Jou programming language

Jou is a programming language that looks like Python but behaves like C. For example:

import "stdlib/io.jou"

def main() -> int:
    printf("Hello World\n")

    # Print numbers 0 to 9
    for i = 0; i < 10; i++:
        printf("%d\n", i)

    return 0

Jou has classes. They can have methods, but that's about it: otherwise Jou classes are just like structs in C.

import "stdlib/io.jou"

class Greeting:
    target: byte*

    def show(self) -> None:
        printf("Hello %s\n", self.target)

def main() -> int:
    g = Greeting{target="World"}
    g.show()    # Output: Hello World
    return 0

See the examples and tests directories for more example programs or read the Jou tutorial.

For now, Jou is great for writing small programs that don't have a lot of dependencies. Here are things I have written in Jou:

• Advent of Code 2023: examples/aoc2023
• Advent of Code 2024: examples/aoc2024
• Advent of Code 2025: examples/aoc2025
• A klondike solitaire card game with curses UI: https://github.com/Akuli/curses-klondike/

I would recommend Jou for:

• People who find C programming fun but like Python's syntax
• Python programmers who want to try programming at a lower level (maybe to eventually learn C or Rust)

See also AI's thoughts on Jou.

Design goals and non-goals

Jou eliminates some surprising things in C. For example:

• In C, char may or may not be signed, depending on your OS, but Jou's byte data type is always unsigned.
• In C, negative % positive is negative or zero, which means that array[i % array_len] doesn't wrap around as expected. In Jou, negative % positive is positive or zero.
• Jou doesn't have strict aliasing. This means that in Jou, memory is just bytes, and you can't get UB by interpreting the same bytes in different ways, like you can in C.
• Jou has Windows support that doesn't suck. You simply download and extract a zip, and add it to PATH. (See instructions below.)

I will try my best to keep Jou simple, and not turn it into yet another big language that doesn't feel like C, such as C++, Zig, Rust, and many others. For example, the recommended way to print things will be C's printf() function, as explained in the Jou tutorial. This also means that I reject many feature requests.

Jou is not intended to be memory safe, because it would make Jou more difficult to use. See Jou's UB documentation for more discussion, including thoughts on Rust.

Setup

These instructions are for using Jou. The instructions for developing Jou are in CONTRIBUTING.md.

Linux

1. Install the dependencies:

   $ sudo apt install git llvm-19-dev clang-19 make python3
   

   Let me know if you use a distro that doesn't have apt, and you need help with this step.
2. Download and compile Jou.

   $ git clone https://github.com/Akuli/jou
   $ cd jou
   $ make
   

3. Run the hello world program to make sure that Jou works:

   $ ./jou examples/hello.jou
   Hello World
   

   You can now run other Jou programs in the same way.
4. (Optional) If you want to run Jou programs with simply jou filename instead of something like ./jou filename or /full/path/to/jou filename, you can add the jou directory to your PATH. To do so, edit ~/.bashrc (or whatever other file you have instead, e.g. ~/.zshrc):

   $ nano ~/.bashrc
   

   Add the following line to the end:

   export PATH="$PATH:/home/yourname/jou/"
   

   Replace /home/yourname/jou/ with the path to the folder (not the executable file) where you downloaded Jou. Note that the ~ character does not work here, so you need to use a full path (or $HOME) instead.

These LLVM/clang versions are supported:

• LLVM 15 with clang 15
• LLVM 16 with clang 16
• LLVM 17 with clang 17
• LLVM 18 with clang 18
• LLVM 19 with clang 19
• LLVM 20 with clang 20

These versions are not covered by automated tests but seem to work anyway:

• LLVM 21 with clang 21

By default, the make command picks the latest available version. You can also specify the version manually by setting the LLVM_CONFIG variable:

$ sudo apt install llvm-15-dev clang-15
$ make clean    # Delete files that were compiled with previous LLVM version
$ LLVM_CONFIG=llvm-config-15 make

MacOS

1. Install Git, make and LLVM 15. If you do software development on MacOS, you probably already have Git and make, because they come with Xcode Command Line Tools. You can use brew to install LLVM:

   $ brew install llvm@15
   

   Instead of LLVM 15, you can also use any other supported LLVM version:
   • llvm@15
   • llvm@16
   • llvm@17
   • llvm@18
   • llvm@19
   • llvm@20
2. Install Python if you don't have it already. Any reasonably new version will work.
3. Download and compile Jou.

   $ git clone https://github.com/Akuli/jou
   $ cd jou
   $ make
   

4. Run the hello world program to make sure that Jou works:

   $ ./jou examples/hello.jou
   Hello World
   

   You can now run other Jou programs in the same way.
5. (Optional) If you want to run Jou programs with simply jou filename instead of something like ./jou filename or /full/path/to/jou filename, you can add the jou directory to your PATH. To do so, edit ~/.bashrc (or whatever other file you have instead, e.g. ~/.zshrc):

   $ nano ~/.bashrc
   

   Add the following line to the end:

   export PATH="$PATH:/Users/yourname/jou/"
   

   Replace /Users/yourname/jou/ with the path to the folder (not the executable file) where you downloaded Jou. Note that the ~ character does not work here, so you need to use a full path (or $HOME) instead.

NetBSD

1. Install the dependencies:

   # pkgin install bash clang git gmake
   

   This should get you also a Python package as a dependency. Any reasonably new version of Python will work with Jou. You may also want to install diffutils for coloured diff outputs.
2. Download and compile Jou.

   $ git clone https://github.com/Akuli/jou
   $ cd jou
   $ gmake
   

3. Run the hello world program to make sure that Jou works:

   $ ./jou examples/hello.jou
   Hello World
   

   You can now run other Jou programs in the same way.
4. (Optional) If you want to run Jou programs with simply jou filename instead of something like ./jou filename or /full/path/to/jou filename, you can add the jou directory to your PATH. Refer to the manual page of your login shell for exact syntax.

NB: Using Clang and LLVM libraries built as a part of the base system is not currently supported.

64-bit Windows

1. Go to releases on GitHub. It's in the sidebar at right.
2. Choose a release (latest is probably good) and download a .zip file whose name starts with jou_windows_64bit_.
3. Extract the zip file somewhere on your computer.
4. You should now have a folder that contains jou.exe, lots of .dll files, and subfolders named stdlib and mingw64. Add this folder to PATH. If you don't know how to add a folder to PATH, you can e.g. search "windows add to path" on youtube.
5. Write Jou code into a file and run jou filename.jou on a command prompt. Try the hello world program, for example.

Updating to the latest version of Jou

Run jou --update.

Bootstrapping

The Jou compiler (in compiler/ folder) is written in Jou. It can compile itself. However, this doesn't help you much if you have nothing that can compile Jou code.

To solve this problem, there are two scripts:

• bootstrap_transpiler.py is a Python script that converts Jou code to C code. It is not intended to be used for anything other than this. For example, it doesn't support all features of the Jou language, and when it doesn't support something, it usually fails with an unhelpful error message.
• bootstrap.sh takes old versions of the Jou compiler from Git history, starting with a commit that is compatible with ./bootstrap_transpiler.py. It then uses the previous Jou compiler to compile the next version of the Jou compiler until it gets a Jou compiler that supports the latest Jou syntax.

On Windows, you can also pass the --small option to ./windows_setup.sh as described in CONTRIBUTING.md. This downloads a release of Jou from GitHub and uses its jou.exe as a starting point instead of converting Jou code to C code. This is useful if you don't have Python installed.

You might be wondering why bootstrap_transpiler.py does not directly support the latest Jou version. The main reason is that it would break frequently when working on the Jou compiler, and when it breaks, it produces horribly bad error messages. This is by design: its only purpose is to compile the subset of Jou used in a specific compiler version. Instead, it is usually better to add a new commit to the end of the list in bootstrap.sh.

See CONTRIBUTING.md for some more details if you want to learn more about the Jou compiler or develop it.

Editor support

Tell your editor to syntax-highlight .jou files as if they were Python files. You may want to copy some other Python settings too, such as how to handle indentations and comments.

If your editor uses a langserver for Python, make sure it doesn't use the same langserver for Jou. For example, vscode uses the Pylance language server, and you need to disable it for .jou files; otherwise you get lots of warnings whenever you edit Jou code that would be invalid as Python code.

Here are some editor-specific instructions. If you get this to work on some other editor, consider adding instructions here :)

Porcupine (my editor)

I use the following configuration with my Porcupine editor:

[Jou]
filename_patterns = ["*.jou"]
pygments_lexer = "pygments.lexers.Python3Lexer"
syntax_highlighter = "pygments"
comment_prefix = '#'
autoindent_regexes = {dedent = 'return( .+)?|break|pass|continue', indent = '.*:'}

To apply this configuration, copy/paste it to end of Porcupine's filetypes.toml (menubar at top --> Settings --> Config Files --> Edit filetypes.toml).

Helix

Add the following to ~/.config/helix/languages.toml:

[[language]]
name = "jou"
scope = "source.python"
file-types = ["jou"]
roots = []
comment-token = "#"
indent = { tab-width = 4, unit = "    " }
language-servers = []
grammar = "python"
language-id = "python"  # needed for helix to find runtime/queries/python/*.scm files