Janet is a functional and imperative programming language and bytecode interpreter. It is a modern lisp, but lists are replaced by other data structures with better utility and performance (arrays, tables, structs, tuples). The language also supports bridging to native code written in C, meta-programming with macros, and bytecode assembly.

There is a repl for trying out the language, as well as the ability to run script files. This client program is separate from the core runtime, so janet could be embedded into other programs. Try janet in your browser at https://janet-lang.org.

Use Cases

Janet makes a good system scripting language, or a language to embed in other programs. Think Lua or Guile.

Features

• Minimal setup - one binary and you are good to go!
• First class closures
• Garbage collection
• First class green threads (continuations)
• Python style generators (implemented as a plain macro)
• Mutable and immutable arrays (array/tuple)
• Mutable and immutable hashtables (table/struct)
• Mutable and immutable strings (buffer/string)
• Lisp Macros
• Byte code interpreter with an assembly interface, as well as bytecode verification
• Tailcall Optimization
• Direct interop with C via abstract types and C functions
• Dynamically load C libraries
• Functional and imperative standard library
• Lexical scoping
• Imperative programming as well as functional
• REPL
• Parsing Expression Grammars built in to the core library
• 300+ functions and macros in the core library
• Embedding Janet in other programs
• Interactive environment with detailed stack traces

Documentation

• For a quick tutorial, see the introduction for more details.
• For the full API for all functions in the core library, see the core API doc

Documentation is also available locally in the repl. Use the (doc symbol-name) macro to get API documentation for symbols in the core library. For example,

(doc doc)

Shows documentation for the doc macro.

To get a list of all bindings in the default environment, use the (all-bindings) function.

Source

You can get the source on GitHub or SourceHut. While the GitHub repo is the official repo, the SourceHut mirror is actively maintained.

Building

macos and Unix-like

The Makefile is non-portable and requires GNU-flavored make.

cd somewhere/my/projects/janet
make
make test
make repl

32-bit Haiku

32-bit Haiku build instructions are the same as the unix-like build instructions, but you need to specify an alternative compiler, such as gcc-x86.

cd somewhere/my/projects/janet
make CC=gcc-x86
make test
make repl

FreeBSD

FreeBSD build instructions are the same as the unix-like build instuctions, but you need gmake to compile. Alternatively, install directly from packages, using pkg install lang/janet.

cd somewhere/my/projects/janet
gmake
gmake test
gmake repl

Windows

1. Install Visual Studio or Visual Studio Build Tools
2. Run a Visual Studio Command Prompt (cl.exe and link.exe need to be on the PATH) and cd to the directory with janet.
3. Run build_win to compile janet.
4. Run build_win test to make sure everything is working.

Emscripten

To build janet for the web via Emscripten, make sure you have emcc installed and on your path. On a linux or macOS system, use make emscripten to build janet.js and janet.wasm - both are needed to run janet in a browser or in node. The JavaScript build is what runs the repl on the main website, but really serves mainly as a proof of concept. Janet will run slower in a browser. Building with emscripten on windows is currently unsupported.

Meson

Janet also has a build file for Meson, a cross platform build system. Although Meson has a python dependency, Meson is a very complete build system that is maybe more convenient and flexible for integrating into existing pipelines. Meson also provides much better IDE integration than Make or batch files, as well as support for cross compilation.

For the impatient, building with Meson is as simple as follows. The options provided to meson setup below emulate Janet's Makefile.

git clone https://github.com/janet-lang/janet.git
cd janet
meson setup build \
          --buildtype release \
          --optimization 2 \
          -Dgit_hash=$(git log --pretty=format:'%h' -n 1)
ninja -C build

# Run the binary
build/janet

# Installation
ninja -C build install

Development

Janet can be hacked on with pretty much any environment you like, but for IDE lovers, Gnome Builder is probably the best option, as it has excellent meson integration. It also offers code completion for Janet's C API right out of the box, which is very useful for exploring.

Installation

See the Introduction for more details. If you just want to try out the language, you don't need to install anything. You can also simply move the janet executable wherever you want on your system and run it.

Usage

A repl is launched when the binary is invoked with no arguments. Pass the -h flag to display the usage information. Individual scripts can be run with ./janet myscript.janet

If you are looking to explore, you can print a list of all available macros, functions, and constants by entering the command (all-bindings) into the repl.

$ ./janet
Janet 0.0.0 alpha  Copyright (C) 2017-2018 Calvin Rose
janet:1:> (+ 1 2 3)
6
janet:2:> (print "Hello, World!")
Hello, World!
nil
janet:3:> (os/exit)
$ ./janet -h
usage: ./janet [options] scripts...
Options are:
  -h Show this help
  -v Print the version string
  -s Use raw stdin instead of getline like functionality
  -e Execute a string of janet
  -r Enter the repl after running all scripts
  -p Keep on executing if there is a top level error (persistent)
  -- Stop handling option
$

If installed, you can also run man janet to get usage information.

Embedding

The C API for Janet is not yet documented but coming soon.

Janet can be embedded in a host program very easily. There is a make target make amalg which creates the file build/janet.c, which is a single C file that contains all the source to Janet. This file, along with src/include/janet.h and src/include/janetconf.h can dragged into any C project and compiled into the project. Janet should be compiled with -std=c99 on most compilers, and will need to be linked to the math library, -lm, and the dynamic linker, -ldl, if one wants to be able to load dynamic modules. If there is no need for dynamic modules, add the define -DJANET_NO_DYNAMIC_MODULES to the compiler options.

Examples

See the examples directory for some example janet code.

Discussion

Feel free to ask questions and join discussion on the Janet Gitter Channel. Alternatively, check out the #janet channel on Freenode

FAQ

Why is my terminal is spitting out junk when I run the repl?

Make sure your terminal supports ANSI escape codes. Most modern terminals will support these, but some older terminals, windows consoles, or embedded terminals will not. If your terminal does not support ANSI escape codes, run the repl with the -n flag, which disables color output. You can also try the -s if further issues ensue.

Why Janet

Janet is named after the almost omniscient and friendly artificial being in The Good Place.