Field validation with nice type safety in TypeScript

validation.ts

/** Valid doesn't really need any metadata */
type Valid = {
  t: true
}

/** Invalid should describe all the problems */
type Invalid = {
  t: false
  errors: Array<{ text: string /** object, in case more fields are needed here */ }>
}

/** Field validator signature */
type ValidateField<T> = (input: T) => Valid | Invalid

/** implementation of string validator */
const validateTitle: ValidateField<string> = input => {
  return input
    ? { t: true }
    : {
        t: false,
        errors: [
          { text: 'Too long error, do something about it' },
          { text: 'Value should be funnier' },
          { text: 'lol asd asd asd asd asd asda sd' },
        ],
      }
}

// render example
const valid = validateTitle('foo')
if (valid.t) {
  // foo
  valid.errors.map // compiler error here!
} else {
  const items = valid.errors.map(x => x.text)
  // <ul>{items}</ul>
}