> Programmer would say I want a sequence here. The compiler would say, I chose a linked list representation. The programmer would look over it, and disagree, sa

> Programmer would say I want a sequence here. The compiler would say, I chose a linked list representation. The programmer would look over it, and disagree, saying, you should put this into an array. To some extent, this is the promise of object-oriented programming, that in this particular instance has failed a bit in mainstream languages. It's true that in, say, Java, you have to massively refactor your code to switch between arrays and linked lists, because you use different syntax ([] vs. method calls) to access elements. It could be a bit better in C++ due to operator overloading; you could hide your actual container type behind a typedef, and as long as both container type A and container type B support the same operations with [] that you actually use, you can freely switch between them. In Smalltalk, every container class derives from a single Collection class, and they have very very similar APIs. There you can, to a large extent, just program against the Collection API and not care much about the actual type of collection you have in your hand. You still have to choose one! The compiler won't do it for you, not in the way you envision. But the idea is that if you program against the generic API, then profile/benchmark your code, it should suffice to change a single line of the program to try a different representation to compare against it. (Of course some things won't work. You can't index into a set.) Other dynamic languages should be similar, Python for example. But I think you have to work harder to achieve full genericity.