaxhighlight>
The word <code>dup</code> duplicates the top item on the stack. The <code>&lt;-></code> stands for filling in that part of the XML document with an item from the stack.

==Implementation and libraries==
Factor includes a large standard library, written entirely in the language. These include
* A cross-platform GUI toolkit, built on top of [[OpenGL]] and various windowing systems, used for the development environment.<ref>{{cite web | url = http://docs.factorcode.org/content/article-ui.html | title = Factor documentation: UI framework | first = Slava | last = Pestov}}</ref>
* Bindings to several database libraries, including [[PostgreSQL]] and [[SQLite]].<ref>{{cite web | url = http://docs.factorcode.org/content/article-db.html | title = Factor documentation: Database library | first = Doug | last = Coleman}}</ref>
* An [[HTTP]] server and client, with the Furnace web framework.<ref>{{cite web | url = http://docs.factorcode.org/content/article-http.server.html | title = Factor documentation: HTTP server | first = Slava | last = Pestov}}</ref>
* Efficient homogeneous arrays of integers, floats and C structs.<ref>{{cite web | url = http://docs.factorcode.org/content/article-specialized-arrays.html | title = Factor documentation: Specialized arrays | first = Slava | last = Pestov}}</ref>
* A library implementing regular expressions, generating machine code to do the matching.<ref>{{cite web | url = http://docs.factorcode.org/content/article-regexp.html | title = Factor documentation: Regular expressions | first1 = Doug | last1 = Coleman | first2 = Daniel | last2 = Ehrenberg}}</ref>

A [[foreign function interface]] is built into Factor, allowing for communication with [[C (programming language)|C]], [[Objective-C]] and [[Fortran]] programs. There is also support for executing and communicating with shaders written in [[GLSL]].<ref name="dls"/><ref>{{cite web | first = Slava | last = Pestov | url = http://factor-language.blogspot.com/2010/07/overhauling-factors-c-library-interface.html | title = Overhauling Factor's C library interface| date = 28 July 2010 }}</ref>

Factor is implemented in Factor and [[C++]]. It was originally bootstrapped from an earlier Java implementation. Today, the parser and the optimizing compiler are written in the language. Certain basic parts of the language are implemented in C++ such as the garbage collector and certain primitives.

Factor uses an [[system image|image]]-based model, analogous to many [[Smalltalk]] implementations, where compiled code and data are stored in an image.<ref>{{cite web | first = Slava | last = Pestov | url = http://factor-language.blogspot.com/2010/01/factors-bootstrap-process-explained.html | title = Factor's bootstrap process explained| date = 10 January 2010 }}</ref>  To compile a program, the program is loaded into an image and the image is saved. A special tool assists in the process of creating a minimal image to run a particular program, packaging the result into something that can be deployed as a standalone application.<ref name="dls"/><ref>{{cite web | first = Slava | last = Pestov | url = http://factor-language.blogspot.com/2008/07/on-shaking-trees.html | title = On shaking trees| date = 5 July 2008 }}</ref>

The Factor compiler implements many advanced optimizations and has been used as a target for research in new optimization techniques.<ref name="dls"/><ref>{{cite web|first=Daniel |last=Ehrenberg |title=Closure elimination as constant propagation |url=http://factorcode.org/littledan/abstract.pdf |year=2010 |url-status=dead |archive-url=https://web.archive.org/web/20110726044425/http://factorcode.org/littledan/abstract.pdf |archive-date=2011-07-26 }}</ref>

==References==
{{Reflist}}

==External links==
*{{Official website|http://factorcode.org}}
*{{cite video |people=Slava Pestov |date=October 27, 2008 |title=Factor: An Extensible Interactive Language |url=https://www.youtube.com/watch?v=f_0QlhYlS8g  |archive-url=https://ghostarchive.org/varchive/youtube/20211222/f_0QlhYlS8g |archive-date=2021-12-22 |url-status=live|format=flv |medium=Tech talk |publisher=[[Google]]}}{{cbignore}}
*{{cite video |people=Zed Shaw |date=2008 |title=The ACL is Dead|url=http://vimeo.com/2723800 |format=flv |medium=CUSEC 2008 |publisher=CUSEC}} – a presentation written in Factor which mentions and praises Factor

{{DEFAULTSORT:Factor (Programming Language)}}
[[Category:Programming languages]]
[[Category:Concatenative programming languages]]
[[Category:Function-level languages]]
[[Category:Stack-oriented programming languages]]
[[Category:Programming languages created in 2003]]
[[Category:Extensible syntax programming languages]]
[[Category:2003 software]]
[[Category:High-level programming languages]]
[[Category:Software using the BSD license]]