Yukiyoshi Kameyama

A sound and complete axiomatization of delimited continuations

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 Felleisens theory for first-class continuations.

Polymorphic delimited continuations

This paper presents a polymorphic type system for a language with delimited control operators, shift and reset. Based on the monomorphic type system by Danvy and Filinski, the proposed type system allows pure expressions to be polymorphic. Thanks to the explicit presence of answer types, our type system satisfies various important properties, including strong type soundness, existence of principal types and an inference algorithm, and strong normalization. Relationship to CPS translation as well as extensions to impredicative polymorphism are also discussed. These technical results establish the foundation of polymorphic delimited continuations.

Shifting the stage: staging with delimited control

It is often hard to write programs that are efficient yet reusable. For example, an efficient implementation of Gaussian elimination should be specialized to the structure and known static properties of the input matrix. The most profitable optimizations, such as choosing the best pivoting or memoization, cannot be expected of even an advanced compiler because they are specific to the domain, but expressing these optimizations directly makes for ungainly source code. Instead, a promising and popular way to reconcile efficiency with reusability is for a domain expert to write code generators. Two pillars of this approach are types and effects. Typed multilevel languages such as MetaOCaml ensure _safety_: a well-typed code generator neither goes wrong nor generates code that goes wrong. Side effects such as state and control ease correctness: an effectful generator can resemble the textbook presentation of an algorithm, as is familiar to domain experts, yet insert let for memoization and if for bounds-checking, as is necessary for efficiency. However, adding effects blindly renders multilevel types unsound. We introduce the first two-level calculus with control effects and a sound type system. We give small-step operational semantics as well as a continuation-passing style (CPS) translation. For soundness, our calculus restricts the code generators effects to the scope of generated binders. Even with this restriction, we can finally write efficient code generators for dynamic programming and numerical methods in direct style, like in algorithm textbooks, rather than in CPS or monadic style.

Closing the stage: from staged code to typed closures

Code generation lets us write well-abstracted programs without performance penalty. Writing a correct code generator is easier than building a full-scale compiler but still hard. Typed multistage languages such as MetaOCaml help in two ways: they provide simple annotations to express code generation, and they assure that the generated code is well-typed and well-scoped. Unfortunately, the assurance only holds without side effects such as state and control. Without effects, generators often have to be written in a continuation-passing or monadic style that has proved inconvenient. It is thus a pressing open problem to combine effects with staging in a sound type system. This paper takes a first step towards solving the problem, by translating the staging away. Our source language models MetaOCaml restricted to one future stage. It is a call-by-value language, with a sound type system and a small-step operational semantics, that supports building open code, running closed code, cross-stage persistence, and non-termination effects. We translate each typing derivation from this source language to the unstaged System F with constants. Our translation represents future-stage code using closures, yet preserves the typing, α-equivalence (hygiene), and (we conjecture) termination and evaluation order of the staged program. To decouple evaluation from scope (a defining characteristic of staging), our translation weakens the typing environment of open code using a term coercion reminiscent of Goedels translation from intuitionistic to modal logic. By converting open code to closures with typed environments, our translation establishes a framework in which to study staging with effects and to prototype staged languages. It already makes scope extrusion a type error.

Calculi of Meta-variables

The notion of meta-variable plays a fundamental role when we define formal systems such as logical and computational calculi. Yet it has been usually understood only informally as is seen in most textbooks of logic. Based on our observations of the usages of meta-variables in textbooks, we propose two formal systems that have the notion of meta-variable.

Undecidability of Type-Checking in Domain-Free Typed Lambda-Calculi with Existence

This paper shows undecidability of type-checking and type-inference problems in domain-free typed lambda-calculi with existential types: a negation and conjunction fragment, and an implicational fragment. These are proved by reducing type-checking and type-inference problems of the domain-free polymorphic typed lambda-calculus to those of the lambda-calculi with existential types by continuation passing style translations.

Axioms for Delimited Continuations in the CPS Hierarchy

A CPS translation is a syntactic translation of programs, which is useful for describing their operational behavior. By iterating the standard call-by-value CPS translation, Danvy and Filinski discovered the CPS hierarchy and proposed a family of control operators, shift and reset, that make it possible to capture successive delimited continuations in a CPS hierarchy.

Axioms for control operators in the CPS hierarchy

Abstract A CPS translation is a syntactic translation of programs, which is useful for describing their operational behavior. By iterating the standard call-by-value CPS translation, Danvy and Filinski discovered the CPS hierarchy and proposed a family of control operators, shift and reset, that make it possible to capture successive delimited continuations in a CPS hierarchy. Although shift and reset have found their applications in several areas such as partial evaluation, most studies in the literature have been devoted to the base level of the hierarchy, namely, to level-1 shift and reset. In this article, we investigate the whole family of shift and reset. We give a simple calculus with level-n shift and level-n reset for an arbitrary n>0. We then give a set of equational axioms for them, and prove that these axioms are sound and complete with respect to the CPS translation. The resulting set of axioms is concise and a natural extension of those for level-1 shift and reset.

Typed dynamic control operators for delimited continuations

We study the dynamic control operators for delimited continuations, control and prompt. Based on recent developments on purely functional CPS translations for them, we introduce a polymorphically typed calculus for these control operators which allows answer-type modification. We show that our calculus enjoys type soundness and is compatible with the CPS translation. We also show that the typed dynamic control operators can macro-express the typed static ones (shift and reset), while the converse direction is not possible, which exhibits a sharp contrast with the type-free case.

A Type-Theoretic Study on Partial Continuations

Partial continuations are control operators in functional programming such that a function-like object is abstracted from a part of the rest of computation, rather than the whole rest of computation. Several different formulations of partial continuations have been proposed by Felleisen, Danvy&Filinski, Hieb et al, and others, but as far as we know, no one ever studied logic for partial continuations, nor proposed a typed calculus of partial continuations which corresponds to a logical system through the Curry-Howard isomorphism. This paper gives a simple type-theoretic formulation of a form of partial continuations (which we call delimited continuations), and study its properties. Our calculus does reflect the intended operational semantics, and enjoys nice properties such as subject reduction and confluence. By restricting the type of delimiters to be atomic, we obtain the normal form property. We also show a few examples.