main.ts

mix({
  foo: {
    bacon: 'stuff'
  },
  baz: {
    bacon: 123,
    lettuce: 'what'
  }
}, {
  bacon: ({foo: string, baz: number}): void => {},
  // bacon: ({foo: string, baz: string}): void => {} doesn't work because baz's bacon was given as a number
  lettuce: ({foo: undefined, baz: string}): void => {}
  // lettuce: ({foo: string, baz: string}): void => {} doesn't work as foo's lettuce is not there
})