Continuations: Multiple Invocations Considered Harmful

In this paper, we propose to replace Scheme rst class continuations by a sequential thread system. Our threads are very close to continuations but they are easier to understand and to use for programmers, and its implementation is simpler and more eecient. We show that with our threads we can recover almost completely the Scheme operator call-with-current-continuation (call/cc for short). The only functionality we dismiss is what makes this operator diicult to understand and to implement: the multiple invocations of the same continuation. We think this diierence will aaect the behavior of only a few programs written in Scheme, but it will allow a faster execution of all programs. Moreover, our threads can be naturally extended to a concurrent version of Scheme, without the known problems of continuations.