Motoko Hello World with permission check

HelloWorld.mo

import Hash "mo:base/Hash";
import Principal "mo:base/Principal";

import Text "mo:base/Text";
import Debug "mo:base/Debug";

import Error "mo:base/Error";

// Motoko Playground:
// https://m7sm4-2iaaa-aaaab-qabra-cai.raw.ic0.app/

// Documentation:
// Role based: https://sdk.dfinity.org/docs/developers-guide/tutorials/access-control.html
// Base type: https://sdk.dfinity.org/docs/base-libraries/principal

actor HelloWorld {

  type Id = Hash.Hash;

  // Stable to maintain value when canister is upgraded.
  // Only compatible with primitive type. Otherwise need hooks to copy and restore values.
  stable var helloWorld: Text = "";

  public shared({ caller }) func set(phrase : Text): async() {
    await require_permission( caller );

    helloWorld := phrase;
  };

  private func require_permission(user: Principal) : async () {
      // getPrincipalText() value
      if (user == Principal.fromText("user-principal")) {
          throw Error.reject("test");
      };
  };

  public query func getHash(): async Text {
      let textHash = Text.hash(helloWorld);

      // Does not output in playground. https://github.com/dfinity/motoko-playground/issues/61
      Debug.print(debug_show(textHash));

      return helloWorld;
  };

  public query({ caller }) func getPrincipalText(): async Text {
      return Principal.toText(caller);
  };

  public query({ caller }) func getPrincipalId(): async Id {
      return getUserId(caller);
  };

  private func getUserId(user: Principal) : Id {
      return Principal.hash(user);
  };

};