Online-and-Offline Partial Evaluation: A Mixed Approach

This paper presents a hybrid method of partial evaluation (PE), which combines the power of online PE and the efficiency of offline PE, for a typed strict functional language. We begin with a naive online partial evaluator, and make it efficient without sacrificing its power. To this end, we (1) use state (instead of continuation) for let-insertion, (2) take a so-called cogen approach, and (3) decrease unnecessary computations-such as unnecessary let-insertions and unused values/expressions-with a type-based use analysis, which subsumes various monovariant binding-time analyses. Our method yields the same residual programs as the naive online partial evaluator, modulo inlining of redundant letbindings. We implemented and compared our method and existing methods, both online and offline. Experiments show that our method is at least twice as fast as any other method (e.g., more than 7 times as fast as Thiemannʹs cogen approach to offline PE in the specialization of the power function, thanks to the reduction of unnecessary let-insertions) when they yield equivalent residual programs.