gistfile1.ts

// see this in the TypeScript Playground: https://goo.gl/pmNV6U

// foo: (x: string) => IterableIterator<Promise<Response> | Promise<string>> 
const foo = function* (x: string) {
    const page: Response = yield fetch(x); // :( can't propagate type to yield https://github.com/Microsoft/TypeScript/issues/2983
    return page.text();
};

// TYPE ERROR HERE:
// expected bar: (a: string) => Promise<Response | String>
const bar = asyncTransact(foo);

// I = input, O = output
function asyncTransact<I, O1, O2>(genFn: (a: I) => IterableIterator<Promise<O1> | Promise<O2>>): (a: I) => Promise<O1 | O2>;

// workaround: fallback to {}, so bar is: (a: string) => Promise<{}>, which isn't ideal but at least doesn't type-error
// function asyncTransact<I>(genFn: (a: I) => IterableIterator<{}>): (a: I) => Promise<{}>;

function asyncTransact<O>(genFn: (...args: {}[]) => IterableIterator<O>) {
    return async function (this: {}, ...args: {}[]) {
        // skeleton of actual code
        const gen: IterableIterator<O> = genFn.apply(this, args);
        return await gen.next().value;
    };
}