Does this differ significantly from fclabels or the upcoming OverloadedRecordFields extension? (Aside from being purely type driven, which has problems in your example if you compose a second Int into it.)

1. Yes, it’s similar to OverloadedRecordFields but doesn’t force you to use a label, and you may use Tagged to label a field if you want.

2. Yes, but again, you can use Tagged to allow same type in different disguise.

I can see a potential problem because you can't hide instances. Once you define a Has-relationship, you can't cheaply change it. That could lead to conflicts, unless you hack around it with orphaned instances in a separate module. But you say you want to solve conflicts with tagging – so it would be reasonable to incorporate the tag in the class from the start. Which brings us back to fclabels I suppose.

MarLinn