Handling union of unions by default

[jgoux]

Hello,
at the moment, the default implementation of Union.dispatch doesn't delegate the dispatching if one of its type is of kind "union".
I propose to include this case in the implementation, so we can use union of unions :

Union.dispatch = function (x) {
    for (var i = 0, len = this.meta.types.length; i < len; i++ ) {
      if (types[i].meta.kind === "union") {
        var type = types[i].dispatch(x);
        if (type) {
          return type;
        }
      } else if (is(x, types[i])) {
        return types[i];
      }
    }
  };

With this implementation, something like this would work by default :

const TodoListAggregate.Command = union([AddTodo, EditTodo])
TodoListAggregate.Command.dispatch(customDispatch) // dispatch to AddTodo or EditTodo based on custom logic
const Command = union([TodoListAggregate.Command, AnotherAggregate.Command]) // union of unions :D
Command(fooBar) // assuming fooBar is of type AddTodo, it will delegate the dispatching to TodoListAggregate.Command.dispatch method

[gcanti]

Hi @jgoux, good idea! I'd just add a check to be sure it's really a union (theorically it could also be a class constructor):

function isStruct(x) {
  ...
}

function isMaybe(x) {
  ...
}

function isUnion(x) {
  return isType(x) && (x.meta.kind === 'union');
}

...

Union.dispatch = function (x) { // default dispatch implementation
  for (var i = 0, len = types.length; i < len; i++ ) {
    var type = types[i];
    if (isUnion(type)) { // dispatch recursively
      return type.dispatch(x);
    }
    if (is(x, type)) {
      return type;
    }
  }
};

Do you want to submit a PR with this change and some tests? I'd love to see you listed as a contributor

[jgoux]

I'll try to push something tomorrow. 👍