  Hybrid programs combine digital control with differential equations, and
naturally appear in a wide range of application domains, from biology and
control theory to real-time software engineering. The entanglement of discrete
and continuous behaviour inherent to such programs goes beyond the established
computer science foundations, producing challenges related to e.g. infinite
iteration and combination of hybrid behaviour with other effects. A systematic
treatment of hybridness as a dedicated computational effect has emerged
recently. In particular, a generic idealized functional language HybCore with a
sound and adequate operational semantics has been proposed. The latter
semantics however did not provide hints to implementing HybCore as a runnable
language, suitable for hybrid system simulation (e.g. the semantics features
rules with uncountably many premises). We introduce an imperative counterpart
of HybCore, whose semantics is simpler and runnable, and yet intimately related
with the semantics of HybCore at the level of hybrid monads. We then establish
a corresponding soundness and adequacy theorem. To attest that the resulting
semantics can serve as a firm basis for the implementation of typical tools of
programming oriented to the hybrid domain, we present a web-based prototype
implementation to evaluate and inspect hybrid programs, in the spirit of GHCi
for Haskell and UTop for OCaml. The major asset of our implementation is that
it formally follows the operational semantic rules.
