I found this quote in "The Joy of Clojure" on p. 32, but someone said the same thing to me over dinner last week and I've heard it other places as well:
[A] downside to object-oriented programming is the tight coupling between function and data.
I understand why unnecessary coupling is bad in an application. Also I'm comfortable saying that mutable state and inheritance should be avoided, even in Object-Oriented Programming. But I fail to see why sticking functions on classes is inherently bad.
I mean, adding a function to a class seems like tagging a mail in Gmail, or sticking a file in a folder. It's an organizational technique that helps you find it again. You pick some criteria, then put like things together. Before OOP, our programs were pretty much big bags of methods in files. I mean, you have to put functions somewhere. Why not organize them?
If this is a veiled attack on types, why don't they just say that restricting the type of input and output to a function is wrong? I'm not sure whether I could agree with that, but at least I'm familiar with arguments pro and con type safety. This sounds to me like a mostly separate concern.
Sure, sometimes people get it wrong and put functionality on the wrong class. But compared to other mistakes, this seems like a very minor inconvenience.
So, Clojure has namespaces. How is sticking a function on a class in OOP different from sticking a function in a namespace in Clojure and why is it so bad? Remember, functions in a class don't necessarily operate just on members of that class. Look at java.lang.StringBuilder - it operates on any reference type, or through auto-boxing, on any type at all.
P.S. This quote references a book which I have not read: Multiparadigm Programming in Leda: Timothy Budd, 1995.