> The Delta Cycle logic is actually quite similar to functional reactive programming. It separates how a value changes from when a process responds to that change.

This is what I use when I play with hardware simulation in Haskell:

  type S a = [a]

  register :: a -> S a -> S a
  register a0 as = a0:as

  -- combinational logic can be represented as typical pure
  -- functions and then glued into "circuits" with register's
  -- and map/zip/unzip functions.

This thing also separates externally visible events recorded in the (infinite) list of values from externally unobservable pure (combinational) logic. But, one can test combinational logic separately, with property based testing, etc.