Falafel: Arrays in a Functional Language

Various methods are used to describe communication protocols. The search for suitable description methods for communication protocols in the 1980s yielded a broad range of approaches out of which some fundamental methods have crystallized. These methods constitute the basis of the semantic models of the various formal description techniques. Therefore we first consider the most important description methods in this chapter before we turn to the formal description techniques. From these, finite state machines, Petri nets, process calculi, and temporal logics have crystallized as the most important and most common ones. Apart from these methods, other approaches were also investigated, such as the use of grammars, data flow languages, and functional languages. They did not receive wider attention because they were not applied to real-life protocols. The use of high-level programming languages was considered likewise. Programming languages support the algorithmic description of protocol procedures as well as the generation of prototypes, but they usually do not possess a formally defined semantics. In addition, programming languages are primarily implementation languages and not specification languages. They often yield descriptions which are close to an implementation. For that reason, high-level programming languages are not used for protocol description. Many formal description techniques, however, use programming language features.

Message Driven Computation (MDC) is a model of computation with which they have been experimenting at the Illinois Institute of Technology. The authors aim to prove the viability of MDC in practice for the expression of parallel algorithms and the implementation of functional and dataflow programming languages. In the paper they discuss their implementation of the Lucid programming language in MDC. The discussion presents a subset of Lucid which illustrates the principles of Lucid, Message Driven Computing, and the translation into and the interpretation of dataflow graphs. >

A framework for compiler construction of functional programming languages is described. The framework has been designed to allow experimenting with functional programming languages and their implementations. Its three main characteristics are: (1) given the existing implementation of a functional programming language (not necessary in a source code), the implementation of a new functional programming language can be easily and efficiently developed, using the framework; (2) the framework enables enhancement of already implemented languages with new features; and (3) implementations of the languages in the framework are transparent, which can be freely exchanged between the languages.

This thesis proposes a novel way of performing type checking, whose results are incremental, depending on the provided local information. This new way of type checking is called co-contextual, where all context information of expressions, methods, classes, etc., is removed. Instead, we introduce corresponding structures using requirements. Standard type systems are translated to the co-contextual ones systematically using dualism as technique. Type systems play an important role to prevent execution errors from occurring during runtime. They are used to check programs statically for potential errors. Programs are type checked against a given set of rules. Depending on these rules programs are well-typed or not. The set of these rules is called typing rules. Each type rule associates types to the constructs of a program given a certain context. There can be different forms of contexts, depending on the features of the typed programming language. Functional languages use a typing context of variables and their types; object-oriented (OO) languages use additional class tables. Class tables are used for example to ensure that method and class declarations are well-typed. Type checking is performed top-down. While traversing the syntax tree of a program, typing contexts are extended with information on the expressions and their types. In case of OO, class tables are extended with clauses from class declarations, including the class members, i.e., fields, methods, or constructors. Contexts are passed through the nodes of the syntax tree in order to coordinate type checking between them. Therefore, while traversing the syntax tree top-down, the type checker creates dependencies between otherwise independent subexpressions. This way, it inhibits incrementalization and parallelization of type checking. That is, a change to a node of the syntax tree would require to redo the type check of the whole syntax tree. In this thesis a novel formulation of type systems is proposed, in order to remove dependencies between subexpressions. We propose a co-contextual formulation of typing rules that depends only on the local program constructs, e.g., expressions, methods, classes. The co-contextual typing rules have as conclusion a type and sets of requirements. That is, contexts and class tables are replaced by the dual concept of context and class table requirements. In addition, operations on contexts and class tables are replaced by new dual operations on requirements. The co-contextual type checker traverses a syntax tree bottom-up and merges context requirements of independently checked subexpressions. We describe a method for systematically constructing a co-contextual formulation of type rules from a regular context-based formulation and we show how co-contextual type rules give rise to incremental type checking. We derive co-contextual type checkers for functional and OO languages. As a representative of functional languages we consider PCF and extensions of it: records, parametric polymorphism, structural subtyping and let-polymorphism. Also, we investigate featherweight java (FJ) as the basis of OO languages and extensions of it: method overloading and generics. We build a1 co-contextual type checker for FJ enabling key features of OO languages: subtype polymorphism, nominal typing and implementation inheritance. The dualism between the co-contextual and contextual type systems preserves the correctness of the contextual calculus. That is, we prove the correctness of the co-contextual calculus via the equivalence between contextual type rules and their co-contextual formulations. We implemented an incremental type checker for PCF along with a performance evaluation showing that co-contextual type checking has performance comparable to standard context-based type checking, and incrementalization can improve performance significantly. Regarding FJ, we implemented a co-contextual type checker with incrementalization and compared its performance against javac on a number of realistic programs. Our performance evaluation shows significant speedups for the co-contextual type checker with incrementalization in comparison to javac.

What will a definitive programming language look like? By definitive language I mean a programming language that gives good solutions at its level of abstraction, allowing computer science researchers to move on and work at higher levels. Given the evolution of computer science as a field with a rising level of abstraction, it is my belief that a small set of definitive languages will eventually exist. But how can we learn something about this set, considering that many basic questions about languages have not yet been settled? In this paper, I give some tentative conclusions about one definitive language. I present four case studies of substantial research projects that tackle important problems in four quite different areas: fault-tolerant programming, secure distributed programming, network-transparent distributed programming, and teaching programming as a unified discipline. All four projects had to think about language design. In this paper, I summarize the reasons why each project designed the language it did. It turns out that all four languages have a common structure. They can be seen as layered, with the following four layers in this order: a strict functional core, then deterministic concurrency, then message-passing concurrency, and finally shared-state concurrency (usually with transactions). This confirms the importance of functional programming and message passing as important defaults; however, global mutable state is also seen as an essential ingredient.KeywordsProgramming LanguageMutable StateLanguage DesignConcurrent ProgrammingFunctional LanguageThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Artificial Intelligence in the American Healthcare Industry: Looking Forward to 2030

In this thesis, we explore techniques for the development and verification of programs in a high-level, expressive, and safe programming language. Our programs can express problems over unbounded domains and over recursive and mutable data structures. We present an implementation language flexible enough to build interesting and useful systems. We mostly maintain a core shared language for the specifications and the implementation, with only a few extensions specific to expressing the specifications. Extensions of the core shared language include imperative features with state and side effects, which help when implementing efficient systems. Our language is a subset of the Scala programming language. Once verified, programs can be compiled and executed using the existing Scala tools. We present algorithms for verifying programs written in this language. We take a layer-based approach, where we reduce, at each step, the program to an equivalent program in a simpler language. We first purify functions by transforming away mutations into explicit return types in the functions' signatures. This step rewrites all mutations of data structures into cloning operations. We then translate local state into a purely functional code, hence eliminating all traces of imperative programming. The final language is a functional subset of Scala, on which we apply verification. We integrate our pipeline of translations into Leon, a verifier for Scala. We verify the core functional language by using an algorithm already developed inside Leon. The program is encoded into equivalent first-order logic formulas over a combination of theories and recursive functions. The formulas are eventually discharged to an external SMT solver. We extend this core language and the solving algorithm with support for both infinite-precision integers and bit-vectors. The algorithm takes into account the semantics gap between the two domains, and the programmer is ultimately responsible to use the proper type to represent the data. We build a reusable interface for SMT-LIB that enables us to swap solvers transparently in order to validate the formulas emitted by Leon. We experiment with writing solvers in Scala; they could offer both a better and safer integration with the rest of the system. We evaluate the cost of using a higher-order language to implement such solvers, traditionally written in C/C++. Finally, we experiment with the system by building fully working and verified applications. We rely on the intersection of many features including higher-order functions, mutable data structures, recursive functions, and nondeterministic environment dependencies, to build concise and verified applications.

Object-oriented programming languages are suitable for describing real-world objects, functional programming languages for algebraic values. In this paper we propose an object-oriented functional programming language, called TOFL, which combines many desired properties of the two paradigms. In particular, TOFL unifies object classes, inheritance, (method) redefinitions and late binding as in object-oriented languages, algebraic data types, higher-order functions, type classes and type inference (without type reconstruction in this paper) as in functional languages. We translate TOFL into a stratified and explicitly typed λ-calculus T with overloaded functions, where redefinitions and late binding become late binding of overloaded functions. The operational semantics of T gives a semantics and a simple prototyping implementation of TOFL.

I will discuss the design of Ct, an API for nested data parallel programming in C++. Ct uses meta-programming and functional language ideas to essentially embed a pure functional programming language in impure and unsafe languages, like C++. I will discuss the evolution of the design into functional programming ideas, how this was received in the corporate world, and how we plan to proliferate the technology in the next year.Ct is a deterministic parallel programming model integrating the nested data parallelism ideas of Blelloch and bulk synchronous processing ideas of Valiant. That is, data races are not possible in Ct. Moreover, performance in Ct is relatively predictable. At its inception, Ct was conceived as a simple library implementation behind C++ template magic. However, performance issues quickly forced us to consider some form of compilation. Using template programming was highly undesirable for this purpose as it would have been difficult and overly specific to C++ idiosyncrasies. Moreover, once compilation for performance was considered, we began to consider a language semantics that would enable powerful optimizations like calculational fusion, synchronization barrier elimination, and so on. The end result of this deliberation is an API that exposes a value-oriented, purely functional vector processing language. Additional benefits of this approach are numerous, including the important ability to co-exist within legacy threading programming models (because of the data isolation inherent in the model). We will show how the model applies to a wide range of important (at least by cycle count) applications. Ct targets both shipping multi-core architectures from Intel as well as future announced architectures.The corporate reception to this approach has (pleasantly) surprised us. In the desktop and high-performance computing space, where C, C++, Java, and Fortran are the only programming models people talk about, we have made serious inroads into advocating advanced programming language technologies. The desperate need for productive, scalable, and safe programming languages for multi-core architectures has provided an opening for functional, type-safe languages. We will discuss the struggles of multi-core manufacturers (i.e. Intel) and their software vendors that have created this opening.For Intel, Ct heralds its first serious effort to champion a technology that borrows functional programming technologies from the research community. Though it is a compromise that accommodates the pure in the impure and safe in the unsafe, this is an important opportunity to demonstrate the power of functional programming to the unconverted. We plan to share the technology selectively with partners and collaborators, and will have a fully functional and parallelizing implementation by year's end. At CUFP, we will be prepared to discuss our long term plans in detail.

Functional programming languages are well-suited for developing compilers, and compilers for functional languages are often themselves written in a functional language. Functional abstractions, such as monads, allow abstracting away some of the repetitive structure of a compiler, removing boilerplate code and making extensions simpler. Even so, functional languages are rarely used to implement compilers for languages of other paradigms.

This thesis presents a refinement calculus for transforming highly abstract specifications into programs written in a functional programming language. Refinement calculi allow the derivation of a program from a formal specification by a sequence of correctness-preserving transformation steps. Most refinement calculi target imperative programming languages. Functional programming languages are often more expressive than imperative programming languages. This expressive power leads to a reduced gap between the concepts used to express a problem (as a specification) and the concepts used to express its solution (as a program). Thus, a refinement calculus which targets a functional programming language can lead to a simpler development process which produces a final product more quickly. We build a refinement calculus for functional programs by adding specification constructs to a functional language and examining transformations over the resulting language.This thesis complements other work in the area in two major ways. Firstly, we investigate the addition of truly-nondeterministic, rather than underdetermined, choice constructs to a functional language. These constructs allow specifications which are more abstract and which admit more implementations. Secondly, most refinement calculi for functional programs add erratic choice constructs to a functional language. We investigate the addition of both demonic and angelic nondeterminism to a functional language. Demonic and erratic choice are similar: given a number of alternatives, they choose any one. In contrast, given a number of alternatives angelic choice always makes the correct choice, if one exists. Angelic choice is difficult to reason about, but allows the concise specification of powerful parallel constructs.

The 1990s saw a hugely productive interaction between database and programming language research. Ideas about type systems from programming languages played a central role in generalizing and adapting relational database systems to new data models. At the same time databases provided some of the best concrete examples of the application of concurrency theory and of the benefits of high-level optimization in functional programming languages. One of the driving ambitions behind this research was the idea that database access should be properly embedded in programming languages: one should not have to be bilingual in order to use a database from a programming language; and that goal has to some extent been realized. In the past fifteen years, new data models, both for data storage and for data exchange have appeared with depressing regularity and with each such model, the inevitable query language. Does programming language research have anything to contribute to these new languages? Should we take the time to to worry about embedding these models in conventional languages? Over the same period, some interesting new connections between databases and programming languages have emerged, notably in the areas of scientific databases, annotation and provenance. Will this provide new opportunities for cross-fertilization?

Removing partial parametrization for efficient implementation of functional languages

It is well known that modern functional programming languages are naturally amenable to parallel programming. Achieving efficient parallelism using functional languages, however, remains difficult. Perhaps the most important reason for this is their lack of support for efficient in-place updates, i.e., mutation, which is important for the implementation of both parallel algorithms and the run-time system services (e.g., schedulers and synchronization primitives) used to execute them. In this paper, we propose techniques for efficient mutation in parallel functional languages. To this end, we couple the memory manager with the thread scheduler to make reading and updating data allocated by nested threads efficient. We describe the key algorithms behind our technique, implement them in the MLton Standard ML compiler, and present an empirical evaluation. Our experiments show that the approach performs well, significantly improving efficiency over existing functional language implementations.

It is well known that modern functional programming languages are naturally amenable to parallel programming. Achieving efficient parallelism using functional languages, however, remains difficult. Perhaps the most important reason for this is their lack of support for efficient in-place updates, i.e., mutation, which is important for the implementation of both parallel algorithms and the run-time system services (e.g., schedulers and synchronization primitives) used to execute them. In this paper, we propose techniques for efficient mutation in parallel functional languages. To this end, we couple the memory manager with the thread scheduler to make reading and updating data allocated by nested threads efficient. We describe the key algorithms behind our technique, implement them in the MLton Standard ML compiler, and present an empirical evaluation. Our experiments show that the approach performs well, significantly improving efficiency over existing functional language implementations.