> My industry experience clearly shows that you're just flat wrong

Please tell me how a code search performed by library author Foo will ensure that library user Bar won't break invariants Foo intended to enforce.

> What kind of nonsense is this?

In ML, I can prove that users my library can't use my library wrong. Maybe they won't be able to use my library at all - they type checker will reject every attempt. But it guarantees that, if they can use my library, they will use it right, in the sense that every invariant I enforce won't (and can't possibly) be broken by users.

For example, there may be multiple ways to realize the same ordered set as a red-black tree (balanced differently), but I can arrange things so that the difference can't be observed by users of the ordered set abstraction.

> The library users in Smalltalk would do such searches.

Library users shouldn't be in the business of enforcing invariants that are only relevant to the library's author. See? This is what I mean by “Smalltalk can't separate concerns”.

I'm not saying all of this to be mean. It's been known for quite a while that parametricity is the mathematics of abstraction and separation of concerns [0]. If you need to insulate users of your code from your design choices, you absolutely need parametricity. (Or “social conventions”, but those don't work in the long run.)

[0] http://www.cse.chalmers.se/edu/year/2010/course/DAT140_Types...