Menene haɗin gida (discriminated union) kuma me ya sa ya kasance mai iko?

Question

Accepted Answer

**Haɗin gida (da alamar) makamantanci** shine haɗi na nau'ikan aiki waɗanda duk suna raba wani yanki na harafin hadde-hadde — **alamar sarrafawa** — wanda compiler yake amfani da shi don bambanta nau'ikan aiki da dakatar da tsayawa cikin amsacin.

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

Kwana daya da daya yana da `status` harafin. Duba shi ya dakatar zuwa daidai nau'i ɗaya, bayyanar waɗannan nau'ikan ginshiki:

```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
  }
}
```

Ko kuwa ka yi ƙoƙari don samun `r.data` a cikin yanayin `loading`, shi ne kuskuren tsarawa — tsarin nau'i ya sa jigon marasa inganci *ba tare da wakilci ba*.

## Binciken ƙarfinoma

```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;
  }
}
```

Allocation na `never` ya tilasta ka gudnar da kowane yanayi — saka sabon status kuma compiler ya nuna maka kowane juyawa da suke buƙatar sabuntawa.

## Me ya sa ya zama mahimmanci

Haɗin gida makamantanci shine hanya ta gidimsawa don tsara jiha (karɓa/nasara/kuskure), abubuwan da suka faru/ayyuka (Redux reducers), da kuma duk "ɗaya daga cikin siffofin da yawa" bayanai.

Suna sa jihohin da ba su yarda ba ba zai yiwu ba kuma, tare da whaƙar `never`, suna ba ka ƙarfinoma na lokaci na tsarawa — ci gaba mai girma a kan tsare waɗanda ba su da tsari `boolean`/filin da aka zaɓa.