Flow 中的通用函数

发表 admin at 2024年8月26日

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Flow从面向对象程序中借鉴的最佳概念之一是泛型的概念。在许多情况下，泛型对于细化类型检查至关重要。

泛型在哪些地方有用？

假设你有一个函数memoize。使用 Flow 类型，它可能看起来像这样：

function memoize(func: (key: any) => any): (key: any) => any {
  const registry = new Map();
  return function(key: any): any {
    let value = registry.get(key);
    if (typeof value === 'undefined') {
      value = func(key);
      registry.set(key, value);
    }
    return value;
  };
}

问题是它会吞噬以下细节func：

// Type is (val: number) => boolean
function isPrime(val: number): boolean {
  // Implementation...
}
// Type is (key: any) => any
const memoizedIsPrime = memoize(isPrime);
// This gives an error, since `Sentinel` is not a number.
isPrime('Sentinel');
// This does not give an error, since 'Sentinel' is any.
memoizedIsPrime('Sentinel');

使其通用

它非常简单，只需在参数前用 V 形符号声明类型并将其用作类型即可：

// We're using `K` and `V` here for convention, but you can name them pretty much anything.
export default function memoize<K, V>(func: (key: K) => V): (key: K) => V {
  const registry = new Map();
  return function(key: K): V {
    let value = registry.get(key);
    if (typeof value === 'undefined') {
      value = func(key);
      registry.set(key, value);
    }
    return value;
  };
}

Flow 将推断其余内容：

// Type is (val: number) => boolean
function isPrime(val: number): boolean {
  // Implementation...
}
// Type is (key: K) => V.  Flow infers that `K` is number and `V` is boolean.
const memoizedIsPrime = memoize(isPrime);
// This gives an error, since `Sentinel` is not a number.
isPrime('Sentinel');
// This gives an error, since 'Sentinel' is a 'K' (number).
memoizedIsPrime('Sentinel');