In Henshin, the metamodels are usually fixed and actually required to be known at design-time. That basically means that rules and transformations in Henshin are statically typed. To overcome this limitation, we recently came up with a new way to support dynamic types and metamodel changes. The idea is to use wrappers for instance models which allow us to connect the instance model and the metamodel levels using ordinary structural features. You should not be surprised that we defined these wrappers as yet another EMF model:
A WObject wraps an EObject, WLinks represent instances of EReferences, and WValues specific data values. The default implementation of the wrappers propagates all consistent changes to the wrappers or the metamodel down to the EObjects. Using this model in Henshin allows us to define dynamically typed transformation rules. Let's have a look at the following two example rules.
The first one is an ordinary (statically typed) rule which matches an object a of type A such that the name attribute of a is x and creates an object b of type B and a link of type ref from a to b. The second rule is semantically equivalent, but here we use wrappers. The top row matches the metamodel elements, i.e. two EClasses named "A" and "B", an EReference named "ref" and an EAttribute named "att". The lower part represents the instances of these metamodel elements (modeled using wrappers).
There are important differences in the expressive power between the two approaches: for statically typed rules, the metamodel is fixed and must be known at design-time. In the dynamic approach, the metamodel elements are matched at run-time. This allows us to dynamically create and modify instances, and even to make changes to the metamodel while there are instances of it, as done in the following rule:
This is only a small example, but I think it already shows the potential of the approach. We have described this approach and its use for defining coupled metamodel and instance model evolution in a paper accepted for ICMT 2013. The wrapper model is shipped with the current release of Henshin (0.9.6) together with two examples: one for copying arbitrary EMF models and one on coupled evolution.