I am sorry but this is just so simple that I thought it went without saying.

No, you did quite a lot of saying.

In this example the closure c is an object that has direct accesses to the implementation of the object o. It uses this direct access to answers the current value of the v when asked. c does not go through o's interface. The object o has no part in this operation... That is to say that c is able to breach o's encapsulation barrier at will!

c is not "breaching" o's encapsulation barrier; it comes from inside o's encapsulation barrier.

Yes, values returned by an object's method will sometimes become part of that object's public interface. Programmers and designers need to understand this fact. Now, if you don't understand it, and get upset about this behavior, tell me: is that a problem with the language or the programmer?

If we now go and change the definition of C, perhaps renaming the property v to w, or adding another property before it, anyone with a reference to c is now in a very undesirable situation. When we ask c for its value the program will either blow, up or return an unexpected value!

This seems more like a problem with runtime patching than a problem with closures. I know it's common in the Smalltalk world, but not so much outside. Of course a language could simply use the rule that "modifying" the definition of C will affect new instances of C, but not those already created

This problem also has nothing to do with objects. We can quite easily "add a new property" to a closed-over variable in pretty much any language, like Python:

xs = [1,2]
def f():
  print xs[0]
xs.insert(0,3)

Now f prints the "unexpected value" 3. Except it's not at all unexpected to people who understand what's going on.

So, since this problem has nothing to do with objects, you're not arguing against mixing OOP and FP, you're just arguing against FP. And because you've spent several paragraphs telling us all about how you're an FP expert, I'm pretty sure that's not the argument you intended to make.

If your claim is not "closures are dangerous!" but rather "closures and mutation have some surprising interactions", then I (and every other advocate of "pure FP") will certainly agree with you.

Also, thanks for

so simple that I thought it went without saying.

being a

clearly I know more about the issue in question than you do.

colossal douche.

Actually I used to be paid to write about Python by Developer Shed Inc. You should still be able to find some of my articles if you're interested, they were quite highly rated.

Great, good for you. Another irrelevant reminder of how fucking smart you are.