Apa iku discriminated union lan kenapa kuwi kuat?

Question

Accepted Answer

A **discriminated (tagged) union** adalah union saka tipe-tipe obyek sing kabeh padha nduweni field literal sing umum — **discriminant** — sing compiler gunakake kanggo bisa bedakake varian-varian lan narrow kanthi aman.

```ts
type Result =
  | { status: "loading" }
  | { status: "success"; data: string }
  | { status: "error"; message: string };
```

Saben anggota nduweni `status` literal. Ngecek iku narrow menyang tepat siji varian, mbukak field-field saka varian kuwu:

```ts
function render(r: Result) {
  switch (r.status) {
    case "loading": return "...";
    case "success": return r.data;     // ✅ data exists only here
    case "error":   return r.message;  // ✅ message exists only here
  }
}
```

Yen sampeyan nyoba ngakses `r.data` ing kasus `loading`, iku compile error — sistem tipe ndadekake kombinasi sing ora valid iku *ora bisa digambarake*.

## Exhaustiveness checking

```ts
function render2(r: Result): string {
  switch (r.status) {
    case "loading": return "...";
    case "success": return r.data;
    case "error":   return r.message;
    default:
      const _exhaustive: never = r; // ✅ if you add a variant and forget a case, this errors
      return _exhaustive;
  }
}
```

Assignment `never` ngekang sampeyan kanggo nangani saben kasus — nambah status anyar lan compiler nunjukne sampeyan ing saben switch sing perlu dianyari.

## Apa Bedane Penting

Discriminated union iku cara idiomatik kanggo model state (loading/success/error), events/actions (Redux reducers), lan saben "siji saka pirang-pirang bentuk" data.

Kuwi ndadekake state sing ora sah iku mustahil lan, karo trick `never`, menehi sampeyan compile-time exhaustiveness — kemenangan safety sing gedhe ketimbang model `boolean`/optional-field sing longgar.