我有2个使用相同鉴别器字段+值的区别联合类型。我正在尝试编写一个可以根据鉴别器将1映射到另一个的函数。
e.g。
输入类型:
type InA = {
type: 'a',
data: string
};
type InB = {
type: 'b',
data: number
};
type In = InA | InB;
输出类型:
type OutA = {
type: 'a',
data: Object
};
type OutB = {
type: 'b',
data: Array<number>
};
type Out = OutA | OutB;
制图功能
// This is the function I'd like to have a better type signature
// for inferring output type based on input type
function map<In, Out>(
in: In
): Out {
// do something
}
用法
// I want the compiler to infer that this is OutB based on the InB
let result = map({ type: 'b', value: 999 });
有没有办法为地图编写功能签名,以便这样做?
在收到答案后更新 我能够使用@ titian-cernicova-dragomir答案的修改版本。这是关于我如何使用它的一般想法+一些额外的背景:
/** Http Request types **/
type RequestA = {
type: 'names',
url: '/names'
};
type RequestB = {
type: 'numbers',
url: '/numbers'
};
type Request = RequestA | RequestB;
/** Response types **/
type ResponseA = {
type: 'names',
data: Array<string>
};
type ResponseB = {
type: 'numbers',
data: Array<number>
};
type Response = ResponseA | ResponseB;
/** Helper from accepted answer */
type GetOut<T, A> = T extends { type: A } ? T : never;
/** Generic function for fetching data */
export function fetchData<
Req extends Request,
Res extends GetOut<Response, Req['type']>
>(request: Req): Promise<Res> {
return fetch(request.url)
.then(response => response.json())
.then(data => {
return <Res>{
type: request.type,
data
}
});
}
// compiler knows that this is of type Promise<ResponseA> based on
// type discriminiator
let names = fetchData({
type: 'names',
url: '/names'
});
// compiler knows that this is of type Promise<ResponseB> based on
// type discriminiator
let numbers = fetchData({
type: 'numbers',
url: '/numbers'
});