Structural recursion for querying ordered graphs
Proceedings of the 18th ACM SIGPLAN international conference on Functional programming
A parameterized graph transformation calculus for finite graphs with monadic branches
Proceedings of the 15th Symposium on Principles and Practice of Declarative Programming
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XML query languages need to provide some mechanism to inspect and manipulate nodes at all levels of an input tree. We investigate the expressive power provided in this regard by structural recursion. In particular, we show that the combination of vertical recursion down a tree combined with horizontal recursion across a list of trees gives rise to a robust class of transformations: it captures the class of all primitive recursive queries. Since queries are expected to be computable in at most polynomial time for all practical purposes, we next identify a restriction of structural recursion that captures the polynomial time queries. We also give corresponding results for list-based complex objects.