如何区分联合与泛型？

Question

我正在努力根据几何形状（在显示某些 GeoJSON 数据的上下文中）自动推断不同种类项目的类型。

我使用的是通用类型，因此我没有设法设置自定义类型保护，因为它可以让我区分“个体”项目和“聚合”，但不能区分不同类型的“个体”项目。

基本上，我需要推理的层次：

从聚合中区分单个项目
区分每个类别中的不同几何形状。

我创建了一个简化示例，在我的真实应用程序中，我有 4 种不同类型的项目，它们可能具有不同的可能几何形状。

Here is a TypeScript playground，以及下面的代码：

type A = {type: "A", a: string}
type B = {type: "B", b: string}
type C = {type: "C", c: string}
type Geometries = A | B | C

type IndividualFeature<G extends A | B = A | B> = { geometry: G, indivAttribute: string}
type AggregateFeature = { geometry: C, aggAttribute: string}

type DisplayableFeature = IndividualFeature | AggregateFeature


const display = (feature: DisplayableFeature) => {
    switch(feature.geometry.type) {
        case "A":
            console.log("type A", feature.geometry.a, feature.indivAttribute);
            return;
        case "B":
            console.log("type B", feature.geometry.b, feature.indivAttribute)
            return;
        case "C": 
            console.log("type C", feature.geometry.c, feature.aggAttribute)
        default:
        // should not happen
    }
}

const indivFeature: IndividualFeature = { geometry: { type: "A", a: "a"}, indivAttribute: "hello indiv"}
const aggFeature: AggregateFeature = { geometry: { type: "C", c: "c"}, aggAttribute: "hello agg"}

几何被正确区分，但不是单独与聚合（feature.indivAttribute/feature.aggAttribute 触发错误）。作为记录，我尝试了类型保护：这让我能够区分“Indiv”和“Aggregates”，但我失去了对几何形状的区分。

我应该如何构造我的 types/code 以便 feature.indivAttribute 在本例中被正确识别为有效属性？

Answer 1

也许只是投射功能？

switch(feature.geometry.type) {
        case "A": console.log("type A", feature.geometry.a, (feature as IndividualFeature).indivAttribute);
            return;
        case "B":
            console.log("type B", feature.geometry.b, (feature as IndividualFeature).indivAttribute)
            return;
        case "C": 
            console.log("type C", feature.geometry.c, (feature as AggregateFeature).aggAttribute)
        default:
        // should not happen
    }

Answer 2

这确实是打字稿的限制，即使没有泛型。这里存在一个 github 问题：microsoft/TypeScript#18758. There is also a PR with some recent activity: microsoft/TypeScript#38839.

目前无法基于嵌套的可区分联合来缩小联合。判别式必须处于同一“级别”。

作为解决方法，您可以像这样编写 custom type guard：

type AllTypes = DisplayableFeature["geometry"]["type"] // "A" | "B" | "C"

type FeatueOfType<T extends AllTypes> = {
    "A": IndividualFeature<A>,
    "B": IndividualFeature<B>,
    "C": AggregateFeature
}[T]

function isFeatueOfType<T extends AllTypes>(
    feature: DisplayableFeature, type: T
): feature is FeatueOfType<T> {
    return feature.geometry.type === type
}

FeatueOfType<T> 将几何类型映射到它的要素类型。例如。 FeatueOfType<"A"> 等于 IndividualFeature<A>。（这种类型可能会直接从 DisplayableFeature 生成，而不是手工编写，但这可能会变得复杂。）

当你像 isFeatueOfType(feature, "A") 这样调用类型保护并且 feature.geometry.type === type 检查成功时，我们告诉打字稿 feature 的类型必须是 FeatueOfType<"A">，即 IndividualFeature<A>.

（请注意，当类型保护中存在错误时，例如上面写的 feature.geometry.type !== type，typescript 无法捕捉到它。因此始终建议对其进行适当的测试。）

用法：

const display = (feature: DisplayableFeature) => {
    if (isFeatueOfType(feature, "A")) {
        doSometingWithA(feature) // typechecks
        console.log("type A", feature.geometry.a, feature.indivAttribute);
    }
    else if (isFeatueOfType(feature, "B")) {
        console.log("type B", feature.geometry.b, feature.indivAttribute)
    }
    else if (isFeatueOfType(feature, "C")) {
        console.log("type C", feature.geometry.c, feature.aggAttribute)
    }
    else {
        throw Error()
    }
}

function doSometingWithA(a: IndividualFeature<A>) {}

(Playground link)

如何区分联合与泛型？

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