A GongFu updater pattern

updater.js

function update1(msg, model, appContext) {
  // This approach uses effects to implement application logic to do other UI actions.
  switch (msg.type) {
    case "ButtonPressed":
      if (userCanDo(msg)) {
        return { 
          model: assocTyped("doo", doo, model),
          effect: Effect(DoSomething(msg.what))
        };
      else 
        return { model, effect: Effect(ShowAnotherCrazyModal()) };
      break;
    case "DoSomething":
        ...
      break;
    case "ShowAnotherCrazyModal":
        ...
      break;
  }
}
  
function update2(msg, model, appContext) {
  // This approach calls the update function directly with the appropriate message.
  switch (msg.type) {
    case "ButtonPressed":
      if (userCanDo(msg)) {
        model = assocTyped("doo", doo, model);
        return update2(DoSomething(msg.what), model, appContext);
      else 
        return update2(ShowAnotherCrazyModal());
      break;
    case "DoSomething":
        ...
      break;
    case "ShowAnotherCrazyModal":
        ...
      break;    
  }
}