Using a Continuation Twice and Its Implications for the Expressive Power of Call/cc

. We study the implications for the expressive power of call/cc of upward continuations, specifically the idiom of using a continuation twice. Although such control effects were known to Landin and Reynolds when they invented J and escape, the forebears of call/cc, they still act as a conceptual pitfall for some attempts to reason about continuations. We use this idiom to refute some recent conjectures about equivalences in a language with continuations, but no other effects. This shows that first-class continuations as given by call/cc have greater expressive power than one would expect from goto or exits. Keywords: call/cc, continuations, upward continuations, expressiveness, program equivalence. 1. Introduction You can enter a room once, and yet leave it twice. (Peter Landin) A common informal explanation of continuations is the comparison with forward goto. This is in some sense a very apt simile: forward gotos obviously do not give rise to loops, and continuations, without some ..