How do I express this in Typescript?

Question

Let\'s say I have an interface A:

interface A {
  foo: number
  bar: string
}

And I have a generic type Option:

Answer 1

Good news: With TypeScript 2.1.0, this is now possible via Mapped Types:

type Option = { map() => T };
type OptionsHash = { [K in keyof T]: Option };

function optionsFor(structure: T): OptionsHash { ... };

let input = { foo: 5, bar: 'X' };
let output = optionsFor(input);
// output is now typed as { foo: { map: () => number }, bar: { map: () => string } }

The opposite is also possible:

function retreiveOptions(hash: OptionsHash): T { ... };

let optionsHash = {
    foo: { map() { return 5; } },
    bar: { map() { return 'x'; } }
};
let optionsObject = retreiveOptions(optionsHash);
// optionsObject is now typed as { foo: number, bar: string }