- [William R. Cook, OOPSLA 2009: On Understanding Data Abstraction, Revisited](https://www.cs.utexas.edu/~wcook/Drafts/2009/essay.pdf).

- This is a nice paper illustrating the difference between *abstract data types* and *objects*.

- In section 4.3: *"In the 1970s [...] Reynolds noticed that abstract data types facilitate adding new operations, while 'procedural data values' (objects) facilitate adding new representations. Since then, this duality has been independently discovered at least three times [18, 14, 33]."* Then: *"The extensibility problem has been solved in numerous ways, and it still inspires new work on extensibility of data abstractions [48, 15]. Multi-methods are another approach to this problem [11]."*

- In section 5.4, on Smalltalk: *"One conclusion you could draw from this analysis is that the untyped λ-calculus was the first object-oriented language."*

- In section 6: *"Academic computer science has generally not accepted the fact that there is another form of data abstraction besides abstract data types. Hence the textbooks give the classic stack ADT and then say 'objects are another way to implement abstract data types'. [...] Some textbooks do better than others. Louden [38] and Mitchell [43] have the only books I found that describe the difference between objects and ADTs, although Mitchell does not go so far as to say that objects are a distinct kind of data abstraction."*