التحقق من اكتمال الحالات باستخدام never

مشكلة اكتمال الحالات

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; side: number };

// If we add "triangle" later, we want every switch to complain.

النوع never

function fail(): never {
  throw new Error("unreachable");
}
// never is assignable to nothing except never itself.

الإسناد إلى never في الحالة الافتراضية

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; side: number };

function area(s: Shape): number {
  switch (s.kind) {
    case "circle": return Math.PI * s.radius ** 2;
    case "square": return s.side ** 2;
    default:
      const _exhaustive: never = s;
      return _exhaustive;
  }
}
console.log(area({ kind: "square", side: 3 }));

ماذا يحدث عند نسيان إحدى الحالات؟

// type Shape = ... | { kind: "triangle"; base: number; height: number };
// Now in default, s is { kind: "triangle"; ... }
// const _exhaustive: never = s; // Error: triangle not assignable to never

الدالة المساعدة assertNever

function assertNever(value: never): never {
  throw new Error("Unhandled case: " + JSON.stringify(value));
}
console.log(typeof assertNever);

استخدام assertNever في switch

function assertNever(value: never): never {
  throw new Error("Unhandled: " + JSON.stringify(value));
}
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; side: number };

function area(s: Shape): number {
  switch (s.kind) {
    case "circle": return Math.PI * s.radius ** 2;
    case "square": return s.side ** 2;
    default: return assertNever(s);
  }
}
console.log(area({ kind: "circle", radius: 1 }).toFixed(2));

أمان وقت الترجمة في مقابل أمان وقت التشغيل

function assertNever(x: never): never {
  throw new Error("Unhandled: " + String(x));
}
// Compile error if a case is missing; runtime throw as a backstop.
console.log("two layers of safety");

اكتمال الحالات دون default

type Light = "red" | "yellow" | "green";
function next(l: Light): Light {
  switch (l) {
    case "red": return "green";
    case "yellow": return "red";
    case "green": return "yellow";
  }
  // No default needed; all cases return.
}
console.log(next("red"));

اكتمال الحالات باستخدام سلاسل if/else

function assertNever(x: never): never { throw new Error("bad"); }
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; side: number };

function name(s: Shape): string {
  if (s.kind === "circle") return "circle";
  else if (s.kind === "square") return "square";
  else return assertNever(s);
}
console.log(name({ kind: "square", side: 2 }));

لماذا يُعد never الأداة المناسبة؟

// Only never is assignable to never.
let x: never;
// x = "hi"; // Error
// Any concrete leftover type fails the same way.
console.log("never catches gaps");

اكتمال الحالات كشبكة أمان عند إعادة الهيكلة

function assertNever(x: never): never { throw new Error("unhandled"); }
type Status = "idle" | "busy";
function render(s: Status): string {
  switch (s) {
    case "idle": return "Idle";
    case "busy": return "Busy";
    default: return assertNever(s);
  }
}
console.log(render("idle"));

تحقق سريع: اكتمال الحالات

مراجعة: اكتمال الحالات باستخدام never

function assertNever(x: never): never { throw new Error("unhandled"); }
type Shape = { kind: "circle"; radius: number } | { kind: "square"; side: number };
function f(s: Shape) {
  switch (s.kind) {
    case "circle": return s.radius;
    case "square": return s.side;
    default: return assertNever(s);
  }
}
console.log(f({ kind: "circle", radius: 5 }));