Effects for Funargs

Abstract: Stack allocation and first-class functions don't naturally mix together. In this paper we show that a type and effect system can be the detergent that helps these features form a nice emulsion. Our interest in this problem comes from our work on the Chapel language, but this problem is also relevant to lambda expressions in C++ and blocks in Objective C. The difficulty in mixing first-class functions and stack allocation is a tension between safety, efficiency, and simplicity. To preserve safety, one must worry about functions outliving the variables they reference: the classic upward funarg problem. There are systems which regain safety but lose programmer-predictable efficiency, and ones that provide both safety and efficiency, but give up simplicity by exposing regions to the programmer. In this paper we present a simple design that combines a type and effect system, for safety, with function-local storage, for control over efficiency.