setter-getter.ts

function f<A>(k:keyof A) {
  return {
    setValue: (a:A, v:A[typeof k]) => Object.assign({}, a, {[k]: v}),
    getValue: (a:A) => a[k]
  };
}

interface B {
  s: string;
  n: number;
}

const bs = f<B>('s');
const bn = f<B>('n');