TECHNICAL REPORT TR99-02 Type-Based Useless Variable Elimination

We show a type-based method for useless variable elimination, i.e., transformation that eliminates variables whose values contribute nothing to the nal outcome of a computation, and prove its correctness. The algorithm is a surprisingly simple extension of the usual type reconstruction algorithm. Our method seems more attractive than Wand and Siveroni's 0CFA-based method in many respects. First, it is e cient: it runs in time almost linear in the size of an input expression for a simply-typed -calculus, while the 0CFA-based method may require a cubic time. Second, our transformation can be shown to be optimal among those that preserve well-typedness, both for the simply-typed language and for an ML-style polymorphically-typed language. On the other hand, the 0CFA-based method is not optimal for the polymophically-typed language. ANY OTHER IDENTIFYING INFORMATION OF THIS REPORT Summary has been submitted for publication. Up-to-date version of this report will be available through http://www.yl.is.s.u-tokyo.ac.jp/ koba/publications.html DISTRIBUTION STATEMENT This technical report is available ONLY via anonymous FTP from ftp.is.s.u-tokyo.ac.jp (directory /pub/tech-reports). SUPPLEMENTARY NOTES REPORT DATE