An approach for facilitating mixed language programming in distributed systems is presented. It is based on adding a generic remote procedure call facility to each language, and the use of a type system to describe procedural interfaces, as well as data to be transferred between procedures. This type scheme also specifies a machine-independent representation for all data. By defining standard mappings for each programming language, the data conversions required for cross-langauge calls may be performed, automatically in most cases, by active agents that provide the interface between program components written in different languages. When necessary, explicit control of the conversation is possible. A prototype implementation of a system based on this approach has been constructed on a collection of machines running Berkeley UNIX®.

Facilitating Mixed Language Programming in Distrbuted Systems

Subroutines for numerical computation have in the past been written mainly in Fortran and in Algol 60, whereas most programming in the future is expected to be in more advanced languages, for example in Ada. It seems to be a great waste if it will become necessary to convert all existing numerical software into the new languages. What is needed is a facility to include subroutines written in the old languages into programs being written in the newer languages.



In this paper an automatic procedure to facilitate the use of mixed languages is suggested, the purpose of the paper is to indicate the desirability for providing the required facilities, and to encourage discussion and further research in the area of mixed language programming.

Mixed language programming

Most common object models of distributed object systems have a limited set of object-oriented features, lacking the advanced features of ‘polymorphism’ to assume different forms) and ‘concurrency’ (the ability to have more than one thread of execution in an object simultaneously). The lack of support, for advanced features is a serious limitation because it restricts the development of new components and limits reuse of existing of components that use these advanced features. As a result, wrappers must be used that hide the advanced features or components must be re-implemented using only the features of the common object model. 
In this dissertation, a new direction of research centered on a subset of object-oriented languages, specifically statically typed languages, is considered. One of the major drawbacks of existing distributed object systems is that they cater to a broad domain of programming languages including both object-oriented as well as non object-oriented languages. Mapping an object model into a non object-oriented language is a complex task and it does not appear natural to a native language user. 
The interoperable common object model (ICOM) proposed in this dissertation is an attempt to elevate common object models (with the advanced features of polymorphism and concurrency) closer to the object models of statically typed object-oriented languages. Specific features of the ICOM object model include: remote inheritance, method overloading, parameterized types, and guard methods. The actor model and reflection techniques are used to develop a uniform implementation framework for the ICOM object model in C++ and Modula-3. Prototype applications were implemented to demonstrate the utility of the advanced features of the ICOM object model. 
The main contributions of this dissertation are: design and implementation of a powerful common object model, an architecture for distributed compilation, and an implementation of a distributed object model using the actor model.

Improving polymorphism and concurrency in common object models

The importance of mixed language programming, and some of the associated problems, are summarized. The paper presents a structured view on the major ways to realize mixed language programming and how to provide the necessary data types.

Mixed language programming realization and the provision of data types

A multi-language-based data interface system for heterogeneous distributed processing is introduced. A prototyped environment based on this system is discussed, and an evaluation of the prototyped system is presented. It is shown that by keeping the syntax of the specification language flexible and close to existing high-level languages, a user can learn the interface language quickly. Semantically, this data interface views structured data as consisting of two parts: the data values themselves and the representation of the structure among the data values. Through this separation, it is possible to have pipelined data type checking and data conversion operations. >

An efficient data interface for heterogeneous distributed environments

As a support for writing software, a comprehensive set of problem oriented languages appears preferable to any so-called universal language, as soon as static checking is sufficient to ensure type correctness of the mixed language program. We lay the basis for a mixed language system where this requirement is fulfilled. The general outline of the system is first sketched. Detailed consideration is then given to our basic constructs for establishing communication between languages, namely “standard” types and “foreign” types. “Abstract” types, such as defined in CLU, are finally shown to be a particular class of “foreign” types.

Ph. Darondeau

Papers

Types in a mixed language system