Practical Programming Language Research Articles

Capturing Types

Type systems usually characterize the shape of values but not their free variables. However, many desirable safety properties could be guaranteed if one knew the free variables captured by values. We describe CC < :◻ , a calculus where such captured variables are succinctly represented in types, and show it can be used to safely implement effects and effect polymorphism via scoped capabilities. We discuss how the decision to track captured variables guides key aspects of the calculus, and show that CC < :◻ admits simple and intuitive types for common data structures and their typical usage patterns. We demonstrate how these ideas can be used to guide the implementation of capture checking in a practical programming language.

Gradual System F

Bringing the benefits of gradual typing to a language with parametric polymorphism like System F, while preserving relational parametricity, has proven extremely challenging: first attempts were formulated a decade ago, and several designs have been recently proposed, with varying syntax, behavior, and properties. Starting from a detailed review of the challenges and tensions that affect the design of gradual parametric languages, this work presents an extensive account of the semantics and metatheory of GSF, a gradual counterpart of System F. In doing so, we also report on the extent to which the Abstracting Gradual Typing methodology can help us derive such a language. Among gradual parametric languages that follow the syntax of System F, GSF achieves a unique combination of properties. We clearly establish the benefits and limitations of the language, and discuss several extensions of GSF towards a practical programming language.

Proving the correctness of the algorithm for building a crystallographic space group

An application of formal verification (using the proof assistant Isabelle/HOL) for ensuring the correctness of scientific data processing software in the crystallographic domain is presented. The proposed process consists of writing a pseudocode that describes an algorithm in a succinct but mathematically unambiguous way, then formulating or reusing necessary Isabelle theories and proving algorithm properties within these theories, and finally implementing the algorithm in a practical programming language. Both the formal proof and the semi-formal algorithm analysis are demonstrated on an example of a simple but important algorithm (widely used in crystallographic computing) that reconstructs a space-group operator list from a subset of symmetry operators. The cod-tools software package that implements the verified algorithm is also presented. On the basis of the reported results, it is argued that broader application of formal methods (e.g. formal verification of algorithm correctness) allows developers to improve the reliability of scientific software. Moreover, the formalized (within the proof assistant) domain-specific theory can be reused and gradually extended, thus continuously increasing the automation level of formal algorithm verification.

The third AI summer: AAAI Robert S. Engelmore Memorial Lecture

The third AI summer: AAAI Robert S. Engelmore Memorial Lecture

Type-level programming with match types

Type-level programming is becoming more and more popular in the realm of functional programming. However, the combination of type-level programming and subtyping remains largely unexplored in practical programming languages. This paper presents match types , a type-level equivalent of pattern matching. Match types integrate seamlessly into programming languages with subtyping and, despite their simplicity, offer significant additional expressiveness. We formalize the feature of match types in a calculus based on System F sub and prove its soundness. We practically evaluate our system by implementing match types in the Scala 3 reference compiler, thus making type-level programming readily available to a broad audience of programmers.

Modal FRP for all: Functional reactive programming without space leaks in Haskell

Abstract Functional reactive programming (FRP) provides a high-level interface for implementing reactive systems in a declarative manner. However, this high-level interface has to be carefully reigned in to ensure that programs can in fact be executed in practice. Specifically, one must ensure that FRP programs are causal and can be implemented without introducing space leaks. In recent years, modal types have been demonstrated to be an effective tool to ensure these operational properties. In this paper, we present $\mathsf{Rattus}$ , a modal FRP language that extends and simplifies previous modal FRP calculi while still maintaining the operational guarantees for productivity, causality, and space leaks. The simplified type system makes $\mathsf{Rattus}$ a practical programming language that can be integrated with existing functional programming languages. To demonstrate this, we have implemented a shallow embedding of $\mathsf{Rattus}$ in Haskell that allows the programmer to write $\mathsf{Rattus}$ code in familiar Haskell syntax and seamlessly integrate it with regular Haskell code. Thus, $\mathsf{Rattus}$ combines the benefits enjoyed by FRP libraries such as Yampa, namely access to a rich library ecosystem (e.g., for graphics programming), with the strong operational guarantees offered by a bespoke type system. To establish the productivity, causality, and memory properties of the language, we prove type soundness using a logical relations argument fully mechanised in the Coq proof assistant.

EVOLUTION OF PROGRAMMING PARADIGMS

The subject of computer programming has witnessed a wonderful journey over a long time, characterized by the continuous evolution of programming paradigms. This overview paper provides an in-depth analysis of the dynamic and difficult method through which programming paradigms have advanced from their inception to their cutting-edge kingdom. The review commences with an ancient perspective, tracing the origins of programming paradigms from the early days of machine code and meeting language to the development of higher-degree languages. It examines the vital paradigm, which turned into most important throughout the early years of computing, and how it gave an upward push to dependent programming and modular programming. A pivotal shift occurred with the arrival of the object-orientated paradigm, which added a brand new way of considering software layout and improvement. This paradigm's impact on cutting-edge software engineering is explored, highlighting its lasting impact on industries and the open-source movement. As software complexity multiplied, so did the need for greater expressive and abstract programming paradigms, main to the emergence of practical programming and its mathematical foundations. The overview discusses how practical programming languages have received traction in recent years and how features drastically impacted parallel and dispensed computing. Furthermore, the paper explores concurrent and parallel programming paradigms, illustrating how they address the developing call for efficient use of multi-middle processors and disbursed structures. It delves into the challenges and possibilities presented through the increasing significance of parallelism in modern-day computing. The review also discusses rising paradigms such as reactive and eventdriven programming, highlighting their programs in actual-time systems, internet improvement, and the Internet of Things. These paradigms are analyzed in the context of modern software program improvement and the everincreasing technology panorama.

A graded dependent type system with a usage-aware semantics

Graded Type Theory provides a mechanism to track and reason about resource usage in type systems. In this paper, we develop GraD, a novel version of such a graded dependent type system that includes functions, tensor products, additive sums, and a unit type. Since standard operational semantics is resource-agnostic, we develop a heap-based operational semantics and prove a soundness theorem that shows correct accounting of resource usage. Several useful properties, including the standard type soundness theorem, non-interference of irrelevant resources in computation and single pointer property for linear resources, can be derived from this theorem. We hope that our work will provide a base for integrating linearity, irrelevance and dependent types in practical programming languages like Haskell.

A Methodological Review of Machine Learning in Applied Linguistics

The traditional linear regression in applied linguistics (AL) suffers from the drawbacks arising from the strict assumptions namely: linearity, and normality, etc. More advanced methods are needed to overcome the shortcomings of the traditional method and grapple with intricate linguistic problems. However, there is no previous review on the applications of machine learning (ML) in AL, the introduction of interpretable ML, and related practical software. This paper addresses these gaps by reviewing the representative algorithms of ML in AL. The result shows that ML is applicable in AL and enjoys a promising future. It goes further to discuss the applications of interpretable ML for reporting the results in AL. Finally, it ends with the recommendations of the practical programming languages, software, and platforms to implement ML for researchers in AL to foster the interdisciplinary studies between AL and ML.

Lake symbols for island parsing

Context: An island parser reads an input text and builds the parse (or abstract syntax) tree of only the programming constructs of interest in the text. These constructs are called islands and the rest of the text is called water, which the parser ignores and skips over. Since an island parser does not have to parse all the details of the input, it is often easy to develop but still useful enough for a number of software engineering tools. When a parser generator is used, the developer can implement an island parser by just describing a small number of grammar rules, for example, in Parsing Expression Grammar (PEG). Inquiry: In practice, however, the grammar rules are often complicated since the developer must define the water inside the island; otherwise, the island parsing will not reduce the total number of grammar rules. When describing the grammar rules for such water, the developer must consider other rules and enumerate a set of symbols, which we call alternative symbols. Due to this difficulty, island parsing seems to be not widely used today despite its usefulness in many applications. Approach: This paper proposes the lake symbols for addressing this difficulty in developing an island parser. It also presents an extension to PEG for supporting the lake symbols. The lake symbols automate the enumeration of the alternative symbols for the water inside an island. The paper proposes an algorithm for translating the extended PEG to the normal PEG, which can be given to an existing parser generator based on PEG. Knowledge: The user can use lake symbols to define water without specifying each alternative symbol. Our algorithms can calculate all alternative symbols for a lake symbol, based on where the lake symbol is used in the grammar. Grounding: We implemented a parser generator accepting our extended PEG and implemented 36 island parsers for Java and 20 island parsers for Python. Our experiments show that the lake symbols reduce 42 % of grammar rules for Java and 89 % of rules for Python on average, excluding the case where islands are expressions. Importance: This work eases the use of island parsing. Lake symbols enable the user to define the water inside the island simpler than before. Defining water inside the island is essential to apply island parsing for practical programming languages.

Machine-checked Proof of the Church-Rosser Theorem for the Lambda Calculus Using the Barendregt Variable Convention in Constructive Type Theory

In this article we continue the work started in [Ernesto Copello, Álvaro Tasistro, Nora Szasz, Ana Bove, and Maribel Fernández. Alpha-structural induction and recursion for the λ-calculus in constructive type theory. Electronic Notes in Theoretical Computer Science, 323:109–124, 2016], deriving in Constructive Type Theory new induction principles for the λ-calculus, using (the historical) first order syntax with only one sort of names for both bound and free variables, and with α-conversion based upon name swapping. The principles provide a flexible framework for mimicking pen-and-paper proofs within the rigorous formal setting of a proof assistant. We here report on one successful application, namely a complete proof of the Church-Rosser Theorem. The whole development has been machine-checked using the system Agda [Ulf Norell. Towards a Practical Programming Language Based on Dependent Type Theory. PhD thesis, Department of Computer Science and Engineering, Chalmers University of Technology, September 2007].

A Categorical Model for a Quantum Circuit Description Language (Extended Abstract)

Quipper is a practical programming language for describing families of quantum circuits. In this paper, we formalize a small, but useful fragment of Quipper called Proto-Quipper-M. Unlike its parent Quipper, this language is type-safe and has a formal denotational and operational semantics. Proto-Quipper-M is also more general than Quipper, in that it can describe families of morphisms in any symmetric monoidal category, of which quantum circuits are but one example. We design Proto-Quipper-M from the ground up, by first giving a general categorical model of parameters and state. The distinction between parameters and state is also known from hardware description languages. A parameter is a value that is known at circuit generation time, whereas a state is a value that is known at circuit execution time. After finding some interesting categorical structures in the model, we then define the programming language to fit the model. We cement the connection between the language and the model by proving type safety, soundness, and adequacy properties.

AI and the Origins of the Functional Programming Language Style

The Lisp programming language is often described as the first functional programming language and also as an important early AI language. In the history of functional programming, however, it occupies a rather anomalous position, as the circumstances of its development do not fit well with the widely accepted view that functional languages have been developed through a theoretically-inspired project of deriving practical programming languages from the lambda calculus. This paper examines the origins of Lisp in the early AI programming work of the mid-to-late 1950s, and in particular in the work of Allen Newell, Cliff Shaw and Herbert Simon. Their 1956 program, the Logic Theory Machine, introduced new ideas about data and program structures that were articulated in response to perceived limitations in existing programming technique. Later writers, notably John Backus, have described these features as constituting a "programming language style" distinct from the traditional style that preceded it. The paper examines the origins of the earlier style in practices of manual computation, analyses the key technical differences between it and the style first manifested in the Logic Theory Machine, and concludes that programming practice and experience play a large and underappreciated role in the development of programming styles and languages.

A Declarative Extension of Parsing Expression Grammars for Recognizing Most Programming Languages

Parsing Expression Grammars are a popular foundation for describing syntax. Unfortunately, several syntax of programming languages are still hard to recognize with pure PEGs. Notorious cases appears: typedef-defined names in C/C++, indentation-based code layout in Python, and HERE document in many scripting languages. To recognize such PEG-hard syntax, we have addressed a declarative extension to PEGs. The "declarative" extension means no programmed semantic actions, which are traditionally used to realize the extended parsing behavior. Nez is our extended PEG language, including symbol tables and conditional parsing. This paper demonstrates that the use of Nez Extensions can realize many practical programming languages, such as C, C\#, Ruby, and Python, which involve PEG-hard syntax.

Closed type families with overlapping equations

Open, type-level functions are a recent innovation in Haskell that move Haskell towards the expressiveness of dependent types, while retaining the look and feel of a practical programming language. This paper shows how to increase expressiveness still further, by adding closed type functions whose equations may overlap, and may have non-linear patterns over an open type universe. Although practically useful and simple to implement, these features go beyond conventional dependent type theory in some respects, and have a subtle metatheory.

Formal Description of Alvis Language with α0 System Layer

The paper presents a formal description of a subset of the Alvis language designed for the modelling and formal verification of concurrent systems. Alvis combines possibilities of a formal models verification with flexibility and simplicity of practical programming languages. Alvis provides a graphical modelling of interconnections among agents and a high level programming language used for the description of agents behaviour. Its semantic depends on the so-called system layer. The most universal system layer α0, described in the paper, makes Alvis similar to other formal languages like Petri nets, process algebras, time automata, etc.

Compilation of Interaction Nets

This paper is about a new implementation technique for interaction nets – a visual programming language based on graph rewriting. We compile interaction nets to C, which offers a robust and efficient implementation, in addition to portability. In the presentation of this work we extend the interaction net programming paradigm to introduce a number of features which make it a practical programming language.

Revised6 Report on the Algorithmic Language Scheme

Programming languages should be designed not by piling feature on top of feature, but by removing the weaknesses and restrictions that make additional features appear necessary. Scheme demonstrates that a very small number of rules for forming expressions, with no restrictions on how they are composed, suffice to form a practical and efficient programming language that is flexible enough to support most of the major programming paradigms in use today.Scheme was one of the first programming languages to incorporate first-class procedures as in the lambda calculus, thereby proving the usefulness of static scope rules and block structure in a dynamically typed language. Scheme was the first major dialect of Lisp to distinguish procedures from lambda expressions and symbols, to use a single lexical environment for all variables, and to evaluate the operator position of a procedure call in the same way as an operand position. By relying entirely on procedure calls to express iteration, Scheme emphasized the fact that tail-recursive procedure calls are essentially gotos that pass arguments. Scheme was the first widely used programming language to embrace first-class escape procedures, from which all previously known sequential control structures can be synthesized. A subsequent version of Scheme introduced the concept of exact and inexact number objects, an extension of Common Lisp's generic arithmetic. More recently, Scheme became the first programming language to support hygienic macros, which permit the syntax of a block-structured language to be extended in a consistent and reliable manner.

2APL: a practical agent programming language

This article presents a BDI-based agent-oriented programming language, called 2APL (A Practical Agent Programming Language). This programming language facilitates the implementation of multi-agent systems consisting of individual agents that may share and access external environments. It realizes an effective integration of declarative and imperative style programming by introducing and integrating declarative beliefs and goals with events and plans. It also provides practical programming constructs to allow the generation, repair, and (different modes of) execution of plans based on beliefs, goals, and events. The formal syntax and semantics of the programming language are given and its relation with existing BDI-based agent-oriented programming languages is discussed.

Meta-programming With Built-in Type Equality

We report our experience with exploring a new point in the design space for formal reasoning systems: the development of the programming language Ωmega. Ωmega is intended as both a practical programming language and a logic. The main goal of Ωmega is to allow programmers to describe and reason about semantic properties of programs from within the programming language itself, mainly by using a powerful type system.We illustrate the main features of Ωmega by developing an interesting meta-programming example. First, we show how to encode a set of well-typed simply typed λ-calculus terms as an Ωmega data-type. Then, we show how to implement a substitution operation on these terms that is guaranteed by the Ωmega type system to preserve their well-typedness.