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Factor is a concatenative, stack-based programming language with high-level features including dynamic types, extensible syntax, macros, and garbage collection. On a practical side, Factor has a full-featured library, supports many different platforms, and has been extensively documented.

The implementation is fully compiled for performance, while still supporting interactive development. Factor applications are portable between all common platforms. Factor can deploy stand-alone applications on all platforms. Full source code for the Factor project is available under a BSD license.

Getting Started

Building Factor from source

If you have a build environment set up, then you can build Factor from git. These scripts will attempt to compile the Factor binary and bootstrap from a boot image stored on

To check out Factor:

  • git clone git://
  • cd factor

To build the latest complete Factor system from git, either use the build script:

  • Unix: ./ update
  • Windows: build.cmd
  • M1 macOS: arch -x86_64 ./ update

or download the correct boot image for your system from, put it in the factor directory and run:

  • Unix: make and then ./factor -i=boot.unix-x86.64.image
  • Windows: nmake /f Nmakefile x86-64 and then

Now you should have a complete Factor system ready to run.

Factor does not yet work on arm64 cpus. There is an arm64 assembler in cpu.arm.assembler and we are working on a port and also looking for contributors.

More information on building factor and system requirements.

To run a Factor binary:

You can download a Factor binary from the grid on The nightly builds are usually a better experience than the point releases.

  • Windows: Double-click factor.exe, or run .\ in a command prompt
  • Mac OS X: Double-click or run open in a Terminal
  • Unix: Run ./factor in a shell

Learning Factor

A tutorial is available that can be accessed from the Factor environment:

"first-program" help

Some other simple things you can try in the listener:

"Hello, world" print

{ 4 8 15 16 23 42 } [ 2 * ] map .

1000 [1..b] sum .

4 <iota> [
    "Happy Birthday " write
    2 = "dear NAME" "to You" ? print
] each

For more tips, see Learning Factor.


The Factor environment includes extensive reference documentation and a short "cookbook" to help you get started. The best way to read the documentation is in the UI; press F1 in the UI listener to open the help browser tool. You can also browse the documentation online.

Command Line Usage

Factor supports a number of command line switches:

Usage: factor [Factor arguments] [script] [script arguments]

Common arguments:
    -help            print this message and exit
    -i=<image>       load Factor image file <image> (default factor.image)
    -run=<vocab>     run the MAIN: entry point of <vocab>
        -run=listener    run terminal listener    run Factor development UI
    -e=<code>        evaluate <code>
    -ea=<code>       evaluate <code> with auto-use
    -no-user-init    suppress loading of .factor-rc
    -roots=<paths>   a list of path-delimited extra vocab roots

    "command-line" help
from within Factor for more information.

You can also write scripts that can be run from the terminal, by putting #!/path/to/factor at the top of your scripts and making them executable.

Source Organization

The Factor source tree is organized as follows:

  • vm/ - Factor VM source code (not present in binary packages)
  • core/ - Factor core library
  • basis/ - Factor basis library, compiler, tools
  • extra/ - more libraries and applications
  • misc/ - editor modes, icons, etc
  • unmaintained/ - now at factor-unmaintained

Source History

During Factor's lifetime, sourcecode has lived in many repositories. Unfortunately, the first import in Git did not keep history. History has been partially recreated from what could be salvaged. Due to the nature of Git, it's only possible to add history without disturbing upstream work, by using replace objects. These need to be manually fetched, or need to be explicitly added to your git remote configuration.

Use: git fetch origin 'refs/replace/*:refs/replace/*'

or add the following line to your configuration file

[remote "origin"]
    url = ...
    fetch = +refs/heads/*:refs/remotes/origin/*
    fetch = +refs/replace/*:refs/replace/*

Then subsequent fetches will automatically update any replace objects.


Factor developers used to meet in the #concatenative channel on Drop by if you want to discuss anything related to Factor or language design in general.

Have fun!