Typescript: Pattern-matching for tagged unions (match only by discriminator tag)

tagged-unions-pattern-matching.ts

export type Matcher<KindField extends string, Kind extends string, Obj, Result> = {
    [K in Kind]: (obj: Extract<Obj, { [F in KindField]: K }>) => Result;
};

export function match<KindField extends string>(field: KindField) {
    return function<
        Obj extends { [K in KindField]: Kind },
        Result,
        Kind extends string = Obj[KindField]
    >(obj: Obj, matcher: Matcher<KindField, Kind, Obj, Result>): Result {
        const fn = matcher[obj[field]];
        return fn(obj as Parameters<typeof fn>[0]);
    };
}

// Example

const matchByKind = match("kind");

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

const shape = { kind: "square", side: 2 } as Shape;
const surface = matchByKind(shape, {
    square: square => square.side ** 2,
    circle: circle => Math.PI * circle.radius ** 2,
});

console.log(surface.toFixed());