Continuation-passing Style Transformation Research Articles

Compiling with continuations, correctly

In this paper we present a novel simulation relation for proving correctness of program transformations that combines syntactic simulations and logical relations. In particular, we establish a new kind of simulation diagram that uses a small-step or big-step semantics in the source language and an untyped, step-indexed logical relation in the target language. Our technique provides a practical solution for proving semantics preservation for transformations that do not preserve reductions in the source language. This is common when transformations generate new binder names, and hence α-conversion must be explicitly accounted for, or when transformations introduce administrative redexes. Our technique does not require reductions in the source language to correspond directly to reductions in the target language. Instead, we enforce a weaker notion of semantic preorder, which suffices to show that semantics are preserved for both whole-program and separate compilation. Because our logical relation is transitive, we can transition between intermediate program states in a small-step fashion and hence the shape of the proof resembles that of a simple small-step simulation. We use this technique to revisit the semantic correctness of a continuation-passing style (CPS) transformation and we demonstrate how it allows us to overcome well-known complications of this proof related to α-conversion and administrative reductions. In addition, by using a logical relation that is indexed by invariants that relate the resource consumption of two programs, we are able show that the transformation preserves diverging behaviors and that our CPS transformation asymptotically preserves the running time of the source program. Our results are formalized in the Coq proof assistant. Our continuation-passing style transformation is part of the CertiCoq compiler for Gallina, the specification language of Coq.

CPS transformation with affine types for call-by-value implicit polymorphism

Transformation of programs into continuation-passing style (CPS) reveals the notion of continuations, enabling many applications such as control operators and intermediate representations in compilers. Although type preservation makes CPS transformation more beneficial, achieving type-preserving CPS transformation for implicit polymorphism with call-by-value (CBV) semantics is known to be challenging. We identify the difficulty in the problem that we call scope intrusion. To address this problem, we propose a new CPS target language Λ open that supports two additional constructs for polymorphism: one only binds and the other only generalizes type variables. Unfortunately, their unrestricted use makes Λ open unsafe due to undesired generalization of type variables. We thus equip Λ open with affine types to allow only the type-safe generalization. We then define a CPS transformation from Curry-style CBV System F to type-safe Λ open and prove that the transformation is meaning and type preserving. We also study parametricity of Λ open as it is a fundamental property of polymorphic languages and plays a key role in applications of CPS transformation. To establish parametricity, we construct a parametric, step-indexed Kripke logical relation for Λ open and prove that it satisfies the Fundamental Property as well as soundness with respect to contextual equivalence.

Reasoning about effect interaction by fusion

Effect handlers can be composed by applying them sequentially, each handling some operations and leaving other operations uninterpreted in the syntax tree. However, the semantics of composed handlers can be subtle---it is well known that different orders of composing handlers can lead to drastically different semantics. Determining the correct order of composition is a non-trivial task. To alleviate this problem, this paper presents a systematic way of deriving sufficient conditions on handlers for their composite to correctly handle combinations, such as the sum and the tensor, of the effect theories separately handled. These conditions are solely characterised by the clauses for relevant operations of the handlers, and are derived by fusing two handlers into one using a form of fold/build fusion and continuation-passing style transformation. As case studies, the technique is applied to commutative and distributive interaction of handlers to obtain a series of results about the interaction of common handlers: (a) equations respected by each handler are preserved after handler composition; (b) handling mutable state before any handler gives rise to a semantics in which state operations are commutative with any operations from the latter handler; (c) handling the writer effect and mutable state in either order gives rise to a correct handler of the commutative combination of these two theories.

On Correspondence between Selective CPS Transformation and Selective Double Negation Translation

A double negation translation (DNT) embeds classical logic into intuitionistic logic. Such translations correspond to continuation passing style (CPS) transformations in programming languages via the Curry-Howard isomorphism. A selective CPS transformation uses a type and effect system to selectively translate only nontrivial expressions possibly with computational effects into CPS functions. In this paper, we review the conventional call-by-value (CBV) CPS transformation and its corresponding DNT, and provide a logical account of a CBV selective CPS transformation by defining a selective DNT via the Curry-Howard isomorphism. By using an annotated proof system derived from the corresponding type and effect system, our selective DNT translates classical proofs into equivalent intuitionistic proofs, which are smaller than those obtained by the usual DNTs. We believe that our work can serve as a reference point for further study on the Curry-Howard isomorphism between CPS transformations and DNTs.

Effect handlers via generalised continuations

AbstractPlotkin and Pretnar’s effect handlers offer a versatile abstraction for modular programming with user-defined effects. This paper focuses on foundations for implementing effect handlers, for the three different kinds of effect handlers that have been proposed in the literature: deep, shallow, and parameterised. Traditional deep handlers are defined by folds over computation trees and are the original construct proposed by Plotkin and Pretnar. Shallow handlers are defined by case splits (rather than folds) over computation trees. Parameterised handlers are deep handlers extended with a state value that is threaded through the folds over computation trees. We formulate the extensions both directly and via encodings in terms of deep handlers and illustrate how the direct implementations avoid the generation of unnecessary closures. We give two distinct foundational implementations of all the kinds of handlers we consider: a continuation-passing style (CPS) transformation and a CEK-style abstract machine. In both cases, the key ingredient is a generalisation of the notion of continuation to accommodate stacks of effect handlers. We obtain our CPS translation through a series of refinements as follows. We begin with a first-order CPS translation into untyped lambda calculus which manages a stack of continuations and handlers as a curried sequence of arguments. We then refine the initial CPS translation by uncurrying it to yield a properly tail-recursive translation and then moving towards more and more intensional representations of continuations in order to support different kinds of effect handlers. Finally, we make the translation higher order in order to contract administrative redexes at translation time. Our abstract machine design then uses the same generalised continuation representation as the CPS translation. We have implemented both the abstract machine and the CPS transformation (plus extensions) as backends for the Links web programming language.

Call-by-name extensionality and confluence

AbstractDesigning rewriting systems that respect functional extensionality for call-by-name languages with effects turns out to be surprisingly challenging. Simply interpreting extensional laws like η as reduction rules easily breaks confluence. We explore these issues in the setting of a sequent calculus. Building on an insight that appears in different aspects of the theory of call-by-name functional languages—confluent rewriting for two independent control calculi and sound continuation-passing style transformations—we give a confluent reduction system for lazy extensional functions. Finally, we consider limitations to this approach when used for strict evaluation and types beyond functions.

Breaking through the normalization barrier: a self-interpreter for f-omega

According to conventional wisdom, a self-interpreter for a strongly normalizing lambda-calculus is impossible. We call this the normalization barrier. The normalization barrier stems from a theorem in computability theory that says that a total universal function for the total computable functions is impossible. In this paper we break through the normalization barrier and define a self-interpreter for System F_omega, a strongly normalizing lambda-calculus. After a careful analysis of the classical theorem, we show that static type checking in F_omega can exclude the proof's diagonalization gadget, leaving open the possibility for a self-interpreter. Along with the self-interpreter, we program four other operations in F_omega, including a continuation-passing style transformation. Our operations rely on a new approach to program representation that may be useful in theorem provers and compilers.

Self-Representation in Girard's System U

In 1991, Pfenning and Lee studied whether System F could support a typed self-interpreter. They concluded that typed self-representation for System F "seems to be impossible", but were able to represent System F in F ω . Further, they found that the representation of F ω requires kind polymorphism, which is outside F ω . In 2009, Rendel, Ostermann and Hofer conjectured that the representation of kind-polymorphic terms would require another, higher form of polymorphism. Is this a case of infinite regress? We show that it is not and present a typed self-representation for Girard's System U, the first for a λ-calculus with decidable type checking. System U extends System F ω with kind polymorphic terms and types. We show that kind polymorphic types (i.e. types that depend on kinds) are sufficient to "tie the knot" -- they enable representations of kind polymorphic terms without introducing another form of polymorphism. Our self-representation supports operations that iterate over a term, each of which can be applied to a representation of itself. We present three typed self-applicable operations: a self-interpreter that recovers a term from its representation, a predicate that tests the intensional structure of a term, and a typed continuation-passing-style (CPS) transformation -- the first typed self-applicable CPS transformation. Our techniques could have applications from verifiably type-preserving metaprograms, to growable typed languages, to more efficient self-interpreters.

Formalizing Operational Semantic Specifications in Logic

We review links between three logic formalisms and three approaches to specifying operational semantics. In particular, we show that specifications written with (small-step and big-step) SOS, abstract machines, and multiset rewriting, are closely related to Horn clauses, binary clauses, and (a subset of) linear logic, respectively. We shall illustrate how binary clauses form a bridge between the other two logical formalisms. For example, using a continuation-passing style transformation, Horn clauses can be transformed into binary clauses. Furthermore, binary clauses can be seen as a degenerative form of multiset rewriting: placing binary clauses within linear logic allows for rich forms of multiset rewriting which, in turn, provides a modular, big-step SOS specifications of imperative and concurrency primitives. Establishing these links between logic and operational semantics has many advantages for operational semantics: tools from automated deduction can be used to animate semantic specifications; solutions to the treatment of binding structures in logic can be used to provide solutions to binding in the syntax of programs; and the declarative nature of logical specifications provides broad avenues for reasoning about semantic specifications.

Two Values Passing CPS Transformation for Call-by-Name Calculus with Constants

为Plotkin带常数传名调用(演算定义了一个新的CPS(continuation-passing-style)变换方法.方法基于求值上下文变换,新颖之处在于,每次传递二值给继续而不是常规的一值.先给出二值CPS变换编码,再在此基础上定义CPS语言,最后建立源语言和CPS语言的一一映射关系并证明Plotkin的模拟定理.与Plotkin的工作比较,工作特点在于,给出了一个CPS归约闭语言,该语言中所有继续都可以用函数形式表达,且模拟定理的可靠性和完备性方向证明更为简单.;In this paper, a new CPS (continuation-passing-style) transformation for Plotkin's call-by-name ( calculus with constants is proposed. It is based on evaluation contexts transformation and the features that two values, instead of one, are passed to the continuation every time. With encodings, a CPS language is introduced. Then, Plotkin's simulation theorem is proved by establishing 1-to-1 correspondence between the source language and CPS language. Compared with Plotkin's work, a reduction closed CPS language is defined in which all continuations are explicitly expressed as functional encodings and it is simpler to prove both the soundness and completeness directions of simulation theorem.

A static simulation of dynamic delimited control

We present a continuation-passing-style (CPS) transformation for some dynamic delimited-control operators, including Felleisen's $\verb|control|$ and $\verb|prompt|$ , that extends a standard call-by-value CPS transformation. Based on this new transformation, we show how Danvy and Filinski's static delimited-control operators $\verb|shift|$ and $\verb|reset|$ simulate dynamic operators, allaying in passing some skepticism in the literature about the existence of such a simulation. The new CPS transformation and simulation use recursive delimited continuations to avoid undelimited control and the overhead it incurs in implementation and reasoning.

The essence of compiling with continuations

In order to simplify the compilation process, many compilers for higher-order languages use the continuation-passing style (CPS) transformation in a first phase to generate an intermediate representation of the source program. The salient aspect of this intermediate form is that all procedures take an argument that represents the rest of the computation (the "continuation"). Since the naïve CPS transformation considerably increases the size of programs, CPS compilers perform reductions to produce a more compact intermediate representation. Although often implemented as a part of the CPS transformation, this step is conceptually a second phase. Finally, code generators for typical CPS compilers treat continuations specially in order to optimize the interpretation of continuation parameters.A thorough analysis of the abstract machine for CPS terms shows that the actions of the code generator invert the naïve CPS translation step. Put differently, the combined effect of the three phases is equivalent to a source-to-source transformation that simulates the compaction phase. Thus, fully developed CPS compilers do not need to employ the CPS transformation but can achieve the same results with a simple source-level transformation.

CPS transformation of flow information

We consider the question of how a Continuation-Passing-Style (CPS) transformation changes the flow analysis of a program. We present an algorithm that takes the least solution to the flow constraints of a program and constructs in linear time the least solution to the flow constraints for the CPS-transformed program. Previous studies of this question used CPS transformations that had the effect of duplicating code, or of introducing flow sensitivity into the analysis. Our algorithm has the property that for a program point in the original program and the corresponding program point in the CPS-transformed program, the flow information is the same. By carefully avoiding both duplicated code and flow-sensitive analysis, we find that the most accurate analysis of the CPS-transformed program is neither better nor worse than the most accurate analysis of the original. Thus a compiler that needed flow information after CPS transformation could use the flow information from the original program to annotate some program points, and it could use our algorithm to find the rest of the flow information quickly, rather than having to analyze the CPS-transformed program.

CPS transformation of flow information, Part II: administrative reductions

We characterize the impact of a linear $\beta$-reduction on the result of a control-flow analysis. (By ‘a linear $\beta$-reduction’ we mean the $\beta$-reduction of a linear $\lambda$-abstraction, i.e., of a $\lambda$-abstraction whose parameter occurs exactly once in its body.) As a corollary, we consider the administrative reductions of a Plotkin-style transformation into Continuation-Passing Style (CPS), and how they affect the result of a constraint-based control-flow analysis and, in particular, the least element in the space of solutions. We show that administrative reductions preserve the least solution. Preservation of least solutions solves a problem that was left open in Palsberg and Wand's article ‘CPS Transformation of Flow Information.’ Together, Palsberg and Wand's article and the present article show how to map in linear time the least solution of the flow constraints of a program into the least solution of the flow constraints of the CPS counterpart of this program, after administrative reductions. Furthermore, we show how to CPS transform control-flow information in one pass.

Syntactic accidents in program analysis: on the impact of the CPS transformation

We show that a non-duplicating transformation into Continuation-Passing Style (CPS) has no effect on control-flow analysis, a positive effect on binding-time analysis for traditional partial evaluation, and no effect on binding-time analysis for continuation-based partial evaluation: a monovariant control-flow analysis yields equivalent results on a direct-style program and on its CPS counterpart, a monovariant binding-time analysis yields less precise results on a direct-style program than on its CPS counterpart, and an enhanced monovariant binding-time analysis yields equivalent results on a direct-style program and on its CPS counterpart. Our proof technique amounts to constructing the CPS counterpart of flow information and of binding times. Our results formalize and confirm a folklore theorem about traditional binding-time analysis, namely that CPS has a positive effect on binding times. What may be more surprising is that the benefit does not arise from a standard refinement of program analysis, as, for instance, duplicating continuations. The present study is symptomatic of an unsettling property of program analyses: their quality is unpredictably vulnerable to syntactic accidents in source programs, i.e., to the way these programs are written. More reliable program analyses require a better understanding of the effect of syntactic change.

A first-order one-pass CPS transformation

We present a new transformation of λ-terms into continuation-passing style (CPS). This transformation operates in one pass and is both compositional and first-order. Previous CPS transformations only enjoyed two out of the three properties of being first-order, one-pass, and compositional, but the new transformation enjoys all three properties. It is proved correct directly by structural induction over source terms instead of indirectly with a colon translation, as in Plotkin's original proof. Similarly, it makes it possible to reason about CPS-transformed terms by structural induction over source terms, directly. The new CPS transformation connects separately published approaches to the CPS transformation. It has already been used to state a new and simpler correctness proof of a direct-style transformation, and to develop a new and simpler CPS transformation of control-flow information.

A Lambda-Revelation of the SECD Machine

We present a simple inter-derivation between lambda-interpreters, i.e., evaluation functions for lambda-terms, and abstract reduction machines for the lambda-calculus, i.e., transition functions. The two key derivation steps are the CPS transformation and Reynolds's defunctionalization. By transforming the interpreter into continuation-passing style (CPS), its flow of control is made manifest as a continuation. By defunctionalizing this continuation, the flow of control is materialized as a first-order data structure.<br /> <br />The derivation applies not merely to connect independently known lambda-interpreters and abstract machines, it also applies to construct the abstract machine corresponding to a lambda-interpreter and to construct the lambda-interpreter corresponding to an abstract machine. In this article, we treat in detail the canonical example of Landin's SECD machine and we reveal its denotational content: the meaning of an expression is a partial endo-function from a stack of intermediate results and an environment to a new stack of intermediate results and an environment. The corresponding lambda-interpreter is unconventional because (1) it uses a control delimiter to evaluate the body of each lambda-abstraction and (2) it assumes the environment to be managed in a callee-save fashion instead of in the usual caller-save fashion.

A Simple CPS Transformation of Control-Flow Information

We build on Danvy and Nielsen's first-order program transformation into continuation-passing style (CPS) to design a new CPS transformation of flow information that is simpler and more efficient than what has been presented in previous work. The key to simplicity and efficiency is that our CPS transformation constructs the flow information in one go, instead of first computing an intermediate result and then exploiting it to construct the flow information. More precisely, we show how to compute control-flow information for CPS-transformed programs from control-flow information for direct-style programs and vice-versa. As a corollary, we confirm that CPS transformation has no effect on the control-flow information obtained by constraint-based control-flow analysis. The transformation has immediate applications in assessing the effect of the CPS transformation over other analyses such as, for instance, binding-time analysis.

CPS Transformation of Flow Information, Part II: Administrative Reductions

We characterize the impact of a linear beta-reduction on the result of a control-flow analysis. (By "a linear beta-reduction'' we mean the beta-reduction of a linear lambda-abstraction, i.e., of a lambda-abstraction whose parameter occurs exactly once in its body.)<br /> <br />As a corollary, we consider the administrative reductions of a Plotkin-style transformation into continuation-passing style (CPS), and how they affect the result of a constraint-based control-flow analysis and, in particular, the least element in the space of solutions. We show that administrative reductions preserve the least solution. Preservation of least solutions solves a problem that was left open in Palsberg and Wand's article "CPS Transformation of Flow Information.''<br /> <br />Together, Palsberg and Wand's article and the present article show how to map in linear time the least solution of the flow constraints of a program into the least solution of the flow constraints of the CPS counterpart of this program, after administrative reductions. Furthermore, we show how to CPS transform control-flow information in one pass.<br /><br />Supersedes BRICS-RS-01-40.

A Simple Correctness Proof of the Direct-Style Transformation

We build on Danvy and Nielsen's first-order program transformation into continuation-passing style (CPS) to present a new correctness proof of the converse transformation, i.e., a one-pass transformation from CPS back to direct style. Previously published proofs were based on, e.g., a one-pass higher-order CPS transformation, and were complicated by having to reason about higher-order functions. In contrast, this work is based on a one-pass CPS transformation that is both compositional and first-order, and therefore the proof simply proceeds by structural induction on syntax.