Interweave with Map should respect existing List type

[rjgotten]

Currently, List interweaves with Map via

Map.setup = function(){
  oldSetup.apply(this, arguments);
  if (!(this.prototype instanceof List)) {
    this.List = Map.List.extend({
      Map: this
    }, {});
  }
};

— can-list.js; lines 936-943

This foregoes any custom, derived type of List that may already have been defined on a particular Map subclass. This would be easily fixed by updating the code in question to check for pre-existence of a List value beforehand:

Map.setup = function(){
  oldSetup.apply(this, arguments);
  if (!(this.prototype instanceof List)) {
    this.List = (this.List || Map.List).extend({
      Map: this
    }, {});
  }
};

Derived generic List classes are very useful when you want to add generic custom behavior to all instances of a particular Map class. E.g. I could (and infact do) have a derived ResizableList with a settable length property that allows growing/shrinking the list, transparantly adding/removing members of the list's Map type based on its default initialization.