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Comment by nessus42 | Hacker News Reader

> Just because it is Functional Programming it does not mean to "throw away encapsulation".

Surely not, as even Clojure has support for OO programming. But Rich Hickey says that 90% of the time that encapsulation is used by OO programmers, it shouldn't have been. Instead, they should have just used a map or a list, etc.

So, does something as complex as representing a book fall into the 90% case or the 10% case?

Or maybe Hickey is really just alluding to the Law of Demeter and saying that methods should return data and not objects. It might very well be that if the Law of Demeter were religiously followed in OO programming, that the use of objects would fall by 90%.

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The ML family of languages have structures, signatures and functors for encapsulation. Yet, it has little to do with OO in the traditional Javaesque kind of sense. Encapsulation does not require an object to achieve. Hickey says OO a'la carte and splits up the conflated concepts.

The reason he says to use a map or a list is because of another conflation danger. If you have an opaque book - you have a book but not its underlying representation - then if you want to represent multiple these, you should make the container "exposed". Because that choice is the modular one. All the functions which operate on my container can now be used. For instance (MAP spellcheck books). But I can also easily do (PARALLEL-MAP spellcheck books) right? Or even (MAP-REDUCE spellcheck id books) if I have petabytes of books and a cluster supporting MAP-REDUCE. This is why templates or generics are so powerful. They expose this modular relation.

But encapsulating the list into a "books" class will mean we have to write lots of extra code for each of the above 3 variants. We naturally don't want to do that.

Even more - encapsulation is not always mean OOP.

Erlang have encapsulation more strong than in any OOP language - all you have is server process and messages thrown to it.

How is that not OO then? OO is not defined by Java. The first OO languages and systems were descended from Lisp or built within Lisp, a functional programming language.

OO without inheritance is still OO. OO with immutability is sill OO. Encapsulation is the heart of OO. Polymorphism is also quite important. Inheritance is a distant third property of OO, but it is one common aspect of OO that is highly over-used. Most OO "best practices" gurus discourage rampant use of inheritance these days and say to prefer composition.

Back to the whole point of this thread, Rich Hickey says specifically NOT to use encapsulation (90% of the time). That would then not be OO. No encapsulation, no OO.

This is a pointless argument, since both views are "correct". Even Joe Armstrong says that "You can take two views, you can say that either erlang isn't object oriented... [or] you can say its more object oriented than all the object oriented languages."

But I do not think it's very useful to say that Erlang is object oriented, since it will confuse a lot of people, since it is not anything like the other "object oriented languages".

http://www.infoq.com/interviews/Erlang-Joe-Armstrong http://www.infoq.com/interviews/johnson-armstrong-oop

>>But I do not think it's very useful to say that Erlang is object oriented, since it will confuse a lot of people

I find it useful to instill this kind of confusion in people around me.

"You talk about encapsulation? Look at erlang"

"Polymorphism? Haskel"

"OOP? Javascript, CLOS"

"Static/dynamic typing? Look here - your java code is in fact dynamic, if you're really want to taste real static - look at ML-family or something near"

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