Abstract

In this paper we study decidable approximations to call by need computations to normal and root-stable forms in term rewriting. We obtain uniform decidability proofs by making use of elementary tree automata techniques. Surprisingly, by avoiding complicated concepts like index and sequentiality we are able to cover much larger classes of term rewriting systems.

The theorem of Huet and Léevy stating that for orthogonal rewrite systems (i) every reducible term contains a needed redex and (ii) repeated contraction of needed redexes results in a normal form if the term under consideration has a normal form, forms the basis of all results on optimal normalizing strategies for orthogonal rewrite systems. However, needed redexes are not computable in general.

In the paper we show how the use of approximations and elementary tree automata techniques allows one to obtain decidable conditions in a simple and elegant way. Surprisingly, by avoiding complicated concepts like index and sequentiality we are able to cover much larger classes of rewrite systems.

We also study modularity aspects of the classes in our hierarchy. It turns out that none of the classes is preserved under signature extension. By imposing various conditions we recover the preservation under signature extension. By imposing some more conditions we are able to strengthen the signature extension results to modularity for disjoint and constructor-sharing combinations.

BibTeX

@article{DM-IC05,
  author      = "Ir{\`e}ne Durand and Aart Middeldorp",
  title       = "Decidable Call-by-Need Computations in Term Rewriting",
  journal     = "Information and Computation",
  volume      = 196,
  number      = 2,
  pages       = "95--126",
  year        = 2005
}