We review a model of computation used in industrial practice in signal processing software environments and experimentally in other contexts. We give this model the name "dataflow process networks," and study its formal properties as well as its utility as a basis for programming language design. Variants of this model are used in commercial visual programming systems such as SPW from the Alta Group of Cadence (formerly Comdisco Systems), COSSAP from Synopsys (formerly Cadis), the DSP Station from Mentor Graphics, and Hypersignal from Hyperception. They are also used in research software such as Khoros from the University of New Mexico and Ptolemy from the University of California at Berkeley, among many others. Dataflow process networks are shown to be a special case of Kahn process networks, a model of computation where a number of concurrent processes communicate through unidirectional FIFO channels, where writes to the channel are non-blocking, and reads are blocking. In dataflow process networks, each process consists of repeated "firings" of a dataflow "actor". An actor defines a (often functional) quantum of computation. By dividing processes into actor firings, the considerable overhead of context switching incurred in most implementations of Kahn process networks is avoided. We relate dataflow process networks to other dataflow models, including those used in dataflow machines, such as static dataflow and the tagged-token model. We also relate dataflow process networks to functional languages such as Haskell, and show that modern language concepts such as higher-order functions and polymorphism can be used effectively in dataflow process networks. A number of programming examples using a visual syntax are given. This research is part of the Ptolemy project, whi...

. Constraints allow programmers and users to state declaratively a relation that should be maintained, rather than requiring them to write procedures to maintain the relation themselves. They are thus useful in such applications as programming languages, user interface toolkits, and simulation packages. In many situations, it is desirable to be able to state both required and preferential constraints. The required constraints must hold. Since the other constraints are merely preferences, the system should try to satisfy them if possible, but no error condition arises if it cannot. A constraint hierarchy consists of a set of constraints, each labeled as either required or preferred at some strength. An arbitrary number of different strengths is allowed. In the discussion of a theory of constraint hierarchies, we present alternate ways of selecting among competing possible solutions, and prove a number of propositions about the relations among these alternatives. We then outline algorit...

This paper studies the semantics of hierarchical finite state machines (FMS's) that are composed using various concurrency models, particularly dataflow, discrete-events, and synchronous/reactive modeling. It is argued that all three combinations are useful, and that the concurrency model can be selected independently of the decision to use hierarchical FSM's. In contrast, most formalisms that combine FSM's with concurrency models, such as Statecharts (and its variants) and hybrid systems, tightly integrate the FSM semantics with the concurrency semantics. An implementation that supports three combinations is described.

Focus is a framework for the systematic formal specification and development of distributed interactive systems and their components. Focus provides models, formalisms and verification calculi for the stepwise specification and development, transformation and verification of such systems. Focus aims at the modular development and implementation of distributed interactive systems through several abstraction levels by stepwise refinement. 1 Chapter 1 Methods for System Development A (distributed) system consists of a family of interacting, conceptually or spatially distributed components. A system development method provides a framework for organizing the stepwise construction of such systems. During the development process several descriptions are produced, that reflect different abstraction levels. Only if formal techniques are used these descriptions can be made as precise and unambiguous as necessary. Moreover, formal techniques allow to establish formal relationships between des...

This paper addresses the design of reactive real-time embedded systems. Such systems are often heterogeneous in implementation technologies and design styles, for example by combining hardware ASICs with embedded software. The concurrent design process for such embedded systems involves solving the specification, validation, and synthesis problems. We review the variety of approaches to these problems that have been taken.

Although detractors of functional programming sometimes claim that functional programming is too difficult or counterintuitive for most programmers to understand and use, evidence to the contrary can be found by looking at the popularity of spreadsheets. The spreadsheet paradigm, a first-order subset of the functional programming paradigm, has found wide acceptance among both programmers and end users. Still, there are many limitations with most spreadsheet systems.

Abstract. Many developments have taken place within dataflow programming languages in the past decade. In particular, there has been a great deal of activity and advancement in the field of dataflow visual programming languages. The motivation for this article is to review the content of these recent developments and how they came

The foundations of functional programming languages are examined from both historical and technical perspectives. Their evolution is traced through several critical periods: early work on lambda calculus and combinatory calculus, Lisp, Iswim, FP, ML, and modern functional languages such as Miranda ’ and Haskell. The fundamental premises on which

Constraints are declarative statements of relations among elements of the language's computational domain, e.g., integers, booleans, strings, and other objects. Orthogonally, the tools of object-oriented programming, including encapsulation, inheritance, and dynamic message binding, provide important mechanisms for extending a language's domain. Although the integration of constraints and objects seems obvious and natural, one basic obstacle stands in the way: objects provide a new, larger, computational domain, which the language's embedded constraint solver must accommodate. In this paper we list some goals and non-goals for an integration of constraints and object oriented language features, outline previous approaches to this integration, and describe the scheme we use in Kaleidoscope'91, our objectoriented constraint imperative programming language. Kaleidoscope'91 uses a class-based object model, multi-methods, and constraint constructors to integrate cleanly the encapsulation an...

Two major paradigms in computer programming languages are imperative and declarative programming. We describe a scheme for languages that integrate specific features from these two paradigms into a new framework: Constraint Imperative Programming. Along with the framework, we discuss the design and implementation of a particular instance of this framework, Kaleidoscope'90. From the imperative paradigm, constraint imperative programming adopts explicit control flow, state, and assignment. From the declarative paradigm, it adopts explicit, systemmaintained constraints (relations that should hold). There is a strong practical motivation for making this integration: in a typical application, some portions are most clearly described using imperative constructs, while other portions are most clearly described using constraints. By using a constraint imperative language, the most suitable paradigm can be used as appropriate. 1 Introduction We propose to combine two simple techniques--- decl...