type UserId = string
type Name = string

type User = {
  id: string,
  name: string,
  active: boolean
}

error UserMissing = { id: string }
error StoreUnavailable = { message: string }

service Users {
  find(id: UserId): Option<User> fails StoreUnavailable
  save(user: User): void fails StoreUnavailable
  all(): User[] fails StoreUnavailable
}

const getUser = (id: UserId) =>
  match Users.find(id) {
    Some(user) => user,
    None => fail(UserMissing({ id }))
  }

const activate = (user: User) =>
  ({ ...user, active: true })

export const saveActive = (id: UserId): User
  fails UserMissing | StoreUnavailable
  uses Users.find | Users.save => {
  const user = getUser(id)
  if (user.active) { user } else {
    const active = activate(user)
    Users.save(active)
    active
  }
}

type Label = string

export const activationLabel = (id: UserId): Label
  fails StoreUnavailable
  uses Users.find | Users.save =>
  try {
    saveActive(id)
    "active"
  } catch {
    UserMissing(error) => "missing: " + error.id
  }