Compiler and runtime support for continuation marks

Abstract

Continuation marks enable dynamic binding and context inspection in a language with proper handling of tail calls and first-class, multi-prompt, delimited continuations. The simplest and most direct use of continuation marks is to implement dynamically scoped variables, such as the current output stream or the current exception handler. Other uses include stack inspection for debugging or security checks, serialization of an in-progress computation, and run-time elision of redundant checks. By exposing continuation marks to users of a programming language, more kinds of language extensions can be implemented as libraries without further changes to the compiler. At the same time, the compiler and runtime system must provide an efficient implementation of continuation marks to ensure that library-implemented language extensions are as effective as changing the compiler. Our implementation of continuation marks for Chez Scheme (in support of Racket) makes dynamic binding and lookup constant-time and fast, preserves the performance of Chez Scheme's first-class continuations, and imposes negligible overhead on program fragments that do not use first-class continuations or marks.