Respect your parents

Attribute grammars describe how to decorate static trees. Rewriting systems describe how to transform trees into new trees. Attribution is undermined by rewriting because a node may appear in both the source and product of a transformation. If an attribute of that node depends on the node’s context, then a previously computed value may not be valid. We explore this problem and formalise it as a question of ancestry: the context of a node is given by the tree’s parent relationships and we must use the appropriate parents to calculate attributes that depend on the context. We show how respecting parents naturally leads to a view of context-dependent attributes as tree-indexed attribute families. Viewed in this way, attribution co-exists easily with rewriting transformations. We demonstrate the practicality of our approach by describing our implementation in the Kiama language processing library.