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 activeNames = (users: User[]) =>
  users
    |> filter((user) => user.active)
    |> map((user) => user.name)

export const listActiveNames = (): Name[]
  fails StoreUnavailable
  uses Users.all =>
  Users.all() |> activeNames