Communications of the ACM

Vviewpoints

of the principle when applied to more complex systems; that paper proposes a software structure that is very different from what is usually found in such systems. It also illustrates how that structure can be documented.

In 1968 and 1969, two famous NATO conferences discussed a new field— software engineering—to help us achieve reliable and trustworthy software systems. Why?

Most of those who attended the two conferences had been educated in either science or mathematics, but drifted into building programs for others to use (software). They realized that their education had taught them how to add to the world’s knowledge, but not how to build products. Noting that tradi-

DAVID LORGE PARNAS is well known for his insights into how best to teach software ngineering. Parnas has been studying software design and development since 1969, and has received more than 25 awards for his contributions. In 2007, he shared the IEEE Computer Society’s 60th anniversary award with computer pioneer Maurice Wilkes. He received B.S., M.S., and Ph.D. degrees in Electrical Engineering from Carnegie Mellon University. He has published more than 285 papers, many of which are considered classics. He designed the CEAB ( Canadian Engineering Accreditation Board) accredited McMaster University Software Engineering program, where he is now professor emeritus.

He and seven colleagues have articulated an approach based on actionable capabilities rather than concepts. Communications columnist Peter J. Denning had a conversation with Parnas about these ideas.

PETER J. DENNING: You have a long-standing interest in methods for producing reliable and safe software. In 1971, you first articulated the principle of information hiding. 2–4 Why is this still important?

DAVID LORGE PARNAS: The concept
of information hiding is based on the
observation that changes outside a
program (such as a revision of other
programs, hardware changes, or re-
quirements changes) only affect the
correctness of that program if informa-
tion that would have to be used when
showing the correctness of that pro-
gram was invalidated by the change.
Consequently, software should be
organized so that dependence on in-
formation that is likely to change is re-
stricted to a small, clearly identified, set
of programs. Applying this principle
results in programs that are easier to
understand, easier to maintain, and
less likely to contain errors.

As expressed in my early papers, the information hiding principle applies to all programs in any programming language. It serves as a guideline for software developers and maintainers. The first of the three papers introduced the basic idea; the second illustrated it for a simple system and the third showed the surprising implications