Behavioural Circuit Design in Lava

The Haskell library Lava is great for describing the structure of digital circuits: large, regular circuits can be captured by short, clear descriptions. However, structural circuit description alone – and hence Lava – has a somewhat limited application domain. Many circuits are more appropriately expressed in terms of their desired behaviour. In this article, I present a Haskell library – called Recipe – that sits on top of Lava and provides a set of behavioural programming constructs, including mutable variables, sequential and parallel composition, iteration and choice. Mutable state is provided in the form of rewrite rules – normal Lava functions from state to state – giving the library a powerful blend of the structural and behavioural styles. The approach taken here is also applicable to software-based embedded systems programming. Indeed, I have developed a simple C backend for Lava, and my final example – a program that controls a brick-sorter robot – runs on a Lego Mindstorms RCX microcontoller.