One of the themes of Barbara Liskov's Turing Award lectue ("CS History 101") was that nobody has invented a better programming concept than abstract data types. William Cook wrote a paper for OOPSLA '09 that looks at how well PLT'ers understand their own vocabulary, in particular abstract data types and concepts that on the syntactical surface blend to all seem like ADTs. The paper is On Understanding Data Abstraction, Revisited.
In 1985 Luca Cardelli and Peter Wegner, my advisor, published an ACM Computing Surveys paper called â€œOn understanding types, data abstraction, and polymorphismâ€. Their work kicked off a flood of research on semantics and type theory for object-oriented programming, which continues to this day. Despite 25 years of research, there is still widespread confusion about the two forms of data abstraction, abstract data types and objects. This essay attempts to explain the differences and also why the differences matter.
The Introduction goes on to say:
What is the relationship between objects and abstract data types (ADTs)? I have asked this question to many groups of computer scientists over the last 20 years. I usually ask it at dinner, or over drinks. The typical response is a variant of â€œobjects are a kind of abstract data typeâ€. This response is consistent with most programming language textbooks.
So what is the point of asking this question? Everyone knows the answer. Itâ€™s in the textbooks.
My point is that the textbooks mentioned above are wrong! Objects and abstract data types are not the same thing, and neither one is a variation of the other.
Ergo, if the textbooks are wrong, then your Dinner Answer to (the) Cook is wrong! The rest of the paper explains how Cook makes computer scientists sing for their supper ;-)
When Iâ€™m inciting discussion of this topic over drinks, I donâ€™t tell the the full story up front. It is more fun to keep asking questions as the group explores the topic. It is a lively discussion, because most of these ideas are documented in the literature and all the basic facts are known. What is interesting is that the conclusions to be drawn from the facts are not as widely known.