判別可能ユニオンによる網羅的なエラー処理

export type Payment =
  | { kind: "Card"; last4: string }
  | { kind: "Cash" }
  | { kind: "Wire"; iban: string }

export function describe(p: Payment): string {
  switch (p.kind) {
    case "Card": return `Card ••••${p.last4}`
    case "Cash": return "Cash"
    case "Wire": return `Wire ${p.iban}`
    default: {
      const _exhaustive: never = p
      return _exhaustive
    }
  }
}

export function exhaustiveCheck(x: never): never {
  throw new Error(`Unhandled case: ${JSON.stringify(x)}`)
}

export function pay(p: Payment): string {
  switch (p.kind) {
    case "Card": return "charged card"
    case "Cash": return "took cash"
    case "Wire": return "initiated wire"
    default: return exhaustiveCheck(p)
  }
}

// errors.ts
export type AppError =
  | { tag: "Auth"; reason: "Unauthorized" | "Forbidden" }
  | { tag: "NotFound"; resource: string }
  | { tag: "Conflict"; resource: string }

// handler.ts
import type { AppError } from "./errors"
import { exhaustiveCheck } from "./exhaustive"

export function toStatus(e: AppError): number {
  switch (e.tag) {
    case "Auth": return e.reason === "Unauthorized" ? 401 : 403
    case "NotFound": return 404
    case "Conflict": return 409
    default: return exhaustiveCheck(e)
  }
}