عمق التكرار وحدوده

لدى TypeScript حدود للتكرار

// Excessively deep recursive types can produce:
// "Type instantiation is excessively deep and possibly infinite."

مصدر أخطاء العمق

type Repeat<N extends number, Acc extends unknown[] = []> =
  Acc["length"] extends N ? Acc : Repeat<N, [...Acc, unknown]>;
// Large N can exceed the instantiation depth.
type Five = Repeat<5>;

تكرار محدود وآمن

type Tuple<N extends number, Acc extends unknown[] = []> =
  Acc["length"] extends N ? Acc : Tuple<N, [...Acc, unknown]>;
type Three = Tuple<3>; // [unknown, unknown, unknown]
const t: Three = [1, 2, 3];
console.log(t.length);

أنماط الأنواع ذات التكرار الذيلي

type Reverse<T extends unknown[], Acc extends unknown[] = []> =
  T extends [infer Head, ...infer Tail]
    ? Reverse<Tail, [Head, ...Acc]>
    : Acc;
type R = Reverse<[1, 2, 3]>; // [3, 2, 1]
const r: R = [3, 2, 1];
console.log(r);

لماذا تساعد المجمّعات

type Join<T extends string[], Acc extends string = ""> =
  T extends [infer H extends string, ...infer R extends string[]]
    ? Join<R, Acc extends "" ? H : Acc>
    : Acc;
type First = Join<["a", "b", "c"]>; // "a"
const f: First = "a";
console.log(f);

تجنب التكرار غير الذيلي

// Non-tail (can be costly): builds nesting around the recursive call
// type Bad<T> = T extends [infer H, ...infer R] ? [H, ...Bad<R>] : [];
// Tail-friendly alternative uses an accumulator parameter.
console.log("prefer accumulators");

تحديد حد العمق صراحةً

type Flatten<T, Depth extends unknown[] = []> =
  Depth["length"] extends 5
    ? T
    : T extends (infer U)[]
      ? Flatten<U, [...Depth, unknown]>
      : T;
type X = Flatten<number[][]>; // number
const x: X = 7;
console.log(x);

العمق العملي يكون مناسبًا عادةً

type Json = string | number | boolean | null | Json[] | { [k: string]: Json };
const v: Json = { a: { b: { c: 1 } } }; // fine, shallow
console.log(v);

التعرف على رسالة الخطأ

// Fixes: add/tighten the base case, switch to tail recursion,
// reduce input size, or cap with a depth counter.
console.log("check base case and tail position");

التكرار أثناء التشغيل منفصل

type TreeNode<T> = { value: T; children: TreeNode<T>[] };
function count<T>(n: TreeNode<T>): number {
  return 1 + n.children.reduce((a, c) => a + count(c), 0);
}
console.log(count({ value: 1, children: [{ value: 2, children: [] }] }));

التصميم ضمن الحدود

type Length<T extends unknown[]> = T["length"];
type N = Length<[1, 2, 3]>; // 3
const n: N = 3;
console.log(n);

تحقق سريع: حدود التكرار

مراجعة: عمق التكرار وحدوده

type Reverse<T extends unknown[], Acc extends unknown[] = []> =
  T extends [infer H, ...infer R] ? Reverse<R, [H, ...Acc]> : Acc;
const r: Reverse<[1, 2]> = [2, 1];
console.log(r);