Janet 1.3.1 Documentation
Most default installs of janet come with a build tool,
jpm, that makes
managing dependencies, building Janet code, and distributing Janet applications
jpm is not required by Janet, it is a very handy build tool
that removes much of the boilerplate for creating new Janet projects, and is
updated in step with Janet.
jpm also works on Windows, linux, and macos
so you don't need to write multiple build scripts and worry about platform
quirks to make your scripts, binaries, and applications cross-platform.
jpm's main functions are installing dependencies and building native janet
modules, but it is meant to be used for much of the life-cycle for janet
projects. Since Janet code doesn't usually need to
be compiled, you don't always need
jpm, especially for scripts, but
comes with some functionality that is difficult to duplicate, like compiling janet source code
and all imported modules into a statically linked executable for distribution.
A self contained unit of Janet source code as recognized by
called a project. A project is a directory containing a
file, which contains build recipes. Often, a project will
correspond to a single git repository, and contain a single library. However, a
project.janet file can contain build recipes for as many
libraries, native extensions, and executables as it wants. Each of these
recipes builds an artifact. Artifacts are the output files that will either be
distributed or installed on the end user or developer's machine.
Building Projects with
Once you have the project to your machine, building the various artifacts should be pretty simple.
sudo jpm deps jpm build jpm test jpm install
(On windows, sudo is not required. Use of sudo on posix systems depends if you installed janet to a directory owned by the root user.)
jpm deps is a command that installs Janet libraries that this software depends on recursively.
It will automatically fetch, build, and install all required dependencies for you.
As of August 2019, this only works with git, which you need to have installed on your
machine to install dependencies. If you don't have git you are free to manually
obtain the requisite dependencies and install them manually with
sudo jpm install.
Next, we use the
jpm build command to build artifacts. All built
artifacts will be created in the
build subdirectory of the current
project. Therefor, it is probably a good idea to exclude the build directory
from source control. For building executables and native modules, you will need
to have a c compiler on your PATH where you ran
jpm build. For posix
systems, the compiler is
If you make changes to the source code after building once,
try to only rebuild what is needed on a rebuild. If this fails for any reason, you
can delete the entire build directory with
jpm clean to reset things.
For windows, the C compiler used by
cl.exe , which is
part of MSVC. You can get it with Visual Studio, or standalone with the C and C++
Build Tools from Microsoft. You will then need to run
jpm build in
a Developer Command Prompt, or source
vcvars64.bat in your shell to add
cl.exe to the PATH.
Once we have built our software, it is a good idea to test it to verify that it works
on the current machine.
jpm test will run all test scripts in the
directory of the project and return a non-zero exit code if any fail.
Finally, once we have built our software and tested that it works, we can
install it on our system. For an executable, this means copying it to the
bin directory, and for libraries it means copying them to the global
syspath. You can optionally install into any directory if you don't wan to
pollute your system or you don't have permission to write the directory
where janet itself was installed. You can specify the path to install modules
to via the
--modpath option, and the path to install binaries to with
--binpath option. These need to given before the subcommand
To create your own software in Janet, it is a good idea to understand what
project.janet file is and how it defines rules for building, testing, and
installing software. The code in
project.janet is normal Janet source code that is run in a special environment.
Functions and macros from the
cook library have been imported, making then
easier and more natural to use. For example,
cook/declare-project can be referenced
project.janet file is loaded by
jpm and evaluated to create
various recipes, or rules. For example,
several rules, including
"test". These are a few of the rules that
to create when executed.
Declaring a Project
declare-project as the first
towards the beginning of your
project.janet file. You can also
pass in any metadata about your project that you want, and add
dependencies on other Janet projects here.
(declare-project :name "mylib" # required :description "a library that does things" # some example metadata. # Optional urls to git repositories that contain required artifacts. :dependencies ["https://github.com/janet-lang.json.git"])
Creating a Module
A 100% janet library is the easiest kind of software to distribute in Janet.
Since it does not need to be built, and installing it is simply moving
the files to a system directory, we only need to specify the files that
comprise the library in
(declare-source # :source is an array or tuple that can contain # source files and directories that will be installed. # Often will just be a single file or single directory. :source ["mylib.janet"])
For information on writing modules, see the modules docpage.
Creating a Native Module
Once you have written you C code that defines your native module
(see the embedding page on how to do this), you
must declare it in
project.janet in order for
jpm to build
the native modules for you.
(declare-native :name "mynative" :source ["mynative.c" "mysupport.c"] :embedded ["extra-functions.janet"])
jpm create a native module called
mynative when build
is run, and the arguments should be pretty straight forward. The
:embedded argument is janet source code that will be embedded as an array
of bytes directly into the C source code. It is not recommended to use the
:embedded argument, as one can simply create multiple artifacts, one for
a pure C native module and one for Janet source code.
Creating an Executable
The declaration for an executable file is pretty simple.
(declare-executable :name "myexec" :entry "main.janet")
jpm is smart enough to figure out from the one entry file what libraries
and other code your executable depends on, and bundles them into the final application
for you. The final executable will be located at
Also note that the entry of an executable file should look different than a
normal janet script. It should define a
main function that can receive
a variable number of parameters, the command-line arguments. It will be called
as the entry to your executable.
(import mylib1) (import mylib2) # This will be printed when you run `jpm build` (print "build time!") (defn main [& args] # You can also get command-line arguments through (dyn :args) (print "args: " ;(interpolate ", " args)) (mylib1/do-thing) (mylib2/do-thing))
It's important to remember that code at the top level will run when you invoke
jpm build, not at executable runtime. This is because in order to create
the executable, we marshal the
main function of the app and write it
to an image. In order to create the main function, we need to actually compile
and run everything that it references, in the above case
This has a number of benefits, but the largest is that we only include bytecode
for the functions that our application uses. If we only use one function from
a library of 1000 functions, our final executable will not include the bytecode
for the other 999 functions (unless our one function references some of those
other functions, of course). This feature, called tree-shaking, only works
for Janet code. Native modules will be linked to the final executable statically
in full if they are used at all. A native module is considered "used" if is imported
at any time during
jpm build. This may change, but it is currently the most reliable
way to check if a native modules needs to be linked into the final executable.
There are some limitations to this approach. Any dynamically required modules will not
always be linked into the final executable. If
import is not called
jpm build, then the code will not be linked into the executable. The module
can still be required if it is available at runtime, though.
For an example Janet executable built with
jpm, see https://github.com/bakpakin/littleserver.