type Predicate = () => boolean

function isTurkey(x: string) {
  return x === "turkey"
}

interface Dog {
  bark: "dog"
}

interface Cat {
  meow: "cat"
}

function isCat(animal): animal is Cat {
  return (animal as Cat).meow !== undefined
}

function makeSound(animal: Cat | Dog) {
  if (isCat(animal)) {
    animal.meow
  } else {
    animal.bark
  }
}