Proving the equivalence of higher-order terms by means of supercompilation

Abstract. One of the applications of supercompilation is proving properties of programs. We focus in this paper on a specific task: proving term equivalence for a higher-order lazy functional language. The “classical” way to prove equivalence of two terms t1 and t2 is to write an equality function equals and to simplify the term (equals t1 t2). However, this works only when certain conditions are met. The paper presents another approach to proving term equivalence by means of supercompilation. In this approach we supercompile both terms and compare supercompiled terms syntactically. Some applications of the technique are discussed. In particular, one of these applications may lead to the development of a more powerful “higher-level ” supercompiler.

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We consider the safety fragment of linear temporal logic with the freeze quantifier. The freeze quantifier is used to store a value from an infinite domain in a register for later comparison with other such values. We show that, for one register, satisfiability, refinement and model checking problems are decidable. The main result in the paper is that satisfiability is EXPSPACE-complete. The proof of EXPSPACE-membership involves a translation to a new class of faulty counter automata. We also show that refinement and model checking are not primitive recursive, and that dropping the safety restriction, adding past-time temporal operators, or adding one more register, each cause undecidability of all three decision problems