[thesz]

To differentiate objects one from another.

http://en.wikipedia.org/wiki/Identity_%28object-oriented_pro...

"identity is the basis for polymorphism in object-oriented programming."

In a purely functional object-oriented programming you don't need identity. But then that practice is almost indistinguishable from usual pure functional programming we have for at least 20 years with Haskell.

[loup-vaillant]

Forgive my insistence, but, what kind of problem does the ability to differentiate objects from one another solves? What kind of programs are made simpler by this ability?

Besides, I don't see how you would need identity in the sense of being structurally-equal-but-not-the-same for class based polymorphism. You could perfectly do this without any side effect, despite what the `final` keyword in Java might mean.

By "identity" Wikipedia probably means something like "you think I'm a Foo, which is right, but underneath, I'm a special kind of Foo: a Bar. By the way, there are others special kinds of Foo: the Baz, the Fiz, the Buz…". In other words, subtyping is the basis for polymorphism in OOP.

[thesz]

>Forgive my insistence, but, what kind of problem does the ability to differentiate objects from one another solves? What kind of programs are made simpler by this ability?

Oh, I don't know. I am not an expert in OO* at all.

All I know is that for something to be true OO it has to have objects with identities. I was told so. ;)

All I know about OO is Liskov substitution principle. Where it is unnecessary I tend to use State monad and immutable objects.

>subtyping is the basis for polymorphism in OOP.

Which can be solved by several means, interfaces and inheritance being two of them.

[babel17]

> All I know is that for something to be true OO it has to have objects with identities. I was told so. ;)

Well, you were told wrong things :-)

[thesz]

Please, list object-oriented systems with objects with identities and object-oriented systems without those. Count both. Discuss. ;)

Alan Kay, who coined term "object-oriented" described object-oriented system as an assembly of objects communicating by message passing. Message passing requires identities (and state, except in severely reduced form).

[babel17]

Message passing does require a sender, a receiver, and a message. You can have those in purely functional programming. No identity needed. But if you really want one, you can have it: Two objects are identical, if they react to all messages in the same way. You can also explicitly define equality ==, and then say that a and b are identical if a == b.

[thesz]

>Message passing does require a sender, a receiver, and a message. You can have those in purely functional programming. No identity needed.

Would you mind show me some code in pure functional language?

I mean, in Haskell or Clean. Those are pure enough.