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This programming language may be used to instruct a computer to perform a task.
Official website
Execution method: Compiled (machine code)
Garbage collected: Yes
Parameter passing methods: By reference
Type safety: Safe
Type compatibility: Duck
Type expression: Implicit
Type checking: Dynamic
Lang tag(s): factor
See Also:
Listed below are all of the tasks on Rosetta Code which have been solved using Factor.

Factor is a stack-based, concatenative, general-purpose programming language with a focus on practicality.

Initially developed by Slava Pestov, Factor began life in 2003 as a scripting language written for a game. The implementation was originally an interpreter written in Java, but has since gained an optimizing compiler and has been rewritten in Factor with a minimal C++ core. Read more about Factor's implementation history here. Factor is still being developed by dozens of contributors, with the latest stable release in August 2023.

Factor is a stack language similar to, but of a higher level than, Forth. Factor is a concatenative language, meaning that rather than applying functions to arguments (applicative languages) to evaluate things, we compose functions to evaluate a single piece of data — the entire program up until that particular point. In Factor, the basic structure of data flow is function composition. That is, foo bar baz is equivalent to baz(bar(foo())) in an applicative language. This offers a nice left-to-right style of reading and data flow.

In Factor, we tend to name data flow operations rather than values. In an applicative language, you might write

var x = ...;
var y = foo(x);
var z = bar(x);

In Factor this is a data flow pattern called bi.

[ foo ] [ bar ] bi

This says, "apply foo to the object at the top of the data stack, and apply bar to it as well." Rather than naming the values x, y, and z, we named the data flow pattern.

Factor comes with many practical features, including a REPL, a self-contained help browser, an object inspector, a debugger/code walker, a deployment tool, editor integration for most popular text editors and IDEs, and introspection capabilities useful for developers. Factor has a fully-featured library, including things such as an HTTP server/client, bindings to graphics libraries and databases, a C FFI, a cross-platform GUI framework, on down to niche things like polynomial arithmetic. Factor features an object system that takes inspiration from Common Lisp and Self.

Most code tends to be expressed naturally in a functional manner. Factor comes with combinators (higher-order functions) typically seen in functional languages, such as map, filter, reduce, and many more. Although most things can be done efficiently without mutation, Factor doesn't shy away from it when it's useful. Mutating words end with exclamation points (by convention). Factor provides lexical and dynamic variables which can make writing imperative code more natural, or allows one to clean up code that performs a lot of stack shuffling.

One of Factor's greatest strengths is its ability to factor words into smaller words. Due to the nature of concatenative programming, this is typically a cut and paste job that can be done almost anywhere there is whitespace. Factor also has impressive metaprogramming capabilities. Since Factor is almost entirely written in Factor, there is full introspection support, including seamless access to Factor's parser, allowing one to define new syntax. Factor also offers Lisp-style macros, and in general, Factor code can be treated like a collection (homoiconicity).

About Factor examples on Rosetta Code

Most of the newer examples are meant to be copied and pasted directly into the listener (Factor's REPL) where they should run without issue. In order to deploy the examples to binaries or run them as scripts, you'll need to put them in a vocabulary (e.g. IN: myvocab and set a MAIN: word which acts as the entry point for the program. If an example doesn't run, it probably means that the example only works in an older version of Factor. Most of the time, this is because certain words have been changed without maintaining backwards compatibility (e.g. iota became <iota> in Factor 0.98).

For this reason, it is advised that examples use the works with template to indicate which version of Factor the example works with. For example,

{{works with|Factor|0.98}}


Works with: Factor version 0.98


Tasks not implemented in Factor



This category has the following 3 subcategories, out of 3 total.

Pages in category "Factor"

The following 200 pages are in this category, out of 1,001 total.

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