A Timed IO monad

Programming with explicit timing information is often tedious and error prone. This is especially visible in music programming where, when played, the specified durations of notes and rests must be shortened in order to compensate the actual duration of all surrounding processing. In this paper, we develop the notion of timed extension of a monad that aims at relieving programmers from such a burden. We show how, under simple conditions, such extensions can be built, and how useful features of monad programming such as asynchronous concurrency with promises or data-flow programming with monadic streams can be uniformly lifted to the resulting timed programming framework. Even though presented and developed in the abstract, the notion of timed extension of a monad is nevertheless illustrated by two concrete instances: a default timed IO monad where programmers specify durations in mi-croseconds, and a musically timed IO monad, where programmers specify durations in number of beats, the underlying tempo, that is, the speed of the music in beats per minute, possibly changed whenever needed

Teaching how to program using automated assessment and functional glossy games (Experience Report)

Our department has long been an advocate of the functional-first school of programming and has been teaching Haskell as a first language in introductory programming course units for 20 years. Although the functional style is largely beneficial, it needs to be taught in an enthusiastic and captivating way to fight the unusually high computer science drop-out rates and appeal to a heterogeneous population of students.This paper reports our experience of restructuring, over the last 5 years, an introductory laboratory course unit that trains hands-on functional programming concepts and good software development practices. We have been using game programming to keep students motivated, and following a methodology that hinges on test-driven development and continuous bidirectional feedback. We summarise successes and missteps, and how we have learned from our experience to arrive at a model for comprehensive and interactive functional game programming assignments and a general functionally-powered automated assessment platform, that together provide a more engaging learning experience for students. In our experience, we have been able to teach increasingly more advanced functional programming concepts while improving student engagement.The authors would like to thank the precursors of the 20-year functional programming culture and FPro unit at our university, and all the instructors and TAs that have been involved in the PLab unit throughout the years. This work is financed by the ERDFs European Regional Development Fund through the Operational Programme for Competitiveness and Internationalisation - COMPETE 2020 Programme within project POCI-01-0145-FEDER-006961, and by National Funds through the Portuguese funding agency, FCT s Fundacao para a Ciencia e a Tecnologia as part of project UID/EEA/50014/2013

Programming with monadic CSP-style processes in dependent type theory

We introduce a library called CSP-Agda for representing processes in the dependently typed theorem prover and interactive programming language Agda. We will enhance processes by a monad structure. The monad structure facilitates combining processes in a modular way, and allows to define recursion as a direct operation on processes. Processes are defined coinductively as non-well-founded trees. The nodes of the tree are formed by a an atomic one step relation, which determines for a process the external, internal choices, and termination events it can choose, and whether the process has terminated. The data type of processes is inspired by Setzer and Hancock's notion of interactive programs in dependent type theory. The operators of CSP will be defined rather than atomic operations, and compute new elements of the data type of processes from existing ones. The approach will make use of advanced type theoretic features: the use of inductive-recursively defined universes; the definition of coinductive types by their observations, which has similarities to the notion of an object in object-oriented programming; the use of sized types for coinductive types, which allow coinductive definitions in a modular way; the handling of finitary information (names of processes) in a coinductive settings; the use of named types for automatic inference of arguments similar to its use in template Meta-programming in C++; and the use of interactive programs in dependent type theory.We introduce a simulator as an interactive program in Agda. The simulator allows to observe the evolving of processes following external or internal choices. Our aim is to use this in order to simulate railway interlocking system and write programs in Agda which directly use CSP processes

Pure functional epidemics

Agent-Based Simulation (ABS) is a methodology in which a system is simulated in a bottom-up approach by modelling the micro interactions of its constituting parts, called agents, out of which the global system behaviour emerges. So far mainly object-oriented techniques and languages have been used in ABS. Using the SIR model of epidemiology, which simulates the spreading of an infectious disease through a population, we demonstrate how to use pure Functional Reactive Programming to implement ABS. With our approach we can guarantee the reproducibility of the simulation at compile time and rule out specific classes of run-time bugs, something that is not possible with traditional object-oriented languages. Also, we found that the representation in a purely functional format is conceptually quite elegant and opens the way to formally reason about ABS

Implementation of an orchestration language as a haskell domain specific language

Even though concurrent programming has been a hot topic of discussion in Computer Science for the past 30 years, the community has yet to settle on a, or a few standard approaches to implement concurrent programs. But as more and more cores inhabit our CPUs and more and more services are made available on the web the problem of coordinating different tasks becomes increasingly relevant. The present paper addresses this problem with an implementation of the orchestration language Orc as a domain specific language in Haskell. Orc was, therefore, realized as a combinator library using the lightweight threads and the communication and synchronization primitives of the Concurrent Haskell library. With this implementation it becomes possible to create orchestrations that re-use existing Haskell code and, conversely, re-use orchestrations inside other Haskell programs. The complexity inherent to distributed computation, entails the need for the classification of efficient, re-usable, concurrent programming patterns. The paper discusses how the calculus of recursive schemes used in the derivation of functional programs, scales up to a distributed setting. It is shown, in particular, how to parallelize the entire class of binary tree hylomorphisms.FCT -Fuel Cell Technologies Program(PTDC/EIA/73252/2006

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

Temporal semantics for a live coding language

Sonic Pi is a music live coding language that has been designed for educational use as a first programming language. However, it is not straightforward to achieve the necessary simplicity of a first language in a music live coding setting, for reasons largely related to the manipulation of time. The original version of Sonic Pi used a `sleep' function for managing time, blocking computation for a specified time period. However, while this approach was conceptually simple, it resulted in badly timed music, especially when multiple musical threads were executing concurrently. This paper describes an alternative programming approach for timing (implemented in Sonic Pi v2.0) which maintains syntactic compatibility with v1.0, yet provides accurate timing via interaction between real time and a "virtual time''. We provide a formal specification of the temporal behaviour of Sonic Pi, motivated in relation to other recent approaches to the semantics of time in live coding and general computation. We then define a monadic model of the Sonic Pi temporal semantics which is sound with respect to this specification, using Haskell as a metalanguage

Behavioural Types

Behavioural type systems in programming languages support the specification and verification of properties of programs beyond the traditional use of type systems to describe data processing. A major example of such a property is correctness of communication in concurrent and distributed systems, motivated by the importance of structured communication in modern software. Behavioural Types: from Theory to Tools presents programming languages and software tools produced by members of COST Action IC1201: Behavioural Types for Reliable Large-Scale Software Systems, a European research network that was funded from October 2012 to October 2016. As a survey of the most recent developments in the application of behavioural type systems, it is a valuable reference for researchers in the field, as well as an introduction to the area for graduate students and software developers