Download Haplo from the GitHub repository: https://github.com/haplo-org/haplo
The Haplo plugin API documentation describes how to write a Haplo plugin.
For the initial open source release, we’ve created a temporary build system which allows you to run the Haplo platform in development mode.
We recommend you try Haplo in a VM running 64 bit Ubuntu Server with at least 4GB of memory: Build and run Haplo on Ubuntu Server
We’ve also ported Haplo to Mac OS X. It’s not an ideal platform, so the instructions can be a bit fiddly: Build and run Haplo on Mac OS X
Here’s how to write a Haplo plugin using a development server built with the instructions above.