A year and a half ago, I wrote a blog post which questioned some statements from Joe Armstrong's 'Why OO Sucks' article. While I still can't agree with all the article, I can see what he *really* meant and what I misunderstood then.
So let's get back to the original article and its statements:
- 1. Data structure and functions should not be bound together.
My response to that went mainly along the lines of "but that's how people tend to think, isn't it". However, what I think now is... well why not? Even in functional languages functions that handle specific types of data know the details of the data, so in order to understand the function you need to know what the data looks like. In order to use that function you need to feed it with the specific type of data it needs. Not so separate anymore?
Binding data and functions in OO languaged serves mainly two purposes: either to give some specific view or attribute of an instance (in which case they're nothing more than a convenient interface pure function calls) or to change the instance's inner state (to which I'll get back later).
- 2. Everything has to be an object.
Again, I said it's comfortable and natural to consider data as objects, but what I didn't know by then was the difference between classes and data types. I didn't know yet about pattern matching and other ways of working with immutable data. All those techniques make working with it equally comfortable and natural (maybe in more mathematical spirit, but I doubt it should be a disadvantage for anyone).
A little wiser now, I'm quite convinced that this argument comes down again to objects having private state. If there was an OO language where all objects would have to be immutable, I think the need of turning every piece of data into an object wouldn't be so painful.
- 3. In an OOPL data type definitions are spread out all over the place.
OK, here I was silly. First, I wrote "OO laguages" while *clearly* thinkng "Java" (well, that was the single OO language I knew then). Second: I totally forgot about inheritance.
What I think now is that there's no much difference here between OO and functional languages. If you consider inheritance as a specific case of composition (which it is) you get an equivalent of nested data types with functions handling one type are using other functions to handle the other. Not much difference from functional languages.
- 4. Objects have private state.
Ah, so here we have it. After playing a bit with functional languages I learnt that state, while unavoidable, is in fact a nasty bastard. Joe Armstrog doesn't like the way the state problem was solved in OO languages. They go along the lines of "yeah... we have state, but don't mind it. As long as you give us correct requests you'll be fine", which isn't true because giving correct requests (calling correct methods) requires to know the state in the first place.
The functional languages tend to use pure functions what lets them access the state only when neccessary (and FPL desingers solve the state problem in several different interesting ways).
To be honest. I kind of agree with that now. I don't like mutable objects. You never know what to expect from them :)
Ah, and my arguments here were also quite silly. First: I felt already that state is not so good, but only in the context of multithreading. Second: I was a fan of information hiding and working with tightly encapsulated data. While I'm not saying that encapsulation is bad (on the contrary!), I think I understand it differently now.
Encapsulation is not putting something in black box and forcing others not to see the its inner parts. It's rather finding a "sweet spot" in control flow: the interface that would be convenient enough for anyone to use without a need to look deeper. In other words: I don't associate encapsulation with information hiding anymore.
Concluding this longish article: I still don't agree with the statements of Joe Armstrong's article - in particular: the quiet assumption that a good language is a functional language. But I have to admit that my former article was based mainly on ignorance and misunderstanding Joe Armstrong's arguments.
And I still don't like the conspiration theory in the last paragraph there :)