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. Each word takes a fixed number of arguments and has a fixed number of return values. Arguments to words are passed on a 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; for this, they are both considered stack languages. For example, below is a snippet of code that prints out "hello world" to the current output stream: "hello world" print print is a word in the io 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). The factorial function can be implemented in Factor in the following way: : factorial ( n -- n! ) dup 1 > [ [1,b] product ] [ drop 1 ] if ; Not all data has to be passed around only with the stack. Lexically scoped local variables let one store and access temporaries used within a procedure. 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. Factor emphasizes flexibility and the ability to extend the language. 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 literals 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: : make-html ( string -- xml ) dup <XML <html> <head><title><-></title></head> <body><h1><-></h1></body> </html> XML> ; The word dup duplicates the top item on the stack. The <-> 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. Bindings