On: Imperative vs Functional Programming

[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 → 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?

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 → thesz]

> 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 → babel17]

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 → thesz]

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 → babel17]

>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.

[loup-vaillant → thesz]

http://www.loup-vaillant.fr/articles/classes-as-syntactic-su...

My Ocaml code is not pure, but we can easy remove any occurrence of `ref`, and restricting oneself to pure methods, which produce a new fresh object instead of modifying `this` in place.

[thesz → loup-vaillant]

Sorry, I am not very fluent in OCaml. Also, I didn't find word "message" in the link above.

I am experiencing limitations of pure objects and message passing almost right now. In my spare time I am currently developing dynamic data flow CPU which is based on immutable "objects" and message sending. It is quite unusual style of programming, very unlike anything else, including Smalltalk, Erlang and everything.

[loup-vaillant → thesz]

It's like Haskell, with a slightly heavier syntax. And it's eager. And you can unsafePerformIO all over the place (but avoid doing so even in Ocaml).

    (* Ocaml *)    --Haskell
    let foo = 42   foo = 42    -- top level declaration
    type bar =     data bar =  -- type declaration

Also, records in Ocaml are a separate kind of type, more like C structs:

    type bar = { x:int;     (* defination *)
                 y:float;
               }
    bar.x                   (* "member" acces *)

"Message passing" and "Method calling" are strictly equivalent, at least when everything is synchronous.