---
name: form-machine
description: Add a form or wizard as an Elm-style state machine (Model/Msg/pure reduce) — the one idiom for anything with validation or submission, one step or many. Use instead of hand-rolled mutable fields + ad-hoc error signals.
---

# Form machine (Model / Msg / reduce)

If you're about to add a second boolean to track state, stop — model a discriminated
union. Fields exist only in the states that need them, so illegal states are
unrepresentable.

## Skeleton

`<context>/domain/<name>.machine.ts` (pure TS, no Angular imports):

```ts
import { Result, assertNever } from '@shared/kernel/fp';

export interface Draft {
  postcode: string;
  uren: string;
} // raw strings as typed
export type StepErrors = Partial<Record<keyof Draft, string>>;
export interface Valid {
  postcode: Postcode;
  uren: Uren;
} // branded, proven valid

export type State =
  | { tag: 'Editing'; step: 1 | 2 | 3; draft: Draft; errors: StepErrors }
  | { tag: 'Submitting'; data: Valid }
  | { tag: 'Submitted'; data: Valid }
  | { tag: 'Failed'; data: Valid; error: string };

export type Msg =
  | { tag: 'SetField'; key: keyof Draft; value: string }
  | { tag: 'Next' }
  | { tag: 'Back' }
  | { tag: 'Submit' }
  | { tag: 'Retry' }
  | { tag: 'SubmitConfirmed' }
  | { tag: 'SubmitFailed'; error: string }
  | { tag: 'Seed'; state: State }; // mount any state (stories, resume)

export const initial: State = { tag: 'Editing', step: 1, draft: emptyDraft, errors: {} };

export function reduce(s: State, m: Msg): State {
  switch (m.tag) {
    /* … pure transitions only … */
    default:
      return assertNever(m); // exhaustiveness enforced
  }
}

export function validate(draft: Draft): Result<StepErrors, Valid> {
  /* calls value-object parsers */
}
```

Rules:

- **Reducer stays pure.** HTTP lives in a command that dispatches `SubmitConfirmed` / `SubmitFailed` (see **mutation-command** skill).
- **Derive, don't store**: anything computable from answers is a pure function (`visibleSteps(answers)`), never a stored field.
- Server-owned thresholds arrive as config values; keep only an offline fallback constant (see `SCHOLING_THRESHOLD_DEFAULT` in the intake machine).
- Co-located `.machine.spec.ts` is **required**: drive `reduce` with messages, assert states. No TestBed.

## Wiring in the UI

The organism holds `createStore(initial, reduce)` (`@shared/application/store`) as a
field initializer, derives view state via `computed` + `whenTag(state, 'Editing')`,
and renders into `<app-wizard-shell>` (`shared/layout/wizard-shell`) — status, steps,
errors, and primary/back/retry outputs map 1:1 onto the machine.

## Worked examples

- `src/app/herregistratie/domain/herregistratie.machine.ts` — canonical multi-step + submit lifecycle.
- `src/app/herregistratie/domain/intake.machine.ts` — progressive disclosure, derive-don't-store, server-owned threshold.
- `src/app/herregistratie/ui/` — the wizard organism + page composition.

## Verify

```bash
npm test && npm run lint && npm run build
```