Erlang (Functional Programming) vs Object Oriented Programming in terms of thinking

Question

I am learning Erlang and I am trying to create a very sample blog program. However my mind currently is trapped in the OO world (var p = new Post(); p.Title = ""; p.Save();). I would like to understand some basic thinkings in Erlang. Instead of creating Post object what I should do in terms of data structure (p.Title, p.DateCreated, p.Body)? Should I be using tuple? I would like to understand the recommended way in doing such things (in both Erlang specific and or Functional Programming specific). Or is what I am doing fundamentally wrong in either Erlang or FP?

Requirement (in OO terms, not sure how to explain in FP terms yet ^_^):

1. create Post object (id, title, date_created, body, IList)
2. create Comment object (id, post_id, created_by (name as string), date_created)
3. one Post can have multiple comments
4. post.AddComment(Comment)

Thanks.

Updated: I am not looking for specific way of doing OOP in Erlang unless it's the recommended way. I am looking for standard/recommended way of doing what's described in the question, however I am not trying to replicate OOP in Erlang.

Answer 1

Erlang is an Object Oriented language. This statement has more power if you look at OOP the way Alan Kay described it:

OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme late-binding of all things.

As you must be aware, Erlang promotes a style of programming called Concurrency Oriented Programming, in which you abstract away objects as independent processes that communicate by message passing. Each process has it's local state, they live in their own parallel world. Dynamic polymorphism is achieved by the fact that, you can define a class of processes that can respond to a common set of messages. As Erlang 'objects' live in their own tiny process, it becomes a natural medium for modelling the real-world. You can make use of your OOP skills better in Erlang than in any other language.

Can't give a full description about OOP in Erlang in such a small space. I suggest that you read the book Programming Erlang: Software for a Concurrent World.

Also see these links:

• Joe Armstrong About Erlang (Interview)
• Erlang as an OO language

Answer 2

Your example doesn't really represent good OO style. Comments appear on already published blog posts, so by then you just have some kind of reference to the post id the comment is posted to.

For OO programming it would make more sense to have some kind of BlogDb object that one send post and comment objects to. A comment object needs to know what post id it is a comment to. You should not create post and comment objects using the 'new' operator, instead the BlogDb interface has methods that return fresh instances of those.

Suddenly you have a passable way to implement just the same thing in Erlang. Start a gen_server that is the blog_db. Do things like

Post = myblog:post(Title, Body),
{ok, Result} = myblog:add_post(BlogDb, Post),
...

You don't need to know the details of the Post value, so how it is constructed is hidden in a "constructor" for it in another module.

Answer 3

I would use records:

-record(post, {title, date_created, body, comments = []}).
-record(comment, {created_by, date_created, content}).

Then if you want to use mnesia as database:

Post = #post{title = "", body = "", date_created = erlang:universaltime()},
mnesia:transaction(fun() -> mnesia:write(Post) end).

To add a comment:

Comment = #comment{created_by = "", content = "", date_created = erlang:universaltime()},
mnesia:transaction(fun() ->
    [Post] = mnesia:read(post, Title),
    PostWithNewComment = Post#post{comments = [Comment | Post#post.comments]},
    mnesia:write(PostWithNewComment)
end).

I haven't tested the code, but this is what I would do. Also I assumed that each title is unique.