type TriState = "yes" | "no" | "unknown"
function logOutState(state: TriState) {
  if (state == "yes") {
    console.log("User selected yes")
  } else if (state == "no") {
    console.log("User selected no")
  } else {
    console.log("User has not made a selection yet")
  }
}

type Foo = {
  kind: "foo" // Literal type
  foo: number
}
type Bar = {
  kind: "bar" // Literal type
  bar: number
}
function doStuff(arg: Foo | Bar) {
  if (arg.kind === "foo") {
    console.log(arg.foo) // OK
    console.log(arg.bar) // Error!
  } else {
    // MUST BE Bar!
    console.log(arg.foo) // Error!
    console.log(arg.bar) // OK
  }
}