A dynamic language and bytecode vm
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bakpakin Remove the callable? predicate.
Many times are callable now in some circumstances, so
the predicate is not that useful.
Latest commit beed839 Feb 21, 2019


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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.

Implemented in mostly standard C99, janet runs on Windows, Linux and macOS. The few features that are not standard C (dynamic library loading, compiler specific optimizations), are fairly straight forward. Janet can be easily ported to new platforms.

For syntax highlighting, there is some preliminary vim syntax highlighting in janet.vim. Generic lisp syntax highlighting should, however, provide good results. One can also generate a janet.tmLanguage file for other programs with make grammar.

Use Cases

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


  • 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 can be found in the doc directory of the repository. There is an introduction section contains a good overview of the language.

API documentation for all bindings can also be generated with make docs, which will create build/doc.html, which can be viewed with any web browser. This includes all forms in the core library except special forms.

For individual bindings from within the REPL, use the (doc symbol-name) macro to get API documentation for 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-symbols) function.


Install a stable version of janet from the releases page. Janet is prebuilt for a few systems, but if you want to develop janet, run janet on a non-x86 system, or get the latest, you must build janet from source. Janet is in alpha and may change in backwards incompatible ways.


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)
janet:2:> (print "Hello, World!")
Hello, World!
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


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/janet.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.

Compiling and Running

Janet only uses Make and batch files to compile on Posix and windows respectively. To configure janet, edit the header file src/include/janet/janet.h before compilation.

macos and Unix-like

On most platforms, use Make to build janet. The resulting binary will be in build/janet.

cd somewhere/my/projects/janet
make test

After building, run make install to install the janet binary and libs. Will install in /usr/local by default, see the Makefile to customize.

It's also recommended to set the JANET_PATH variable in your profile. This is where janet will look for imported libraries after the current directory.


FreeBSD build instructions are the same as the unix-like build instuctions, but you need gmake and gcc to compile.

cd somewhere/my/projects/janet
gmake CC=gcc
gmake test CC=gcc


  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.


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.


See the examples directory for some example janet code.


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

Why Janet

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

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