principle-model-the-domain

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# Model the Domain

Read [the workflow contract](../../references/workflow-contract.md).

## Foundations first

Get data structures and ownership right before logic: trace dominant reads,
writes and transitions, then choose types matching those access patterns. DRY
the shared model, not every similar statement. Before sharing mutable state,
ask what another actor can change concurrently; isolate independent state first.
Lay useful test/type infrastructure before dependent features, but only after
subtracting dead complexity. Each increment should deepen a coherent abstraction,
not scatter special-case coordination across callers.

Encode the real domain in a data structure instead of scattering it across conditionals. A structure that matches the domain makes invalid states unrepresentable and deletes branches. Choosing it at write time is cheap. Recovering it later reads as a refactor and gets deferred.

Reach for:

- A state machine instead of scattered booleans, phases, or lifecycle checks.
- A typed model instead of loose parameters or repeated shape assumptions.
- A map, registry, lookup table, or discriminated union instead of branching spread across files.
- A reducer or command/event model instead of ad hoc state mutations.
- A module organized around one body of domain knowledge instead of a sequence such as load, validate, transform, and save. Execution order is not ownership.
- A small module boundary that gathers repeated behavior, ownership, or invariants.
- A queue, cache, index, graph, tree, or normalized collection where the access pattern calls for it.
- Any other structure that fits. When none fits, work out what the code must never allow and how the data gets read, then find the structure that encodes exactly that.

Do not force an abstraction. Prefer boring code if the current shape is already clear, local, and unlikely to grow. Be skeptical of an abstraction that adds indirection without removing branches, duplicated rules, invalid states, or lifecycle risk.

The sign that you skipped this is a new feature that grows an existing if/else chain by one more branch, or a second boolean that must stay in sync with the first. Temporal decomposition is another sign. Phase-named modules repeat the same domain rules across steps.

From matthewyjiang/rho-pstack, MIT.

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