Introducing Ada 95

Peter Kitson

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Copyright © John Barnes

1 Evolution of Ada 95

Ada is a modern programming language suitable for those application areas which benefit from the discipline of organized development, that is, Software Engineering ; it is a general purpose language with special applicability to real-time and embedded systems. Ada was originally developed by an international design team in response to requirements issued by the United States Department of Defense [DoD 78].

Ada 95 is a revised version of Ada updating the 1983 ANSI Ada standard [ANSI 83] and the equivalent 1987 ISO standard [ISO 87] in accordance with ANSI and ISO procedures. (ANSI is the American National Standards Institute and ISO is the International Standards Organization.) This present document outlines the overall Rationale for the revision and illustrates the main new features.

1.1 The Revision Process

Although Ada was originally designed to provide a single flexible yet portable language for real-time embedded systems to meet the needs of the US DoD, its domain of application has expanded to include many other areas, such as large-scale information systems, distributed systems, scientific computation, and systems programming. Furthermore, its user base has expanded to include all major defense agencies of the Western world, the whole of the aerospace community and increasingly many areas in civil and private sectors such as telecommunications, process control and monitoring systems. Indeed, the expansion in the civil sector is such that civil applications now generate the dominant revenues of many vendors.

After some years of use and many implementations, a decision was made in 1988 to undertake a revision to Ada, leading to an updated ANSI/ISO standard. It is normal practice to automatically reconsider such standards every 5 or 10 years and to determine whether they should be abandoned, reaffirmed as they are, or updated. In the case of Ada, an update was deemed appropriate.

To understand the process it should be explained that ANSI, as the US national standards body, originally proposed that Ada become an ISO standard. It is normal practice that ANSI, as the originating body, should sponsor a revised standard as necessary. The US DoD acted as the agent of ANSI in managing the development of the revised standard. Within the US DoD, the Ada Joint Program Office (AJPO) is responsible for Ada and the Ada Board is a federal advisory committee which advises the AJPO.

The revision effort began in January 1988 when the Ada Board was asked to prepare a recommendation to the AJPO on the most appropriate standardization process to use in developing a revised Ada standard, known in the interim as Ada 9X. The recommendation [DoD 88] was delivered in September 1988 to Virginia Castor, the then Director of the Ada Joint Program Office, who subsequently established the Ada 9X Project for conducting the revision of the Ada Standard. Christine M Anderson was appointed Project Manager in October 1988. Close consultation with ISO was important to ensure that the needs of the whole world-wide Ada community (and not just the defense community) were taken into account and to ensure the timely adoption by ISO of the new standard. Accordingly a Memorandum of Understanding was established between the US DoD and ISO [ISO 90].

The Ada 9X program consists of three main phases: the determination of the Requirements for the revised language; the actual Development of the definition of the revised language; and the Transition into use from Ada 83 to Ada 9X.

The output of the Requirements Definition phase was the Ada 9X Requirements document [DoD 90], which specified the revision needs which had to be addressed. The Mapping/Revision phase defined the changes to the standard to meet those requirements; it achieved this in practice of course by defining the new standard.

The scope of the changes was guided by the overall objective of the Ada 9X effort [DoD 89a]:

Revise ANSI/MIL-STD-1815A-1983 to reflect current essential requirements with minimum negative impact and maximum positive impact to the Ada community.

It is probably fair to observe that the changes which were deemed necessary are somewhat greater than might have been foreseen when the project first started in 1988. However, technology and man's understanding do not stand still and the changes now incorporated are very appropriate to the foreseen needs of a modern language for Software Engineering into the 21st Century.

1.2 The Requirements

The development of the requirements was a somewhat iterative process with a number of sub-phases. First, the community of users was invited to submit Revision Requests, secondly, these were sorted and analyzed in order to determine what was really implied by the requests, and finally, a coherent set of requirements was written.

Establishing the level of the requirements needed care. Quite naturally an individual user would often perceive a need in a manner which reflected a symptom of an underlying problem rather than the problem itself. It was very important that the requirements reflected the essence of the problem rather than what might in effect be simply one user's suggested solution to a part of a problem. One reason for this concern was to ensure that better, perhaps deeper and simpler, changes were not excluded by shallow requirements.

In some cases a complete understanding of a requirement could not be established and this led to the introduction of Study Topics. As its name implies a Study Topic described an area where something ought to be done but for some reason the feasibility of a solution was in doubt; perhaps the needs were changing rapidly or there were conflicting needs or implementing technology was unstable or it was simply that we had an incomplete understanding of some basic principles.

The general goal of the revision was thus to satisfy all of the Requirements and to satisfy as many and as much of the Study Topics as possible. Any unsatisfied Study Topics would be set aside for further consideration for a future Ada 0X (that is the next revision due perhaps around 2005).

The requirements document described 41 Requirements and 22 Study Topics. These were divided into a number of chapters which themselves form two broad groups. The first group covered topics of widespread applicability, whereas the second group addressed more specialized topics.

The first group consisted of the need to support International Character Sets (originally identified by several nations in the 1987 ISO standardization process), support for Programming Paradigms (Object Orientation was included in this category), Real-Time requirements, requirements for System Programming (such as Unsigned Integers), and General requirements. This last topic included specific matters such as error detection and general considerations of efficiency, simplicity and consistency; examples of a number of individually minor but irritating aspects of Ada 83 which fall into this category were given in an appendix.

The second, specialized, group consisted of requirements for Parallel Processing, Distributed Processing, Safety-Critical Applications, Information Systems, and Scientific and Mathematical Applications.

The breadth of these specialized requirements raised the specter of a language embracing so many application areas that it would become too costly to implement in its entirety for every architecture. On the other hand, one of the strengths of Ada is its uniformity and the last thing desired was a plethora of uncontrolled subsets. Accordingly, the Requirements document recommended the concept of a Core language plus a small number of Specialized Needs Annexes as a sensible way forward (with emphasis on keeping the number of such annexes very small). All validated compilers would have to implement the Core language and vendors could choose to implement zero, one or more annexes according to the needs of their selected market places.

The Requirements document also included general guidance on the approach to be taken to avoid unnecessary disruption to the Ada community. This covered vital matters such as the ease of implementation, object code efficiency, keeping a balance between change and need, and upward compatibility. It finally stressed that support for reliability, safety and long-term maintainability should continue to take precedence over short-term coding convenience.

Of all these requirements there was strong emphasis on the overriding goal of upward compatibility in order to preserve existing investment by the Ada community as a whole.

1.3 The Main User Needs

The Requirements reflected a number of underlying User Needs which had become apparent over the years since Ada was first defined. Apart from a small number of very specific matters such as the character set issue, the specialized areas and generally tidying up minor inconsistencies, four main areas stood out as needing attention:

Interfacing. Ada 83 recognizes the importance of being able to interface to external systems by the provision of features such as representation clauses and pragmas. Nevertheless, it was sometimes not easy to interface certain Ada 83 programs to other language systems. A general need was felt for a more flexible approach allowing, for instance, the secure manipulation of references and the passing of procedures as parameters. An example arises when interfacing into GUI's where it is often necessary to pass a procedure as a parameter for call-back.
Programming by Extension. This is closely allied to reusability. Although Ada's package and generic capability are an excellent foundation, nevertheless experience with the OO paradigm in other languages had shown the advantages of being able to extend a program without any modification to existing proven components. Not only does this save disturbing existing software thus eliminating the risk of introducing errors but it also reduces recompilation costs.
Program Libraries. The Ada program library brings important benefits by extending the strong typing across the boundaries between separately compiled units. However, the flat nature of the Ada 83 library gave problems of visibility control; for example, it prevented two library packages from sharing a full view of a private type. A common consequence of the flat structure was that packages become large and monolithic. This hindered understanding and increased the cost of recompilation. A more flexible and hierarchical structure was necessary.
Tasking. The Ada rendezvous paradigm is a useful model for the abstract description of many tasking problems. But experience had shown that a more static monitor-like approach was also desirable for common shared-data access applications. Furthermore, the Ada priority model needed revision in order to enable users to take advantage of the greater understanding of scheduling theory which had emerged since Ada 83 was defined.

The main changes incorporated into Ada 95 reflect the response to the Requirements in meeting these four key needs while at the same time meeting an overriding need for upward compatibility in order to minimize the effort and cost of transition.

1.4 The Approach

In responding to the revision requirements, the Revision team followed the inspiration of Jean Ichbiah (who led the original design team), both in remaining faithful to the principles underlying the original Ada design, and in the approach to the revision process. To quote Dr Ichbiah in his introduction to John Barnes' textbook on Ada [Barnes 82]:

Clearly, further progress can only come by a reappraisal of implicit assumptions underlying certain compromises. Here is the major contradiction in any design work. On the one hand, one can only reach an harmonious integration of several features by immersing oneself into the logic of the existing parts; it is only in this way that one can achieve a perfect combination. On the other hand, this perception of perfection, and the implied acceptance of certain unconscious assumptions, will prevent further progress.

Wherever possible, enhanced functionality in Ada 95 has been achieved by seeking and eliminating such unnecessary assumptions, thereby permitting the generalization of features already in Ada, and the removal of special cases and restrictions. A careful analysis was made of Ada 83, of language study notes prepared during the original design process and of the many Ada Issues and other comments that have since been received. Based on this analysis, and on the Ada community's experience in implementing and using Ada during the past ten years, the team identified limitations that, while they were included to simplify implementations and/or to lower risk when the language was first standardized, are no longer necessary. They also drew upon the wealth of practical experience gained during the 1980's in the use of object-oriented design methods, object-oriented programming languages, and real-time programming made possible by Ada.

The resulting Ada 95 revision is upwardly compatible for virtually all existing Ada 83 applications. Most incompatibilities are restricted to pathological combinations of features that are rarely used in practice. Total upward compatibility would not have allowed the correction of certain errors and certainly would not have allowed the enhancements needed to satisfy many of the requirements. Indeed, as discussed in Chapter 4 in more detail, no language revision has ever achieved total upward compatibility. The careful attention to this issue in the design of Ada 95 means that the expected transition costs for existing Ada programs are anticipated to be very low indeed.

Following the guidance of the Requirements document, Ada 95 comprises a Core language, which must be implemented in its entirety, plus several Specialized Needs Annexes which provide extended features for specific application areas. These Annexes provide standard definitions for application-specific packages, pragmas, attributes, and capacity and performance characteristics of implementations. The Annexes address the following areas: Systems Programming, Real-Time Systems, Distributed Systems, Information Systems, Numerics, and Safety and Security.

It should be noted that the Core language includes a considerably extended predefined environment covering important functionality such as mathematical functions and string handling. Much of the functionality of this predefined environment and the specialized annexes is already provided by implementations of Ada 83 but in non-standard ways; providing this within the standard will thus increase portability between implementations.

1.5 Using this Document

This document provides a description of the main features of Ada 95 and the rationale for the changes from Ada 83. It follows the inspiration of the rationale for Ada 83 [IBFW 86] by blending exposition with explanation.

Chapter 2 highlights the main changes primarily the four key areas outlined in Section 1.3. The intent is to provide a technically oriented management overview illustrating, with examples, the prime benefits of Ada 95 with respect to Ada 83.

By contrast, Chapter 3 provides an overview of the whole language in a discursive style and introduces new terminology where appropriate. For the most part, this overview is equally applicable to both Ada 83 and to Ada 95.

Finally, Chapter 4 summarizes the incompatibilities thereby giving guidance to existing Ada 83 programmers in order to smooth their transition to Ada 95.

For further information the reader should turn to the Ada 95 Rationale [Rat 95] of which this document forms the introductory part and the Ada 95 Reference Manual [ISO 95] which is the formal standard itself. The rationale for minute detail is admirably addressed in the Annotated Ada 95 Reference Manual [AARM] which is a version of [ISO 95] including embedded non-normative discussion mainly of interest to language experts and compiler writers.