Janet is a functional and imperative programming language and bytecode interpreter. It is a lisp-like language, but lists are replaced by other data structures (arrays, tables (hash table), struct (immutable hash table), 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 can be embedded in other programs. Try Janet in your browser at https://janet-lang.org.
If you'd like to financially support the ongoing development of Janet, consider sponsoring its primary author through GitHub.
Janet makes a good system scripting language, or a language to embed in other programs. It's like Lua and Guile in that regard. It has more built-in functionality and a richer core language than Lua, but smaller than GNU Guile or Python.
- 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)
- Byte code interpreter with an assembly interface, as well as bytecode verification
- Tail call 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
- Parsing Expression Grammars built into the core library
- 400+ functions and macros in the core library
- Embedding Janet in other programs
- Interactive environment with detailed stack traces
- 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.
(doc symbol-name) macro to get API
documentation for symbols in the core library. For example,
Shows documentation for the
To get a list of all bindings in the default
environment, use the
(all-bindings) function. You
can also use the
(doc) macro with no arguments if you are in the REPL
to show bound symbols.
macOS and Unix-like
The Makefile is non-portable and requires GNU-flavored make.
cd somewhere/my/projects/janet make make test make repl
Find out more about the available make targets by running
32-bit Haiku build instructions are the same as the UNIX-like build instructions,
but you need to specify an alternative compiler, such as
cd somewhere/my/projects/janet make CC=gcc-x86 make test make repl
FreeBSD build instructions are the same as the UNIX-like build instructions,
but you need
gmake to compile. Alternatively, install directly from
pkg install lang/janet.
cd somewhere/my/projects/janet gmake gmake test gmake repl
NetBSD build instructions are the same as the FreeBSD build instructions.
Alternatively, install directly from packages, using
pkgin install janet.
- Install Visual Studio or Visual Studio Build Tools
- Run a Visual Studio Command Prompt (cl.exe and link.exe need to be on the PATH) and cd to the directory with janet.
build_winto compile janet.
build_win testto make sure everything is working.
To build an
.msi installer executable, in addition to the above steps, you will have to:
- Install, or otherwise add to your PATH the WiX 3.11 Toolset
Now you should have an
.msi. You can run
build_win install to install the
.msi, or execute the file itself.
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 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 \ --libdir /usr/local/lib \ -Dgit_hash=$(git log --pretty=format:'%h' -n 1) ninja -C build # Run the binary build/janet # Installation ninja -C build install
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. VSCode, Vim, Emacs, and Atom will have syntax packages for the Janet language, though.
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 move the
janet executable wherever you want on your system and run it.
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
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 1.7.1-dev-951e10f Copyright (C) 2017-2020 Calvin Rose janet:1:> (+ 1 2 3) 6 janet:2:> (print "Hello, World!") Hello, World! nil janet:3:> (os/exit) $ janet -h usage: build/janet [options] script args... Options are: -h : Show this help -v : Print the version string -s : Use raw stdin instead of getline like functionality -e code : 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) -q : Hide prompt, logo, and REPL output (quiet) -k : Compile scripts but do not execute (flycheck) -m syspath : Set system path for loading global modules -c source output : Compile janet source code into an image -n : Disable ANSI color output in the REPL -l path : Execute code in a file before running the main script -- : Stop handling options
If installed, you can also run
man janet to get usage information.
Janet can be embedded in a host program very easily. The normal build
will create a file
build/janet.c, which is a single C file
that contains all the source to Janet. This file, along with
src/conf/janetconf.h can be dragged into any C
project and compiled into the project. Janet should be compiled with
on most compilers, and will need to be linked to the math library,
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.
See the Embedding Section on the website for more information.
See the examples directory for some example janet code.
Feel free to ask questions and join the discussion on the Janet Gitter Channel. Gitter provides Matrix and irc bridges as well.
Why is my terminal 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
-n flag, which disables color output. You can also try the
-s if further issues
Where is (favorite feature from other language)?
It may exist, it may not. If you want to propose major language features, go ahead and open an issue, but they will likely by closed as "will not implement". Often, such features make one usecase simpler at the expense of 5 others by making the language more complicated.
Where is the example code?
In the examples directory.
Is this a Clojure port?
No. It's similar to Clojure superficially because I like Lisps and I like the asthetics. Internally, Janet is not at all like Clojure.
Are the immutable data structures (tuples and structs) implemented as hash tries?
No. They are immutable arrays and hash tables. Don't try and use them like Clojure's vectors and maps, instead they work well as table keys or other identifiers.
Why can't we add (feature from Clojure) into the core?
Usually, one of a few reasons:
- Often, it already exists in a different form and the Clojure port would be redundant.
- Clojure programs often generate a lot of garbage and rely on the JVM to clean it up. Janet does not run on the JVM. We admittedly have a much more primitive GC.
- We want to keep the Janet core small. With Lisps, usually a feature can be added as a library without feeling "bolted on", especially when compared to ALGOL like languages.
Why is it called "Janet"?
Janet is named after the almost omniscient and friendly artificial being in The Good Place.