A submission workflow where each state is its own struct.

%Validating{} has a violations field. %Quoted{} has quotes and selected. %Bound{} has quote and bound_at. Each struct carries only the fields that exist in that state. A %Quoted{} can't have a violations field because the struct doesn't define one. The compiler enforces this.

This is Scott Wlaschin's "Making Illegal States Unrepresentable" pattern. It works in Crank without any special support because Machine.state is term() -- structs are valid states.

Validating ──validate──→ Quoted ──bind──→ Bound
    │                      │
    └──decline──→ Declined ←──decline──┘

State structs carry state-specific data. The data map carries cross-cutting concerns shared across all states (parameters, audit).

When a field on the current state struct changes (adding a violation to %Validating{}), the return is {:next_state, %Validating{updated}, data}. The state value changed, so it's a state transition. :keep_state is reserved for changes to data only.

Every event is handled in every state (total function).

Type annotations

The @type state union below lists every valid state struct. Today these annotations serve as documentation and Dialyzer input. When Elixir's set-theoretic type system (introduced in v1.17) can check them, unhandled state variants will produce compiler warnings without any code changes.