Piglets to the rescue Declarative User Interface Specification with Pluggable View Models

Abstract This paper introduces Pluggable Interactive GUI-lets (or Piglets, for short) as a mean for constructing reusable, reactive graphical user interfaces that can be instantiated over different view models and presentation layers. Piglets therefore provide an attractive alternative for pragmatic user interface specification that can target multiple content delivery channels from the same declarative, type-safe user interface definition, keeping programmers happy and highly productive

In the Age of Web: Typed Functional-First Programming Revisited

Most programming languages were designed before the age of web. This matters because the web changes many assumptions that typed functional language designers take for granted. For example, programs do not run in a closed world, but must instead interact with (changing and likely unreliable) services and data sources, communication is often asynchronous or event-driven, and programs need to interoperate with untyped environments. In this paper, we present how the F# language and libraries face the challenges posed by the web. Technically, this comprises using type providers for integration with external information sources and for integration with untyped programming environments, using lightweight meta-programming for targeting JavaScript and computation expressions for writing asynchronous code. In this inquiry, the holistic perspective is more important than each of the features in isolation. We use a practical case study as a starting point and look at how F# language and libraries approach the challenges posed by the web. The specific lessons learned are perhaps less interesting than our attempt to uncover hidden assumptions that no longer hold in the age of web.Comment: In Proceedings ML/OCaml 2014, arXiv:1512.0143

Could tierless languages reduce IoT development grief?

Internet of Things (IoT) software is notoriously complex, conventionally comprising multiple tiers. Traditionally an IoT developer must use multiple programming languages and ensure that the components interoperate correctly. A novel alternative is to use a single tierless language with a compiler that generates the code for each component and ensures their correct interoperation. We report a systematic comparative evaluation of two tierless language technologies for IoT stacks: one for resource-rich sensor nodes (Clean with iTask), and one for resource-constrained sensor nodes (Clean with iTask and mTask). The evaluation is based on four implementations of a typical smart campus application: two tierless and two Python-based tiered. (1) We show that tierless languages have the potential to significantly reduce the development effort for IoT systems, requiring 70% less code than the tiered implementations. Careful analysis attributes this code reduction to reduced interoperation (e.g. two embedded domain-specific languages (DSLs) and one paradigm versus seven languages and two paradigms), automatically generated distributed communication, and powerful IoT programming abstractions. (2) We show that tierless languages have the potential to significantly improve the reliability of IoT systems, describing how Clean iTask/mTask maintains type safety, provides higher order failure management, and simplifies maintainability. (3) We report the first comparison of a tierless IoT codebase for resource-rich sensor nodes with one for resource-constrained sensor nodes. The comparison shows that they have similar code size (within 7%), and functional structure. (4) We present the first comparison of two tierless IoT languages, one for resource-rich sensor nodes, and the other for resource-constrained sensor nodes

Improving interoperability in distributed multi-tier software stacks

Distributed multi-tier software stacks organise and deploy software components as a hierarchy of interacting tiers. The components are typically heterogeneous, i.e. each component may be written in a different language and may interoperate using a variety of protocols. Tiered software is modular but leads to a range of interoperability challenges including the following. (1) Interoperating components in multiple languages and paradigms increases developer cognitive load since they must simultaneously reason in multiple languages and paradigms. (2) There must be correct interoperation of components, e.g. adherence to the API or communication protocols between components. (3) Interoperation between different components can lead to diverse modes of failure as each component can fail in unique ways. Many of these challenges are the result of contributing factors like tight coupling or polyglot programmming. This thesis investigates techniques to improve heterogeneous interoperability in distributed multi-tier software stacks. Some common approaches include microservices and tierless languages. Microservices are perceived to offer better reliability than components in multi-tier software stacks through the loose coupling of services. The reliability of microservices is investigated by combining the established properties of dependence and state with reliability. This defines a new three-dimensional space: the Microservices Dependency State Reliability (MDSR) classification with six classes. The feasibility of statically identifying MDSR classes is demonstrated with a prototype analyser that identifies all six classes in Flask microservices web applications. The reliability implications of the different MDSR classes are evaluated by running three case study applications (Hipster-Shop, JPyL & WordPress) against a fault injector. Key results are as follows. (1) All applications fail catastrophically if a critical microservice fails. (2) Applications survive the failure of individual minor microservice(s). (3) The failure of any chain of microservices in JPyL & Hipster is catastrophic. (4) Individual microservices do not necessarily have minor reliability implications. In a tierless language, the compiler generates the code for each component and ensures their correct interoperation. They are mainly used to implement web stacks. However, their use in implementing IoT stacks is less common. This investigation compares interoperation in tiered and tierless IoT stacks through the systematic evaluation of four implementations of the prototype UoG smart campus IoT system: two tierless and two Python-based tiered. Key results of the study are as follows. (1) Tierless languages have the potential to significantly reduce the development effort for IoT systems, requiring 70% less code than the tiered implementations. (2) Tierless languages have the potential to significantly improve the reliability of IoT systems. (3) The first comparison of a tierless codebase for resource-rich sensor nodes and one for resourceconstrained sensor nodes shows that they have very similar functional structure and code sizes - within 7%. Tier elimination is a technique that removes a tier/component by integrating two tiers. Specifically, this thesis investigates the implications of eliminating the Apache web server in a 4-tier web stack: Jupyter Notebook, Apache, Python, Linux (JAPyL) and replacing it with PHP libraries in the frontend webpage to get the 3-tier (JPL). The study reveals the following. (1) The JPL 3-tier web stack requires that the developer uses fewer programming languages and paradigms than JAPyL, i.e two compared with four languages and two compared with three paradigms. (2) JPL requires 42% less code than JAPyL. (3) In JPL, some of the functionalities can be automated due to the decreased abstraction levels at the upper layers of the stack. (4) However, the latency in JPL is two to three times greater than that of JAPyL. So while tier elimination reduces developer effort and semantic friction the tradeoffs are high performance overhead & resource consumption and increasing code complexity

Behavioural Types: from Theory to Tools

This book presents research produced by members of COST Action IC1201: Behavioural Types for Reliable Large-Scale Software Systems (BETTY), a European research network that was funded from October 2012 to October 2016. The technical theme of BETTY was the use of behavioural type systems in programming languages, to specify and verify properties of programs beyond the traditional use of type systems to describe data processing. A significant area within behavioural types is session types, which concerns the use of type-theoretic techniques to describe communication protocols so that static typechecking or dynamic monitoring can verify that protocols are implemented correctly. This is closely related to the topic of choreography, in which system design starts from a description of the overall communication flows. Another area is behavioural contracts, which describe the obligations of interacting agents in a way that enables blame to be attributed to the agent responsible for failed interaction. Type-theoretic techniques can also be used to analyse potential deadlocks due to cyclic dependencies between inter-process interactions. BETTY was organised into four Working Groups: (1) Foundations; (2) Security; (3) Programming Languages; (4) Tools and Applications. Working Groups 1–3 produced “state-of-the-art reports”, which originally intended to take snapshots of the field at the time the network started, but grew into substantial survey articles including much research carried out during the network [1–3]. The situation for Working Group 4 was different. When the network started, the community had produced relatively few implementations of programming languages or tools. One of the aims of the network was to encourage more implementation work, and this was a great success. The community as a whole has developed a greater interest in putting theoretical ideas into practice. The sixteen chapters in this book describe systems that were either completely developed, or substantially extended, during BETTY. The total of 41 co-authors represents a significant proportion of the active participants in the network (around 120 people who attended at least one meeting). The book is a report on the new state of the art created by BETTY in xv xvi Preface the area of Working Group 4, and the title “Behavioural Types: from Theory to Tools” summarises the trajectory of the community during the last four years. The book begins with two tutorials by Atzei et al. on contract-oriented design of distributed systems. Chapter 1 introduces the CO2 contract specifi- cation language and the Diogenes toolchain. Chapter 2 describes how timing constraints can be incorporated into the framework and checked with the CO2 middleware. Part of the CO2 middleware is a monitoring system, and the theme of monitoring continues in the next two chapters. In Chapter 3, Attard et al. present detectEr, a runtime monitoring tool for Erlang programs that allows correctness properties to be expressed in Hennessy-Milner logic. In Chapter 4, which is the first chapter about session types, Neykova and Yoshida describe a runtime verification framework for Python programs. Communication protocols are specified in the Scribble language, which is based on multiparty session types. The next three chapters deal with choreographic programming. In Chap- ter 5, Debois and Hildebrandt present a toolset for working with dynamic condition response (DCR) graphs, which are a graphical formalism for choreography. Chapter 6, by Lange et al., continues the graphical theme with ChorGram, a tool for synthesising global graphical choreographies from collections of communicating finite-state automata. Giallorenzo et al., in Chapter 7, consider runtime adaptation. They describe AIOCJ, a choreographic programming language in which runtime adaptation is supported with a guarantee that it doesn’t introduce deadlocks or races. Deadlock analysis is important in other settings too, and there are two more chapters about it. In Chapter 8, Padovani describes the Hypha tool, which uses a type-based approach to check deadlock-freedom and lock-freedom of systems modelled in a form of pi-calculus. In Chapter 9, Garcia and Laneve present a tool for analysing deadlocks in Java programs; this tool, called JaDA, is based on a behavioural type system. The next three chapters report on projects that have added session types to functional programming languages in order to support typechecking of communication-based code. In Chapter 10, Orchard and Yoshida describe an implementation of session types in Haskell, and survey several approaches to typechecking the linearity conditions required for safe session implemen- tation. In Chapter 11, Melgratti and Padovani describe an implementation of session types in OCaml. Their system uses runtime linearity checking. In Chapter 12, Lindley and Morris describe an extension of the web programming language Links with session types; their work contrasts with the previous two chapters in being less constrained by an existing language design. Continuing the theme of session types in programming languages, the next two chapters describe two approaches based on Java. Hu’s work, presented in Chapter 13, starts with the Scribble description of a multiparty session type and generates an API in the form of a collection of Java classes, each class containing the communication methods that are available in a particular state of the protocol. Dardha et al., in Chapter 14, also start with a Scribble specification. Their StMungo tool generates an API as a single class with an associated typestate specification to constrain sequences of method calls. Code that uses the API can be checked for correctness with the Mungo typechecker. Finally, there are two chapters about programming with the MPI libraries. Chapter 15, by Ng and Yoshida, uses an extension of Scribble, called Pabble, to describe protocols that parametric in the number of runtime roles. From a Pabble specification they generate C code that uses MPI for communication and is guaranteed correct by construction. Chapter 16, by Ng et al., describes the ParTypes framework for analysing existing C+MPI programs with respect to protocols defined in an extension of Scribble. We hope that the book will serve a useful purpose as a report on the activities of COST Action IC1201 and as a survey of programming languages and tools based on behavioural types

Behavioural Types: from Theory to Tools

This book presents research produced by members of COST Action IC1201: Behavioural Types for Reliable Large-Scale Software Systems (BETTY), a European research network that was funded from October 2012 to October 2016. The technical theme of BETTY was the use of behavioural type systems in programming languages, to specify and verify properties of programs beyond the traditional use of type systems to describe data processing. A significant area within behavioural types is session types, which concerns the use of type-theoretic techniques to describe communication protocols so that static typechecking or dynamic monitoring can verify that protocols are implemented correctly. This is closely related to the topic of choreography, in which system design starts from a description of the overall communication flows. Another area is behavioural contracts, which describe the obligations of interacting agents in a way that enables blame to be attributed to the agent responsible for failed interaction. Type-theoretic techniques can also be used to analyse potential deadlocks due to cyclic dependencies between inter-process interactions. BETTY was organised into four Working Groups: (1) Foundations; (2) Security; (3) Programming Languages; (4) Tools and Applications. Working Groups 1–3 produced “state-of-the-art reports”, which originally intended to take snapshots of the field at the time the network started, but grew into substantial survey articles including much research carried out during the network [1–3]. The situation for Working Group 4 was different. When the network started, the community had produced relatively few implementations of programming languages or tools. One of the aims of the network was to encourage more implementation work, and this was a great success. The community as a whole has developed a greater interest in putting theoretical ideas into practice. The sixteen chapters in this book describe systems that were either completely developed, or substantially extended, during BETTY. The total of 41 co-authors represents a significant proportion of the active participants in the network (around 120 people who attended at least one meeting). The book is a report on the new state of the art created by BETTY in xv xvi Preface the area of Working Group 4, and the title “Behavioural Types: from Theory to Tools” summarises the trajectory of the community during the last four years. The book begins with two tutorials by Atzei et al. on contract-oriented design of distributed systems. Chapter 1 introduces the CO2 contract specifi- cation language and the Diogenes toolchain. Chapter 2 describes how timing constraints can be incorporated into the framework and checked with the CO2 middleware. Part of the CO2 middleware is a monitoring system, and the theme of monitoring continues in the next two chapters. In Chapter 3, Attard et al. present detectEr, a runtime monitoring tool for Erlang programs that allows correctness properties to be expressed in Hennessy-Milner logic. In Chapter 4, which is the first chapter about session types, Neykova and Yoshida describe a runtime verification framework for Python programs. Communication protocols are specified in the Scribble language, which is based on multiparty session types. The next three chapters deal with choreographic programming. In Chap- ter 5, Debois and Hildebrandt present a toolset for working with dynamic condition response (DCR) graphs, which are a graphical formalism for choreography. Chapter 6, by Lange et al., continues the graphical theme with ChorGram, a tool for synthesising global graphical choreographies from collections of communicating finite-state automata. Giallorenzo et al., in Chapter 7, consider runtime adaptation. They describe AIOCJ, a choreographic programming language in which runtime adaptation is supported with a guarantee that it doesn’t introduce deadlocks or races. Deadlock analysis is important in other settings too, and there are two more chapters about it. In Chapter 8, Padovani describes the Hypha tool, which uses a type-based approach to check deadlock-freedom and lock-freedom of systems modelled in a form of pi-calculus. In Chapter 9, Garcia and Laneve present a tool for analysing deadlocks in Java programs; this tool, called JaDA, is based on a behavioural type system. The next three chapters report on projects that have added session types to functional programming languages in order to support typechecking of communication-based code. In Chapter 10, Orchard and Yoshida describe an implementation of session types in Haskell, and survey several approaches to typechecking the linearity conditions required for safe session implemen- tation. In Chapter 11, Melgratti and Padovani describe an implementation of session types in OCaml. Their system uses runtime linearity checking. In Chapter 12, Lindley and Morris describe an extension of the web programming language Links with session types; their work contrasts with the previous two chapters in being less constrained by an existing language design. Continuing the theme of session types in programming languages, the next two chapters describe two approaches based on Java. Hu’s work, presented in Chapter 13, starts with the Scribble description of a multiparty session type and generates an API in the form of a collection of Java classes, each class containing the communication methods that are available in a particular state of the protocol. Dardha et al., in Chapter 14, also start with a Scribble specification. Their StMungo tool generates an API as a single class with an associated typestate specification to constrain sequences of method calls. Code that uses the API can be checked for correctness with the Mungo typechecker. Finally, there are two chapters about programming with the MPI libraries. Chapter 15, by Ng and Yoshida, uses an extension of Scribble, called Pabble, to describe protocols that parametric in the number of runtime roles. From a Pabble specification they generate C code that uses MPI for communication and is guaranteed correct by construction. Chapter 16, by Ng et al., describes the ParTypes framework for analysing existing C+MPI programs with respect to protocols defined in an extension of Scribble. We hope that the book will serve a useful purpose as a report on the activities of COST Action IC1201 and as a survey of programming languages and tools based on behavioural types