The Henshin transformation meta-model defines the concepts used for the specification of model transformations in Henshin.
Model transformations are defined in modules that comprise a set of units, being either transformation rules - atomic units - or composite units that allow to define a control flow.
Basic building blocks: Rules
Dangling condition: A common beginner-level problem is that rules are deemed unexecutable by the Henshin interpreter despite looking correct at the surface. This behavior is often connected to the dangling condition: A rule execution may not leave behind dangling edges, being edges with a missing source or target. It is possible to turn the gluing condition check off using the checkDangling Boolean flag.
Injective matching: The injectiveMatching flag specifies whether the match-finder is allowed to assign two or more LHS nodes to the same model element. Per default, matching is set injective (e.g., it is not).
Advanced concepts: Rule refinement
Application conditions are graph patterns which restrict the LHS of a given rule during application. They may require the presence of additional elements or relationships not included in the LHS. In this case, they are referred to as positive application conditions (PACs). They may also forbid the presence of elements or relationships in the LHS. In this case, they are referred to as negative application conditions (NACs). Application conditions can be arbitarily nested using the propositional operators Or, And, Xor, and Not. Whether an application condition graph represents a NAC or a PAC is determined by its containment in a Not condition.
Rule-nesting is a powerful concept providing a for-each operator for rules. In nested rules, the outer rule is referred to as kernel rule and an inner rule as multi rule. During execution of a nested rule, the kernel rule is matched once and used as a starting point to apply the multi rule as often as possible. Multi-node mappings allow to specify identity between kernel and multi rule nodes.
Control flow: Units
In Henshin, control flow is specified using units. Units have a fixed number of sub-units, allowing for arbitrary nesting. The following unit types are available:
Sequential units have an arbitrary number of sub-units that are executed in the given order. The Boolean flag strict determines the behavior if one of the sub-units cannot be executed: In strict mode, the execution stops, otherwise, the next sub-unit is executed. The Boolean flag rollback determines whether in the case of a stopped execution previous executions are reverted.
Priority Units have an arbitrary number of sub-units that are checked in the given order for executability. One sub-unit - the first one found to be executable - is executed.
Independent Units have an arbitrary number of sub-units that are checked in nondeterministic order for executability. One sub-unit - the first one found to be executable - is executed.
Loop Units have one sub-unit. The sub-unit is executed as often as it is executable.
Iterated Units have one sub-unit. The sub-unit is executed as often as specified in the iterations property.
ConditionalUnits have either two or three sub-units: if, then, (and else). If a match for the if unit can be found, the then unit is executed. Otherwise, if present, the else unit is executed. Note that any changes specified in the if unit are ignored; it is used for matching only.
Execution-time variability: Parametrization
Parameters allow to shape the behavior of units and rules with variable information that is typically not present before execution time. Parameters are defined as part of the rule and can then be used in elements of the rule, e.g., as attributes and in attribute conditions. Parameters can be set from the environment using the API or the graphical interpreter wizard. Parameters values set from the environment are used during matching. In turn, parameters that were not are set by the matchfinder to the values found in the respective objects. This can be used to propagate values between LHS and RHS elements.
Parameter mappings: In order to pass parameter values from outer to inner units, it is required to define parameter mappings. A parameter mapping assigns a source parameter to a target parameter.