Message.ts

export function addSignature(signature: string) {
  return (text: Text) => {
    const $text = MessageUnion.Text.iso.unwrap(text); // get private type
    const $newText = {
      …$text,
      text: `${$message.text} - ${signature}`,
    };
    return MessageUnion.Text.iso.wrap($newText); // get opaque type
  };
}

// Iso even offers shortcuts, the same result can be achieved like so:

export const addSignature = (signature: string) => MessageUnion.Text.iso.modify(
  ($text) => ({ ...$text, text: `${$text.text} - ${signature}` })
);