The tiny discrete-time Elerea

In order to not feel overwhelmed, we'll begin with one of the simplest formulations of FRP, Elerea. Elerea is a very minimalist implementation, which restricts itself to discrete-time semantics and sampling. There are no events and everything is computed on demand only. Furthermore, the API consists of high-level constructs so that it's exceedingly difficult to shoot yourself in the foot with this library.

Time-varying values of type a are represented as Signal a in Elerea:

data Signal a
-- instances incl. Monad, Eq

Signals can be thought of as functions Nat → a, though obviously they are represented differently.

Signals are made inside signal generators, SignalGen a:

data SignalGen a
-- instances incl. Monad, MonadFix, MonadIO

Signal generators have a MonadFix instance, which will later allow us to build mutually recursive signals.

The minimal API provides just a few signal building blocks. Excluding some trivial extensions, the core combinators...