Keycursieve is an experiment in building a flexible keystroke library for keyboard firmware and other usages. The goal is to create a powerful, compositional framework to allow complicated use-cases to be created quickly.
As a contributor to QMK, one of the difficulties I found was replicating a lot of the same functionality for every feature I created such as one-shot and mod-tap. In addition, custom functions were incompatible with sophisticated features like chording and leaders.
Keycursieve aims to make creating custom functionality easier by composing complicated functionality and allowing for reprocessing of modified keystrokes in a new context. It's name is a portmanteau of key, recursive, and sieve.
To see how Keycursieve works it helps to look at an example, mod-tap. The concept behind mod-tap is that if you hold down a key it performs one action such as <Ctrl> and a different action if you tap it, such as <Esc>.
In QMK mod-tap is limited to modifiers and standard key codes. If you want to add mod-tap functionality to your new action you have to implement it yourself.
How this might be implemented in Keycursieve is to have a handler for the key press. When pressed down, the handler sets a timeout--the delay before the mod goes active--and yields back to the main loop. If the timeout occurs, the handler will be invoked and then returns a data structure to set the modifier and resume processing of the pipeline. On key-up the handler will check to see if the timeout occurred, and if so unset the modifier. If not, then the handler will ask the system to restart the pipeline with a new key code.
The yield, resume, and restart semantics are the core concept here and represent the bulk of the experiment.