io-ts-brand-example.ts

type AgeBrand = {
  readonly Age: unique symbol
}

const Age = t.brand(
  t.number, // A codec representing the new type, here it's a number
  (n): n is t.Branded<number, AgeBrand> => n >= 0, // A type guard
  'Age' // The name of the codec
)

// Using our brand
const result = Age.decode(25) // Right-side Either, since 25 is a valid age
if (E.isRight(result)) {
  const age = result.right // age is of type Age
  console.log(`Age is ${age > 18 ? 'greater' : 'less'} than 18`)
} else {
  console.log(result.left.map(({ message }) => message).join('\n'))
}

// Or in a more functional way with fp-ts
pipe(
  20,
  Age.decode,
  E.fold(
    flow(
      A.map(({ message }) => message),
      msgs => msgs.join('\n')
    ),
    (age) => `Age is ${age > 18 ? 'greater' : 'less'} than 18`
  ),
  console.log
)