Elerea (Eventless reactivity) is a tiny discrete time FRP implementation without the notion of event-based switching and sampling, with first-class signals (time-varying values). Reactivity is provided through various higher-order constructs that also allow the user to work with arbitrary time-varying structures containing live signals. Signals have precise and simple denotational semantics.

Stateful signals can be safely generated at any time through a monadic interface, while stateless combinators can be used in a purely applicative style. Elerea signals can be defined recursively, and external input is trivial to attach. The library comes in two major variants, one of which you need to import:

• FRP.Elerea.Simple: signals are plain discrete streams isomorphic to functions over natural numbers;
• FRP.Elerea.Param: adds a globally accessible input signal for convenience;

Elerea is a minimal library that defines only some basic primitives, and you are advised to install elerea-examples as well to get an idea how to build non-trivial systems with it. The examples are separated in order to minimise the dependencies of the core library. The dow package contains a full game built on top of the simple variant.

The basic idea of the implementation is described in the WFLP 2010 paper Efficient and Compositional Higher-Order Streams (http://sgate.emt.bme.hu/documents/patai/publications/PataiWFLP2010.pdf).

In short, the basic idea is to define completely dynamic data-flow networks through a pure combinator-style monadic interface. The network can be turned into an I/O action that samples it sequentially by the start function. Under the hood, the network is represented by mutable variables whose interconnections are encapsulated in closures, and consistency is ensured by a two-phase update process (essentially double buffering). The library keeps track of the variables through weak pointers, so all of the live variables can be updated (this is necessary to ensure referential transparency), and unused ones can be garbage collected.