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.

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 factorcode.org.

To check out Factor:

• git clone https://github.com/factor/factor.git
• cd factor

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

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

or download the correct boot image for your system from https://downloads.factorcode.org/images/master/, put it in the factor directory and run:

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

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.64.assembler and we are working on a port and also looking for contributors.

More information on building factor and system requirements.

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

• Windows: Double-click factor.exe, or run .\factor.com in a command prompt
• macOS: Double-click Factor.app or run open Factor.app in a Terminal
• Unix: Run ./factor in a shell

The current stable version of Factor uses GTK2 and gtkglext.
On Debian 13 “Trixie” and newer Ubuntu releases (25.10 “Questing Quokka” and the 26.04 “Resolute Raccoon” development branch), the gtkglext library is no longer available in the official repositories.

Factor's GUI depends on this library, so a fresh install cannot start the GUI on these systems.

You can manually install the legacy Debian package and add symbolic links:

wget -c http://http.us.debian.org/debian/pool/main/g/gtkglext/libgtkglext1_1.2.0-11_amd64.deb
sudo apt install ./libgtkglext1_1.2.0-11_amd64.deb

sudo ln -s /usr/lib/x86_64-linux-gnu/libgtkglext-x11-1.0.so.0 \
           /usr/lib/x86_64-linux-gnu/libgtkglext-x11-1.0.so

sudo ln -s /usr/lib/x86_64-linux-gnu/libgdkglext-x11-1.0.so.0 \
           /usr/lib/x86_64-linux-gnu/libgdkglext-x11-1.0.so

This workaround has been tested in clean containers for:

• Debian 13

You can install the libgtkglext1 .deb package from a previous Ubuntu release (e.g. 25.04 “Plucky Pangolin”) and manually add symbolic links expected by Factor:

# Install .deb from Ubuntu 25.04:
wget http://archive.ubuntu.com/ubuntu/pool/universe/g/gtkglext/libgtkglext1_1.2.0-11_amd64.deb
sudo apt install ./libgtkglext1_1.2.0-11_amd64.deb

# Add missing symlinks manually:
cd /usr/lib/x86_64-linux-gnu
sudo ln -s libgtkglext-x11-1.0.so.0.0.0 libgtkglext-x11-1.0.so
sudo ln -s libgdkglext-x11-1.0.so.0.0.0 libgdkglext-x11-1.0.so

This workaround has been tested in clean containers for:

• Ubuntu 25.10
• Ubuntu 26.04 (development branch)

On Debian 12, Ubuntu 22.04, 24.04 and 25.04 libgtkglext1 is still available in the repositories and no workaround is required.

The development branch of Factor has switched from GTK2 to GTK3 for the GUI backend. If you're building or running a binary from the development branch, make sure the GTK3 development library is installed.

If this library is missing, you may see the following error when launching Factor from a terminal:

DlError: libgtk-3.so: cannot open shared object file: No such file or directory

To fix this, install the required package:

• Debian/Ubuntu:

  sudo apt install libgtk-3-dev

• Fedora:

  sudo dnf install gtk3-devel

• Arch:

  sudo pacman -S gtk3

If the library is installed but still not found, make sure it is in your LD_LIBRARY_PATH, or run sudo ldconfig.

# Example:
export LD_LIBRARY_PATH=/usr/lib/x86_64-linux-gnu:$LD_LIBRARY_PATH

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

"first-program" help

Take a look at a guided tour of Factor:

"tour" help

Some demos that are included in the distribution to show off various features:

"demos" run

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.

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=ui.tools    run Factor development UI
    -e=<code>        evaluate <code>
    -no-user-init    suppress loading of .factor-rc
    -roots=<paths>   a list of path-delimited extra vocab roots

Enter
    "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.

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

During Factor's lifetime, source code 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 are quite active in the Factor Discord server. Drop by if you want to discuss anything related to Factor or language design in general.

• Factor homepage
• Concatenative languages wiki
• Join the mailing list
• Search for "factorcode" on Gitter

Have fun!