A New Sharing Mechanism for the TIM

Abstract

In this paper we introduce a new method of implementing lazy evaluation efficiently in the TIM. A major advantage of the TIM [4] over other abstract machines is that it can take advantage of the results of sharing analysis. In this paper, we demonstrate that despite this advantage the sharing mechanism of the original TIM suffers from two serious flaws: expensive shared partial applications (in both space and time); and redundant updates. We devise a more general notation for expressing sharing than that introduced in [4]. This allows us to demonstrate that the sole reason for the defects in the TIM sharing mechanism is its treatment of sharing as a local property of arguments. Consequently we propose a new scheme where sharing is treated as a global property of expressions. This eliminates the problem of redundant updates and opens the way for several improvements including: cheap shared partial applications (in both space and time); and better synergy between the caller and the callee.