# This is a pure function.
->(foo, bar) {
  div {
    h1 foo
    p bar
  }
}

# This is not a pure function.
-> {
  div {
    h1 "The time is:"
    p Time.now.to_s
  }
}

# This is not a pure function.
->(id) {
  user = DB.query(
    "select * from users where id = ?", id
  ).first

  h2 "User name: #{user[:name]}"
}

# This is not a pure function.
-> {
  LOGFILE.info("Rendering my awesome template...")
  h2 "Oops, I just caused a side effect"
}