Aster
Stratego -- Strategies for Program Transformation
Aster is an attribute grammar system based on Stratego. It makes use of the standard Stratego facilities such as pattern matching and concrete syntax to specify attribute equations and associate them with nodes of an abstract syntax tree. To abstract from these basic equations, users can specify attribute decorators that provide an abstract evaluation mechanism for attributes. Inspired by Stratego's rewriting strategies, these can be used to provide separation of concerns between the basic equations and high-level global search or traversal patterns to be taken on the tree for their evaluation. For example, these can be used to specify automatic copy rules, such as a downward copy rule that copies an attribute value from the parent node if it is not defined at the current node, and recursively higher up in the tree.
Attribute decorators can be used to describe different various high-level attribution patterns, and are not limited to basic copy rule specifications. For example, these include providing support for collection attributes, declaration lookup attributes, and attributes that require fix-point evaluation of circular dependencies (e.g., for data-flow equations). These patterns are often implemented as extensions of the basic attribute grammar paradigm in other systems, "baked into" different attribute evaluator systems. Rather than supporting these in the attribute evaluation system itself, Aster allows to provide these as a library of decorators, and lets users specify their own, often domain-specific decorators by hand.