Run-time Type Research Articles

Block Structure and Object-Oriented Languages

Programming languages that support the object-oriented perspective on programming may be divided into two groups. One group of languages originating from Simula follows the Algol tradition with respect to block structure, static name binding and compile-time type checking. Another group of languages originating from Smalltalk is more in the style of the Lisp tradition with a flat set of definitions (classes), dynamic name binding and run-time type checking. The purpose of this paper is to analyze the role of block structure in object-oriented languages. It will be demonstrated that block structure is useful from both a conceptual and technical viewpoint.

Tags and type checking in LISP: hardware and software approaches

One of the major factors that distinguishes LISP from many other languages (Pascal, C, Fortran, etc.) is the need for run-time type checking. Run-time type checking is implemented by adding to each data object a tag that encodes type information. Tags must be compared for type compatibility, removed when using the data, and inserted when new data items are created. This tag manipulation, together with other work related to dynamic type checking and generic operations, constitutes a significant component of the execution time of LISP programs. This has led both to the development of LISP machines that support tag checking in hardware and to the avoidance of type checking by users running on stock hardware. To understand the role and necessity of special-purpose hardware for tag handling, we first measure the cost of type checking operations for a group of LISP programs. We then examine hardware and software implementations of tag operations and estimate the cost of tag handling with the different tag implementation schemes. The data shows that minimal levels of support provide most of the benefits, and that tag operations can be relatively inexpensive, even when no special hardware support is present.

Tags and type checking in LISP: hardware and software approaches

One of the major factors that distinguishes LISP from many other languages (Pascal, C, Fortran, etc.) is the need for run-time type checking. Run-time type checking is implemented by adding to each data object a tag that encodes type information. Tags must be compared for type compatibility, removed when using the data, and inserted when new data items are created. This tag manipulation, together with other work related to dynamic type checking and generic operations, constitutes a significant component of the execution time of LISP programs. This has led both to the development of LISP machines that support tag checking in hardware and to the avoidance of type checking by users running on stock hardware. To understand the role and necessity of special-purpose hardware for tag handling, we first measure the cost of type checking operations for a group of LISP programs. We then examine hardware and software implementations of tag operations and estimate the cost of tag handling with the different tag implementation schemes. The data shows that minimal levels of support provide most of the benefits, and that tag operations can be relatively inexpensive, even when no special hardware support is present.

Tags and type checking in LISP: hardware and software approaches

One of the major factors that distinguishes LISP from many other languages (Pascal, C, Fortran, etc.) is the need for run-time type checking. Run-time type checking is implemented by adding to each data object a tag that encodes type information. Tags must be compared for type compatibility, removed when using the data, and inserted when new data items are created. This tag manipulation, together with other work related to dynamic type checking and generic operations, constitutes a significant component of the execution time of LISP programs. This has led both to the development of LISP machines that support tag checking in hardware and to the avoidance of type checking by users running on stock hardware. To understand the role and necessity of special-purpose hardware for tag handling, we first measure the cost of type checking operations for a group of LISP programs. We then examine hardware and software implementations of tag operations and estimate the cost of tag handling with the different tag implementation schemes. The data shows that minimal levels of support provide most of the benefits, and that tag operations can be relatively inexpensive, even when no special hardware support is present.

Support for dynamic binding in strongly typed languages

Dynamic binding is a facility usually found only in weakly typed languages. A strongly typed, block-structured language can, however, also support dynamic binding if its implementation is modified according to a set of widely applicable principles. The result of applying these principles to a strongly typed base language is a language that has much of the flexibility of weakly typed languages without requiring extensive run-time type checking.The primary principles in this general approach to adding dynamic binding to a language are providing separate compilation for the units of dynamic binding, establishing a link between a name in one unit and its implementation in another, and assuring that programs using dynamic binding remain consistent during their execution. There are different methods for implementing each of these principles; two languages to which dynamic binding has been added serve to illustrate some of the alternatives.

Type-checking Smalltalk

Although most attempts to speed-up Smalltalk have focused on providing more efficient interpreters, code optimization is probably necessary for further increases in speed. A type-system for Smalltalk is a prerequisite for building an optimizing compiler. Unfortunately, none of the type-systems so far proposed for Smalltalk are adequate; they either cause nearly all Smalltalk programs to be type incorrect, allow run-time type errors, or do not provide enough information for optimization. This paper presents a type-system for Smalltalk that is suitable for code optimization.

Block structure and object oriented languages

Programming languages that support the object-oriented perspective on programming may be divided into two groups. One group of languages originating from Simula follows the Algol tradition with respect to block structure, static name binding and compile-time type checking. Another group of languages originating from Smalltalk is more in the style of the Lisp tradition with a flat set of definitions (classes), dynamic name binding and run-time type checking. The purpose of this paper is to analyze the role of block structure in object-oriented languages. It will be demonstrated that block structure is useful from both a conceptual and technical viewpoint.

A language-oriented instruction set for the BALM language

The availability of microprogramming has increased interest in the design of instruction sets which are oriented towards particular higher-level languages (see (1), for example). We will refer to these as language-oriented instruction sets (or LOISs). In this paper we give a formal definition of a LOIS for the extendable language BALM. This LOIS, which we call MBALM, is used as the basis of the implementation of the current version of the language, BALM4. We will also compare two implementations of the MBALM instruction set, and outline briefly our experiences with them. The external description of BALM4 is given in detail elsewhere (2,3). Briefly, it is an extendable language with an Algol-like syntax and data-types integer, real, logical, string, pair (list), vector, identifier, procedure, and label. Type information is carried with all data-objects, and run-time type testing and conversion is done when necessary. Garbage collection and a run-time compiler are significant attributes of the system. Outlined below are some of the properties of the implementation, which was designed initially for reasons of simplicity and flexibility, but which illustrate some of the advantages of a LOIS-based system.