Abstract. Landin’s SECD machine was the first abstract machine for applicative expressions, i.e., functional programs. Landin’s J operator was the first control operator for functional languages, and was specified by an extension of the SECD machine. We present a family of evaluation functions corresponding to this extension of the SECD machine, using a series of elementary transformations (transformation into continuation-passing style (CPS) and defunctionalization, chiefly) and their left inverses (transformation into direct style and refunctionalization). To this end, we modernize the SECD machine into a bisimilar one that operates in lockstep with the original one but that (1) does not use a data stack and (2) uses the caller-save rather than the callee-save convention for environments. We also identify that the dump component of the SECD machine is managed in a callee-save way. The caller-save counterpart of the modernized SECD machine precisely corresponds to Thielecke’s doublebarrelled continuations and to Felleisen’s encoding of J in terms of call/cc. We then variously characterize the J operator in terms of CPS and in terms of delimited-control operators in the CPS hierarchy. As a byproduct, we also present several reduction semantics for applicative expressions

The shift and reset operators, proposed by Danvy and Filinski, are powerful control primitives for capturing delimited continuations. Delimited continuation is a similar concept as the standard (unlimited) continuation, but it represents part of the rest of the computation, rather than the whole rest of computation. In the literature, the semantics of shift and reset has been given by a CPS-translation only. This paper gives a direct axiomatization of calculus with shift and reset, namely, we introduce a set of equations, and prove that it is sound and complete with respect to the CPS-translation. We also introduce a calculus with control operators which is as expressive as the calculus with shift and reset, has a sound and complete axiomatization, and is conservative over Sabry and Felleisen's theory for first-class continuations.

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Delimited continuations are more expressive than traditional abortive continuations and they apparently require a framework beyond traditional continuation-passing style (CPS). We show that this is not the case: standard CPS is sufficient to explain the common control operators for delimited continuations. We demonstrate this fact and present an implementation as a Scheme library. We then investigate a typed account of delimited continuations that makes explicit where control effects can occur. This results in a monadic framework for typed and encapsulated delimited continuations, which we design and implement as a Haskell library.

Operators for delimiting control and for capturing composable continuations litter the landscape of theoretical programming language research. Numerous papers explain their advantages, how the operators explain each other (or don’t), and other aspects of the operators’ existence. Production programming languages, however, do not support these operators, partly because their relationship to existing and demonstrably useful constructs—such as exceptions and dynamic binding—remains relatively unexplored. In this paper, In this paper, we report on our effort of translating the theory of delimited and composable control into a viable implementation for a production system. The report shows how this effort involved a substantial design element, including work with a formal model, as well as significant practical exploration and engineering. The resulting version of PLT Scheme incorporates the expressive combination of delimited and composable control alongside dynamic-wind, dynamic binding, and exception handling. None of the additional operators subvert the intended benefits of existing control operators, so that programmers can freely mix and match control operators.

Abstract. Functional and delimited continuations are more expressive than traditional abortive continuations and they apparently seem to require a framework beyond traditional continuation or monadic semantics. We show that this is not the case: standard continuation semantics is sufficient to explain directly the common control operators for delimited continuations. This implies a monadic framework for typed and encapsulated functional and delimited continuations which we design and implement as a Haskell library.

Abstract. We describe the first implementation of multi-prompt delimited control operators in OCaml that is direct in that it captures only the needed part of the control stack. The implementation is a library that requires no changes to the OCaml compiler or run-time, so it is perfectly compatible with existing OCaml source code and byte-code. The library has been in fruitful practical use for four years. We present the library as an implementation of an abstract machine derived by elaborating the definitional machine. The abstract view lets us distill a minimalistic API, scAPI, sufficient for implementing multiprompt delimited control. We argue that a language system that supports exception and stack-overflow handling supports scAPI. Our library illustrates how to use scAPI to implement multi-prompt delimited control in a typed language. The approach is general and can be used to add multi-prompt delimited control to other existing language systems. 1

Compositional reasoning about any behavioral property of a system depends, first, on the ability to express that property for both individual components and systems constructed from them. Expressiveness problems arise when considering compositional reasoning about performance in the presence of complex user-defined types (as opposed to simpler built-in types). There are interesting implications not just for compositional reasoning but for language design and for formal specification.

We describe the first implementation of multi-prompt delimited control operators in OCaml that is direct in that it captures only the needed part of the control stack. The implementation is a library that requires no changes to the OCaml compiler or run-time, so it is perfectly compatible with existing OCaml source and binary code. The library has been in fruitful practical use since 2006. We present the library as an implementation of an abstract machine derived by elaborating the definitional machine. The abstract view lets us distill a minimalistic API, scAPI, sufficient for implementing multi-prompt delimited control. We argue that a language system that supports exception and stack-overflow handling supports scAPI. With byte- and native-code OCaml systems as two examples, our library illustrates how to use scAPI to implement multi-prompt delimited control in a typed language. The approach is general and has been used to add multi-prompt delimited control to other existing language systems.