# Functional programming, The Elm Architecture & atomic design — a guide

A **progressive learning guide** for developers who are strong programmers but have
done little or no **functional programming (FP)** in the frontend. It teaches three
ideas and shows they are one idea:

1. **FP for the frontend** — pure functions, immutability, types that can't lie.
2. **The Elm Architecture (TEA)** — one state, one direction, one pure update.
3. **Atomic design** — small pure components composed into bigger ones.

The claim of Part 5 is that **TEA and atomic design are the same principle at two
scales**, and this app already lives that way.

**How to read it.** A junior can read top-to-bottom and arrive at "I can add a
feature." A senior can skim Parts 1–4 and jump to **Part 5** (FP × atomic design),
**Part 6** (why it reduces complexity), and **Part 7** (the recipes). Every term is
defined in plain words on first use and again in the **glossary** (Part 8).

This guide is the _teaching_ layer. For the reference deep-dives it points to
[`ARCHITECTURE.md`](./ARCHITECTURE.md) and
[ADR-0001](./architecture/0001-bff-lite-decision-dtos.md) rather than repeating them.
Every code snippet below is real code from this repo, with its file path.

---

## Part 1 — Why this exists (the complexity problem)

Most UI bugs are not algorithmic. They come from **state that can lie**:

- Two booleans that disagree — `isLoading` is `true` _and_ `data` is set.
- A screen that shows an error _and_ a success at the same time.
- A "Submit" that fires while a field is still invalid.
- A wizard whose "next step" field drifts out of sync with the answers.
- The 3am question: _"who changed this value, and when?"_

The root cause is the same each time: state is **scattered** across many mutable
variables, and it changes from **many places**. The number of states explodes, and
most of them are nonsense the compiler still lets you write.

The promise of this architecture, in one line:

> **One state. One direction. Pure logic. Predictable everything.**

Keep all the state in a single value; change it only by sending a message to one pure
function; let the view be a function of that state; push side effects (HTTP, timers)
to the edges. The illegal states stop being reachable, the update logic becomes a unit
test with no mocks, and onboarding becomes "learn one small pattern, apply it
everywhere." The rest of this guide builds that up from first principles.

---

## Part 2 — FP fundamentals for the frontend

FP here is not category theory. It is four habits that make state predictable. Each is
shown in **Elm** (a tiny, canonical functional UI language — our teaching device) and
then in **this app's real TypeScript**.

### 2a. Pure functions

A **pure function**'s output depends _only_ on its inputs, and it changes nothing else
— no network, no writing to outside variables, no clock. Same input → same output,
every time. That is what makes it trivially testable (no mocks) and easy to reason
about.

```elm
-- Elm: pure by default — there is no way to do I/O inside this
add : Int -> Int -> Int
add a b = a + b
```

In this app, the parsers and reducers are pure. For example
(`src/app/registratie/domain/value-objects/uren.ts`):

```ts
export function parseUren(raw: string): Result<string, Uren> {
  const t = raw.trim();
  const n = Number(t);
  if (t === '' || !Number.isInteger(n) || n < 0) {
    return err('Vul een geheel aantal in (0 of meer).');
  }
  return ok(n as Uren);
}
```

Give it `"4160"`, you always get the same `ok(4160)`. No surprises. **Why it helps:**
its unit test is one line per case and never flakes.

### 2b. Immutability

Never mutate a value in place; produce a **new** value instead. The spread `{ ...s, x }`
copies the old fields and overrides one.

```elm
-- Elm: { model | count = model.count + 1 } makes a NEW record
```

This app's reducers always return a fresh object — e.g.
`src/app/herregistratie/domain/herregistratie.machine.ts`:

```ts
export function setField(s: WizardState, key: keyof Draft, value: string): WizardState {
  if (s.tag !== 'Editing') return s;
  return { ...s, draft: { ...s.draft, [key]: value } };
}
```

**Why it helps:** because old states are never overwritten, back-navigation and
"resume where you left off" are free (the previous value still exists), and Angular's
change detection can tell something changed by identity. Time-travel/replay is possible
_because_ nothing is destroyed.

### 2c. Unidirectional data flow

Data flows **down**; events flow **up**; there is exactly one loop. A view never reaches
sideways to mutate another component's state — it emits an event, which becomes a
message, which goes through the one update function, which produces the next state,
which flows back down. Part 3 makes this loop concrete.

### 2d. Modelling state with types (make illegal states unrepresentable)

Two kinds of type do most of the work:

- **Product type** — a record that holds several things _at once_ (`interface Draft { uren; jaren; punten }`).
- **Sum type / discriminated union** — a value that is _exactly one of_ several
  labelled shapes, where a `tag` says which, and **each shape carries only the data that
  makes sense for it**.

The decisive move is choosing types so that **illegal states can't be written down**.
Compare three booleans (2³ = 8 combinations, most nonsense) with one union of the 4 real
states — see [`ARCHITECTURE.md` §2a](./ARCHITECTURE.md#2a-remotedata--one-value-instead-of-three-booleans)
for the full `RemoteData` treatment and diagram. The wizard's own Model is the same
idea (`herregistratie.machine.ts`):

```ts
export type WizardState =
  | { tag: 'Editing'; step: 1 | 2; draft: Draft; errors: Partial<Record<keyof Draft, string>> }
  | { tag: 'Submitting'; data: Valid }
  | { tag: 'Submitted'; data: Valid }
  | { tag: 'Failed'; data: Valid; error: string };
```

Because `step` and `errors` exist **only** on `Editing`, and `Submitting`/`Submitted`
carry already-validated `Valid` data and _no_ error field, "submitting while a field is
invalid" or "success screen with errors still set" cannot be constructed. The bug class
is gone at compile time.

> **FP term — sum type / discriminated union:** one value that is one-of-several
> labelled shapes. The `tag` discriminates; the compiler then knows which fields exist.

### 2e. Side effects at the edges (functional core, imperative shell)

A **side effect** is anything beyond computing a return value: HTTP, timers,
`localStorage`, focus. Pure code can't do them. So we keep a **pure core** (parsers,
reducers, `visibleSteps`) and push every effect to a thin **imperative shell** (the
Angular component/service). The core decides _what the state is_; the shell _goes and
does things_, then feeds the result back in as a message. Part 4d shows exactly how.

> **FP term — pure core / imperative shell:** all decisions in pure functions; all I/O
> in a thin outer layer that calls them.

---

## Part 3 — The Elm Architecture (TEA)

TEA is four pieces and one loop. In Elm:

- **Model** — the single source of truth (all your state, one value).
- **Msg** — every thing that can happen, as a union.
- **`update : Msg -> Model -> Model`** — the _only_ place state changes; pure.
- **`view : Model -> Html Msg`** — a pure function of the state that emits messages.

```elm
type alias Model = { count : Int }

type Msg = Increment | Decrement

update : Msg -> Model -> Model
update msg model =
    case msg of
        Increment -> { model | count = model.count + 1 }
        Decrement -> { model | count = model.count - 1 }

view : Model -> Html Msg
view model =
    div []
        [ button [ onClick Decrement ] [ text "-" ]
        , text (String.fromInt model.count)
        , button [ onClick Increment ] [ text "+" ]
        ]
```

The runtime wires it into a loop:

```mermaid
graph LR
    S["Model (state)"] --> V["view(Model)"]
    V -->|"user event"| M["Msg"]
    M --> U["update(Msg, Model) — PURE"]
    U -->|"next Model"| S
    classDef l fill:#e5f1fb,stroke:#007bc7,color:#00567d;
    class S,V,M,U l;
```

**Effects** don't break the loop. In Elm, `update` can return a `Cmd` (a _description_
of an effect — "go do this HTTP call"); the runtime performs it and feeds the result
back in as another `Msg`. **Subscriptions** are the same for incoming events (time,
websockets). The key property survives: `update` itself stays pure — it only ever
_describes_ effects, never performs them. This app does the same with a small twist
(Part 4d): the effect lives in the component, and its outcome is dispatched as a `Msg`.

---

## Part 4 — How we do TEA in Angular with signals

This app implements TEA with Angular **signals**. There is no extra state library. One
important shape difference from textbook Elm: **state is per-wizard, not one global
Model** — each flow (`herregistratie`, `intake`, `registratie`) has its own little
store. Cross-page state that _must_ be shared lives in one root singleton
(`BigProfileStore`, see [`ARCHITECTURE.md` §2e](./ARCHITECTURE.md#2e-optimistic-update--rollback-and-shared-state-across-pages)).

### 4a. The store — TEA's runtime in ~10 lines

`src/app/shared/application/store.ts`:

```ts
export interface Store<Model, Msg> {
  /** The current state, as a read-only Angular signal. */
  readonly model: Signal<Model>;
  /** Send a message; the model becomes update(model, msg). */
  dispatch(msg: Msg): void;
}

export function createStore<Model, Msg>(
  init: Model,
  update: (model: Model, msg: Msg) => Model,
): Store<Model, Msg> {
  const model = signal(init);
  return {
    model: model.asReadonly(),
    dispatch: (msg) => model.set(update(model(), msg)),
  };
}
```

This _is_ the Elm runtime: a `signal` holds the Model, and `dispatch` is the only way
to change it — it runs the pure `update` and `set`s the new value.

> **Naming note (read the code, not the textbook):** the factory parameter is called
> `update` (the Elm word), but each feature exports its reducer as **`reduce`** and
> passes it in: `createStore(initial, reduce)`. "update" and "reduce" are the same role.

### 4b. Model + Msg + reduce

Mapping the four TEA pieces to real code, using the herregistratie wizard (the smallest
machine) as the example — `src/app/herregistratie/domain/herregistratie.machine.ts`:

- **Model** → `WizardState` (the discriminated union from §2d).
- **Msg** → `WizardMsg`, every event as one union:

```ts
export type WizardMsg =
  | { tag: 'SetField'; key: keyof Draft; value: string }
  | { tag: 'Next' }
  | { tag: 'Back' }
  | { tag: 'Submit' }
  | { tag: 'Retry' }
  | { tag: 'SubmitConfirmed' }
  | { tag: 'SubmitFailed'; error: string }
  | { tag: 'Seed'; state: WizardState }; // mount a specific state (stories/showcase)
```

- **update** → the pure `reduce(state, msg)` — no injection, no HTTP, no mutation:

```ts
export function reduce(s: WizardState, m: WizardMsg): WizardState {
  switch (m.tag) {
    case 'SetField':
      return setField(s, m.key, m.value);
    case 'Next':
      return next(s);
    case 'Back':
      return back(s);
    case 'Submit':
      return submit(s);
    case 'Retry':
      return s.tag === 'Failed' ? { tag: 'Submitting', data: s.data } : s;
    case 'SubmitConfirmed':
      return s.tag === 'Submitting' ? { tag: 'Submitted', data: s.data } : s;
    case 'SubmitFailed':
      return s.tag === 'Submitting' ? { tag: 'Failed', data: s.data, error: m.error } : s;
    case 'Seed':
      return m.state;
    default:
      return assertNever(m); // compiler error if a Msg is unhandled
  }
}
```

`assertNever` (`src/app/shared/kernel/fp.ts`) makes the switch **exhaustive**: add a new
`Msg` variant and forget to handle it, and the build fails. (`intake.machine.ts` and
`registratie-wizard.machine.ts` have larger unions, same exact shape.)

### 4c. view → template + `computed()` + `dispatch`

The container component (`src/app/herregistratie/ui/herregistratie-wizard/herregistratie-wizard.component.ts`)
creates the store and derives view values with `computed()`:

```ts
private store = createStore<WizardState, WizardMsg>(initial, reduce);
readonly state = this.store.model;        // a read-only signal of the Model
protected dispatch = this.store.dispatch; // the only way to change it

private editing = computed(() => (this.state().tag === 'Editing' ? (this.state() as Extract<WizardState, { tag: 'Editing' }>) : null));
protected step    = computed(() => this.editing()?.step ?? 1);
protected draft   = computed<Draft>(() => this.editing()?.draft ?? { uren: '', jaren: '', punten: '' });
protected errUren = computed(() => this.editing()?.errors.uren ?? '');
```

The template is a **function of the state**: it reads those `computed()` signals and
sends messages on events — it never mutates:

```html
<app-text-input
  [ngModel]="draft().uren"
  (ngModelChange)="dispatch({ tag: 'SetField', key: 'uren', value: $event })"
  ...
/>
...
<app-button (click)="dispatch({ tag: 'Back' })">Vorige</app-button>
```

That is the loop: `state → template → event → dispatch(Msg) → reduce → new state →
template`.

### 4d. Effects → a command that dispatches the outcome

`reduce` is pure, so it can't call the network. The component holds a small **command**
method. It does the impure work, then dispatches a `Msg` describing what happened — the
result re-enters through the same pure loop:

```ts
private async runIfSubmitting() {
  const s = this.state();
  if (s.tag !== 'Submitting') return;
  this.profile.beginHerregistratie();              // optimistic flag (shared store)
  const r = await submitHerregistratie(s.data);    // the actual I/O — a Result
  if (r.ok) { this.dispatch({ tag: 'SubmitConfirmed' }); this.profile.confirmHerregistratie(); }
  else      { this.dispatch({ tag: 'SubmitFailed', error: r.error }); this.profile.rollbackHerregistratie(); }
}
```

The command itself (`src/app/herregistratie/application/submit-herregistratie.ts`)
returns a `Result` — success-or-error as a value, never a thrown exception:

```ts
export async function submitHerregistratie(data: Valid): Promise<Result<string, void>> {
  await new Promise((r) => setTimeout(r, 800));
  if (data.uren === 0) return err('Aanvraag afgewezen: geen gewerkte uren geregistreerd.');
  return ok(undefined);
}
```

The split, in one line: **reducer = "what the new state is"; command = "go do the
thing, then say what happened."** Incoming effects (an arriving HTTP value, a
server-owned config) are wired with `effect()` and `untracked()` so the dispatch
doesn't loop on its own write — see the BRP prefill and policy-threshold effects in
`registratie-wizard.component.ts` / `intake-wizard.component.ts`.

### 4e. Because state is one value, you can watch it

Each wizard exposes `state` as a **read-only signal**, deliberately public so the
teaching page can highlight the live state. See it on the in-app showcase
(`src/app/showcase/concepts.page.ts`, route `/concepts`): section 4 lights up the
current `WizardState` among `Editing → Submitting → Submitted/Failed` as you drive the
form, and section 5 shows the intake steps re-deriving as you type.

> **Discrepancy with the PRD — open question.** The PRD refers to a dedicated "state
> debug view" / inspector. **No such feature exists** in the code today. What exists is
> the `/concepts` showcase (live state highlight) and the `?scenario=slow|loading|empty|error`
> interceptor (`src/app/shared/infrastructure/scenario.ts`) for exercising async states.
> A JSON state inspector _would be trivial here_ — single one-way state means you could
> render `JSON.stringify(state())` in a panel and watch every transition — precisely
> because of everything in Part 6. Treat building one as a future task, not documented
> reality.

---

## Part 5 — FP × atomic design (the unifying chapter)

The central idea of this guide:

> **FP and atomic design are the same principle at two scales — composition of pure
> pieces, with state pushed to the boundary.**

### 5a. Atoms & molecules _are_ view functions

A pure function maps inputs → output with no side effects. A **presentational
component** does exactly that: it maps **inputs → DOM**, emits events, and has **no
injected services, no internal mutable state, no effects**. Same inputs → same DOM.
That is referential transparency at the component scale.

In this codebase the form **atoms** (`text-input`, `radio-group`) are thin wrappers over
the design system. They take config via `input()` and — because they implement
Angular's `ControlValueAccessor` — emit changes through `[ngModel]` / `(ngModelChange)`.
The `form-field` **molecule** composes a label + projected control + error. The
`address-fields` **organism** (`src/app/registratie/ui/address-fields/address-fields.component.ts`)
composes three `form-field`s and emits with `output()`:

```ts
export class AddressFieldsComponent {
  value = input.required<AdresValue>(); // data in
  errors = input<AdresErrors>({}); // data in
  fieldChange = output<{ key: keyof AdresValue; value: string }>(); // events out
}
```

> **Read the code, not the slogan:** "events up" has two real forms here. Design-system
> form atoms emit via `ControlValueAccessor`/`ngModel`; higher composites use `output()`.
> Both are "data down, events up" — just different Angular mechanisms.

### 5b. Composition stays pure

Molecules compose atoms; organisms compose molecules — exactly like composing pure
functions, where the composite is still pure. `address-fields` is pure because the
`form-field` and `text-input` it's built from are pure. Each atomic level only uses the
level(s) below it (see the hierarchy diagram in
[`ARCHITECTURE.md` §1](./ARCHITECTURE.md#1-the-big-picture-three-contexts-four-layers)).

### 5c. Pages / containers are the TEA runtime (the shell)

The boundary between "pure presentational" and "stateful container" **is** the
functional-core / imperative-shell line from §2e. The container (e.g.
`herregistratie-wizard.component.ts`) is where the Model signal lives, where
`dispatch`/`reduce` run, and where effects are wired. Everything below it is pure view.

### 5d. The loop, overlaid on the atomic layers

```mermaid
graph TD
    subgraph shell["Container = TEA runtime (imperative shell)"]
        ST["Model signal + dispatch + reduce + effects"]
    end
    subgraph pure["Pure presentational (functional core)"]
        O["Organism (e.g. address-fields)"]
        M["Molecules (form-field, async)"]
        A["Atoms (text-input, radio-group, button)"]
    end
    ST -->|"state flows DOWN as inputs"| O
    O --> M --> A
    A -.->|"events flow UP (output / ngModelChange)"| O
    O -.->|"(fieldChange, click)"| ST
    ST -->|"dispatch(Msg) → reduce → new state"| ST
    classDef l fill:#e5f1fb,stroke:#007bc7,color:#00567d;
    classDef c fill:#e8f5e9,stroke:#39870c,color:#1b5e20;
    class ST l; class O,M,A c;
```

It is the identical Elm loop of Part 3 — just composed through the atomic hierarchy.

---

## Part 6 — How this reduces complexity (concrete payoffs)

Each property maps to a tangible benefit you can point at in this repo:

- **Single source of truth.** All wizard state is one `WizardState` value. To learn
  every way the screen can change, you read **one** function (`reduce`). There is no
  "who else mutates this?"

- **Pure, exhaustive `reduce`.** It's a unit test with **no mocks** — pass a state and a
  msg, assert the next state (`herregistratie.machine.spec.ts`). `assertNever` makes the
  compiler reject an unhandled `Msg`, so adding an event can't silently do nothing.

- **Illegal states won't compile.** `Submitting` carries `Valid` data and has no `errors`
  field, so "submit with errors showing" is unwritable. A whole bug class disappears
  before runtime — contrast the 8-state boolean soup in
  [`ARCHITECTURE.md` §2a](./ARCHITECTURE.md#2a-remotedata--one-value-instead-of-three-booleans).

- **Pure presentational components.** `address-fields` is tested by inputs → DOM and
  reused in two call-sites (the registratie wizard and the change-request form) with no
  hidden state to surprise you.

- **Isolated effects.** Every async path is a `submit-*` command returning a `Result`,
  invoked from one `runIf*` method. Async reasoning happens in one place, not sprinkled
  through the view.

- **Debuggability.** One value flowing one way means you can render and watch it (§4e) —
  reproduction is "set the Model to this," nothing more.

- **Onboarding.** It's one small pattern repeated everywhere. Learn it once; the recipes
  below are that pattern written down for the four common tasks.

---

## Part 7 — Recipes

Each recipe follows the existing pattern and naming, and ends with the same reminder:
**this is the same loop, again.**

### Recipe A — Add an atomic component (atom / molecule / organism)

**When:** you genuinely need a new building block (not a one-off; reuse must earn it —
see [CLAUDE.md §2](../CLAUDE.md)).

**Where:** `shared/ui/` if generic; a context's `ui/` if domain-specific. Pick the level
by composition: composes nothing → **atom**; composes atoms → **molecule**; composes
molecules into a domain block → **organism**.

**Steps:** build it **pure/presentational** — `input()`s for data/config, `output()`s
for events, `computed()` for derived display; **no inject, no state, no effects**. Theme
only with design tokens (no hardcoded hex — CI checks via `npm run check:tokens`).
Add a co-located `*.stories.ts` titled `Layer/Name`.

```ts
// shape — see src/app/registratie/ui/address-fields/address-fields.component.ts
export class AddressFieldsComponent {
  value = input.required<AdresValue>();
  errors = input<AdresErrors>({});
  fieldChange = output<{ key: keyof AdresValue; value: string }>();
}
```

**Tests:** Storybook stories + the a11y addon are the UI coverage (repo convention — no
component DOM tests; pure logic gets a `.spec.ts`, presentational components don't).

_This is the same loop, again: data down via `input()`, events up via `output()`._

### Recipe B — Add state + a state update (Elm-style)

**When:** a new thing can happen to a feature's state.

**Steps:** extend the `Model` immutably; add a `Msg` variant; handle it in the pure
`reduce` returning a **new** model (keep the union exhaustive — `assertNever` guards
you); expose derived values with `computed()`; `dispatch` the `Msg` where the event
originates. Keep effects **out** of `reduce`.

```ts
// 1. Msg variant (herregistratie.machine.ts)
| { tag: 'Reset' }
// 2. reduce arm
case 'Reset': return initial;
// 3. dispatch from the view
(click)="dispatch({ tag: 'Reset' })"
```

**Tests:** `reduce(model, msg)` → expected model. Pure, no mocks
(`herregistratie.machine.spec.ts`).

_This is the same loop, again._

### Recipe C — Add a field + a validation rule

**When:** the form needs a new input with its own rule.

**Steps:** combine A and B. Add the field to the `Draft`; add/extend a `SetField`-style
`Msg`; handle it in `reduce`. Write validation as a **pure** function returning `Result`
(model it on `parseUren` / `parsePostcode`). **Derive** the error/validity with
`computed()` — don't store what you can compute. Render with the `form-field` molecule +
a field atom, and wire validity into the step/submit gating via a `computed()`.

```ts
// pure rule (value-objects/) — Result<error, branded value>
export function parsePostcode(raw: string): Result<string, Postcode> {
  /* ... */
}
// reduce uses it; the view shows the message via <app-form-field [error]="...">
```

**Tests:** the parser's `.spec.ts` (each accept/reject case) + a `reduce` spec for the
new field.

_This is the same loop, again — the rule is just another pure function._

### Recipe D — Add a wizard step

**When:** a flow needs another step.

**Steps:** compose A–C. Model the step's state in the Model; **derive** the visible steps
rather than storing "next" — copy `visibleSteps(answers)` from
`src/app/herregistratie/domain/intake.machine.ts`:

```ts
export function visibleSteps(a: Answers): StepId[] {
  const steps: StepId[] = ['buitenland'];
  if (a.buitenlandGewerkt === 'ja') steps.push('buitenlandDetails');
  steps.push('uren');
  if (lageUren(a)) steps.push('scholing');
  steps.push('punten', 'review');
  return steps;
}
```

Build the step as a presentational component (Recipe A), composed from atoms/molecules.
Derive "can advance" from a `computed()` over the step's validity; back-navigation keeps
earlier answers for free (immutability). Effects (BRP/DUO/submit) go through `submit-*`
commands in the shell, with results dispatched as `Msg`s (Part 4d).

**Tests:** `reduce` specs for the step's messages; a story for the step component; a
`visibleSteps`/machine spec as the acceptance check (`intake.machine.spec.ts`).

_This is the same loop, again — now nested inside the wizard._

---

## Part 8 — Glossary

- **Pure function** — output depends only on inputs; no side effects. Trivial to test.
- **Immutability** — never change a value in place; produce a new one (`{ ...s, x }`).
- **Side effect** — anything beyond returning a value: HTTP, timers, `localStorage`, focus.
- **Unidirectional data flow** — data down (inputs), events up (outputs); one loop.
- **Product type** — a record holding several values at once (`interface Draft { ... }`).
- **Sum type / discriminated (tagged) union** — a value that is exactly one of several
  labelled shapes; a `tag` field says which, and each shape carries only its own data.
- **Make illegal states unrepresentable** — choose types so nonsense states can't be written.
- **`Model`** — the single value holding all of a feature's state.
- **`Msg`** — a union of every event that can happen to the state.
- **`update` / `reduce`** — the one pure function mapping `(Model, Msg) → next Model`.
  (This codebase calls the factory parameter `update`; features export it as `reduce`.)
- **`dispatch`** — send a `Msg`; the store runs `reduce` and updates the signal.
- **Command** — an impure function that does I/O and returns a `Result`, after which the
  caller dispatches a `Msg` with the outcome.
- **`Result<E, T>`** — success-or-error as a value: `{ ok: true, value }` or `{ ok: false, error }`.
- **Value object / `Brand`** — a type whose validity is guaranteed by its parser
  (e.g. `Postcode`); a `Brand<string, 'Postcode'>` is only mintable through `parsePostcode`.
- **`signal`** — Angular's reactive container for a value.
- **`computed`** — a derived signal; recomputes automatically when its inputs change.
- **Presentational (pure) component** — inputs in, events out, `computed()` for display;
  no inject, no state, no effects. A view function.
- **Container component** — holds the Model signal, runs `dispatch`/`reduce`, wires
  effects. The TEA runtime / imperative shell.
- **Functional core / imperative shell** — all decisions in pure functions; all I/O in a
  thin outer layer.
- **TEA (The Elm Architecture)** — Model / Msg / update / view + the one-directional loop.

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_See also:_ [`ARCHITECTURE.md`](./ARCHITECTURE.md) (reference deep-dive on RemoteData,
the store, parse-don't-validate, and the .NET backend seam) and
[ADR-0001](./architecture/0001-bff-lite-decision-dtos.md) (the BFF-lite + decision-DTO
decision). Live demo: `/concepts` in the running app.
