Matthew Flatt

Classes and mixins

While class-based object-oriented programming languages provide a flexible mechanism for re-using and managing related pieces of code, they typically lack linguistic facilities for specifying a uniform extension of many classes with one set of fields and methods. As a result, programmers are unable to express certain abstractions over classes.In this paper we develop a model of class-to-class functions that we refer to as mixins. A mixin function maps a class to an extended class by adding or overriding fields and methods. Programming with mixins is similar to programming with single inheritance classes, but mixins more directly encourage programming to interfaces.The paper develops these ideas within the context of Java. The results are 1. an intuitive model of an essential Java subset; 2. an extension that explains and models mixins; and 3. type soundness theorems for these languages.

Units: cool modules for HOT languages

A module system ought to enable assembly-line programming using separate compilation and an expressive linking language. Separate compilation allows programmers to develop parts of a program independently. A linking language gives programmers precise control over the assembly of parts into a whole. This paper presents models of program units, MzSchemes module language for assembly-line programming. Units support separate compilation, independent module reuse, cyclic dependencies, hierarchical structuring, and dynamic linking. The models explain how to integrate units with untyped and typed languages such as Scheme and ML.

DrScheme: a programming environment for Scheme

DrScheme is a programming environment for Scheme. It fully integrates a graphics-enriched editor, a parser for multiple variants of Scheme, a functional read-eval-print loop, and an algebraic printer. The environment is especially useful for students, because it has a tower of syntactically restricted variants of Scheme that are designed to catch typical student mistakes and explain them in terms the students understand. The environment is also useful for professional programmers, due to its sophisticated programming tools, such as the static debugger, and its advanced language features, such as units and mixins. Beyond the ordinary programming environment tools, DrScheme provides an algebraic stepper, a context-sensitive syntax checker, and a static debugger. The stepper reduces Scheme programs to values, according to the reduction semantics of Scheme. It is useful for explaining the semantics of linguistic facilities and for studying the behavior of small programs. The syntax checker annotates programs with font and color changes based on the syntactic structure of the program. On demand, it draws arrows that point from bound to binding occurrences of identifiers. It also supports α-renaming. Finally, the static debugger provides a type inference system that explains specific inferences in terms of a value-flow graph, selectively overlaid on the program text.

Jiazzi: new-age components for old-fasioned Java

We present Jiazzi, a system that enables the construction of large-scale binary components in Java. Jiazzi components can be thought of as generalizations of Java packages with added support for external linking and separate compilation. Jiazzi components are practical becuase they are constructed out of standard Java source code. Jiazzi requires neither extensions to the Java language nor special conventions for writing Java source code that will go inside a component. Our components are expressive becuase Jiazzi supports cyclic component linking and mixins, which are used together in an open class pattern that enables the modular addition of new features to existing classes. This paper describes Jiazzi, how it enhances Java with components, its implementation, and how type checking works. An implementation of Jiazzi is available for download.

Modular object-oriented programming with units and mixins

Module and class systems have evolved to meet the demand for reuseable software components. Considerable effort has been invested in developing new module and class systems, and in demonstrating how each promotes code reuse. However, relatively little has been said about the interaction of these constructs, and how using modules and classes together can improve programs. In this paper, we demonstrate the synergy of a particular form of modules and classes---called units and mixins, respectively---for solving complex reuse problems in a natural manner.

A Programmer's Reduction Semantics for Classes and Mixins

While class-based object-oriented programming languages provide a flexible mechanism for re-using and managing related pieces of code, they typically lack linguistic facilities for specifying a uniform extension of many classes with one set of fields and methods. As a result, programmers are unable to express certain abstractions over classes. In this paper we develop a model of class-to-class functions that we refer to as mixins. A mixin function maps a class to an extended class by adding or overriding fields and methods. Programming with mixins is similar to programming with single inheritance classes, but mixins more directly encourage programming to interfaces. The paper develops these ideas within the context of Java. The results are 1. an intuitive model of an essential Java subset; 2. an extension that explains and models mixins; and 3. type soundness theorems for these languages.

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 Lisps 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.

Composable and compilable macros:: you want it when?

Many macro systems, especially for Lisp and Scheme, allow macro transformers to perform general computation. Moreover, the language for implementing compile-time macro transformers is usually the same as the language for implementing run-time functions. As a side effect of this sharing, implementations tend to allow the mingling of compile-time values and run-time values, as well as values from separate compilations. Such mingling breaks programming tools that must parse code without executing it. Macro implementors avoid harmful mingling by obeying certain macro-definition protocols and by inserting phase-distinguishing annotations into the code. However, the annotations are fragile, the protocols are not enforced, and programmers can only reason about the result in terms of the compilers implementation. MzScheme---the language of the PLT Scheme tool suite---addresses the problem through a macro system that separates compilation without sacrificing the expressiveness of macros.

Catching bugs in the web of program invariants

MrSpidey is a user-friendly, interactive static debugger for Scheme. A static debugger supplements the standard debugger by analyzing the program and pinpointing those program operations that may cause run-time errors such as dereferencing the null pointer or applying non-functions. The program analysis of MrSpidey computes value set descriptions for each term in the program and constructs a value flow graph connecting the set descriptions. Using the set descriptions, MrSpidey can identify and highlight potentially erroneous program operations, whose cause the programmer can then explore by selectively exposing portions of the value flow graph.

DrScheme: A Pedagogic Programming Environment for Scheme

Teaching introductory computing courses with Scheme elevates the intellectual level of the course and thus makes the subject more appealing to students with scientific interests. Unfortunately, the poor quality of the available programming environments negates many of the pedagogic advantages. To overcome this problem, we have developed DrScheme, a comprehensive programming environment for Scheme. It fully integrates a graphics-enriched editor, a multi-lingual parser that can process a hierarchy of syntactically restrictive variants of Scheme, a functional read-eval-print loop, and an algebraically sensible printer. The environment catches the typical syntactic mistakes of beginners and pinpoints the exact source location of run-time exceptions.