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Here we answer some frequently asked questions about Henshin.

How can I execute transformations?

Use the Henshin Interpreter to execute transformations (available via a wizard or programatically).

My rule cannot be applied, even though I expect that it should.

The two most common problems are as follows:

Dangling edges: This is the most common problem if some of your rules work, but some rules, in particular those that delete something, do not. The problem is that a rule can become inapplicable if the deletion of a particular model element would leave behind dangling edges, that is, references pointing to a non-existing model element. This is because, per default, Henshin follows the Double-Pushout Approach to graph transformations, which requires that all deletions, including deletions of edges, are specified explicitly in the rule. To ensure validity of the transformation, the interpreter performs a check that the rule application does not leave behind dangling edges.
To make the rule applicable, you can either modify the rule to specify the problematic edges to be deleted as well (potentially using multi-rules, to establish a for-all semantics), or switch to Single-Pushout semantics (by setting the checkDangling property of the rule to false).

Metamodel schizophrenia: This problem is the most common if you cannot get any of your rules to work. The term metamodel schizophrenia refers to situations where the same metamodel is referenced differently from the Henshin module than from the input model. For instance, the Henshin module might refer to the metamodel dynamically, via a file path in the workspace, whereas the input model refers to it statically, via the package URI for the EMF-generated code. Or, you might have two different copies of the meta-model stored in your workspace, and both are referenced from your Henshin module and input model. If metamodel schizophrenia applies to your case, the metamodel will be loaded twice and considered as different entities, which makes it natural behavior that the rule cannot be applied.
To solve this issue, you need to fix either your Henshin module or your input model, so that they agree on the way in which they reference the used metamodel. See also: How can I use dynamic EMF with Henshin?.

If you have ruled out these issues, we recommend to check out the Debugging Support.

Does Henshin support automatic tracing for exogenous transformations?

Henshin follows a rewrite approach. Tracing is not build into the transformation language but can be easily realized using a generic Henshin Trace Model.

How can I use dynamic EMF with Henshin?

Define your EMF models as usual. Make sure you set the namespace URIs and prefixes for all packages. To use the packages in Henshin, right-click in the Henshin editor and select Import Package -- From Workspace and choose the packages from your Ecore files. To create an dynamic instance model, right-click on an Ecore file and select Henshin -- Create Dynamic Instance. This will create an XMI file which you can then edit in the Sample Reflective Model Editor of EMF.

If you don't want to use dynamic EMF, import the packages from the registry and use the runtime version (not the development version).

I get NullPointerExceptions when I try to execute my transformation

Make sure all node, edge and attribute types are correctly set. Also, it is important that you reference the same version of the metamodels in your Henshin files as in your instance models. For example, if you have a UML instance model created with the UML editors, you must make sure that you use the same UML metamodel in your transformation. In this case, you would have to import the UML metamodel from the registry and use the runtime version. See also the entries before and after this one.

I get exceptions during state space generation

Try setting the property collectMissingRoots=true.

How can I define a higher-order (HO) transformation in Henshin?

Import the Henshin and the Ecore metamodel into your transformation (import from the registry and use the runtime versions). Check out the examples repository on our website for more details.

Why is my rule with a String constant not matched even though the constant is correct?

The attribute in the Henshin rule must look like this: "CONSTANT" where CONSTANT is the string to be matched. Make sure you use double quotes here. In the instance model, the object's attribute must have the value CONSTANT -- in the EMF editor you must not use any quotes here!

What sort of automatic analysis is supported?

You can use the Henshin State Space Explorer to generate a state space for a transformation system, check structural invariants and do model checking.

The Critical Pair Analysis allows to uncover conflicts and dependencies in a rule set, which can be used for many purposes, including validation and testing.

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