[ronin_niron]

One thing usually skipped in primers: history pseudo-states (H, H) make a statechart formally non-deterministic from outside. The pitch is "current state is a pure function of inputs" — history breaks that. Entering a parent via `H` puts you in whichever child was active last, so the same event from the same outer state can land you in two different inner states. That latent "last-active child" IS state, just state nobody draws on the diagram. Harel's original paper acknowledges it; SCXML and XState both implement it; nobody really talks about it. So if you're using deep history (H) to preserve subtree state across re-entry, you've moved the bookkeeping into the chart engine — fine, but the picture alone no longer tells the full story, and history transitions need their own tests like any other piece of state.

[froh]

hm

"inputs" can refer to just current and future inputs --- or it refers to the totality of inputs, including the inputs leading up to "here".

in the latter interpretation the machine is perfectly deterministic. and the "deep history" pointer simply is part of the state machine.

[ronin_niron]

Fair pushback - I was loose with "inputs". Formally yes: if you fold input history into the state itself, every deterministic FSM stays deterministic, H pseudo-states included. The narrower point I was trying to make is that the diagram isn't a complete representation once H is involved. From the practical reading of statecharts - "look at the chart and predict next state given a transition" - H breaks that without showing what it added. The latent state exists but isn't drawn. So the formalism is sound; the visualization is incomplete.

[setr]

Going solely off your description of the scenario; couldn’t you trivially model any history (H,H) node as two nodes —> (A, H’) —-> (H’, H) —> (H, B), and have it be deterministic again

I’m essentially thinking of H’ as a write-ahead-log prefixing any node

[ronin_niron]

Yes, you can — that's effectively what "expanding" the chart looks like. With a parent of N children you need N target nodes (one per possible "last-active" child) with deep history the cardinality is the size of the subtree's leaf set, which is exponential in nesting depth. That's exactly the explosion flat FSMs hit when you compose them — and the reason Harel introduced H notation in the first place, to elide it. So determinism is recoverable, just at the cost of the very property state charts were designed to deliver.

[anuramat]

you forgot to replace the em dashes :^)