Editing Factor (programming language) (section)

==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.