Here is a unique solution that utilizes a factory to construct the matcher and enforce rules during the build process. This approach might be suitable for your specific situation if you are open to a rewrite.
interface IMatcherFactoryEnd<Item> {
getMatcher: () => (v: Item) => Item;
};
interface IMatcherFactory<Item, Keys, CheckedKeys extends Keys> {
when: <K extends Exclude<Keys, CheckedKeys>>(
key: K,
fn: (v: Item) => Item,
) => (Exclude<Keys, CheckedKeys | K> extends never ? IMatcherFactoryEnd<Item> : IMatcherFactory<Item, Keys, CheckedKeys | K>);
};
class CustomMatcherFactory<Item, Keys, CheckedKeys extends Keys>
implements
IMatcherFactory<Item, Keys, CheckedKeys>,
IMatcherFactoryEnd<Item>
{
constructor (
private getKeyFunc: (v: Item) => Keys,
private caseArray: [CheckedKeys, (v: Item) => Item][],
) { }
when <K extends Exclude<Keys, CheckedKeys>>(
key: K,
fn: (v: Item) => Item,
) {
return new CustomMatcherFactory<Item, Keys, CheckedKeys | K>(
this.getKeyFunc,
[...this.caseArray, [key, fn]],
);
}
getMatcher () {
const getKeyFunc = this.getKeyFunc;
const caseArray = this.caseArray;
return (v: Item) => {
const key = getKeyFunc(v);
const b = caseArray.find(([k, _]) => k === key);
if (b) return b[1](v);
throw new Error("Hit unreacheable code");
};
}
}
export function customMatcherFactory <Item, Keys> (
getKeyFunc: (v: Item) => Keys,
): IMatcherFactory<Item, Keys, never> {
return new CustomMatcherFactory<Item, Keys, never>(
getKeyFunc,
[],
);
}
const getKeyValue = (v: boolean) => v ? "true" as const : "false" as const;
export const matcher_OK = customMatcherFactory(getKeyValue)
.when("true", () => false)
.when("false", () => true)
.getMatcher();
export const matcher_KO1 = customMatcherFactory(getKeyValue)
.when("true", () => false)
.getMatcher();
export const matcher_KO2 = customMatcherFactory(getKeyValue)
.when("false", () => true)
.getMatcher();
export const matcher_KO3 = customMatcherFactory(getKeyValue)
.when("true", () => false)
.when("false", () => true)
.when("true", () => false)
.getMatcher();
export const matcher_KO4 = customMatcherFactory(getKeyValue)
.when("true", () => false)
.when("false", () => true)
.when(true, () => false)
.getMatcher();