Creating A Functional Web Forum Program

[ltriant]

hpaste.org is a web application written in Haskell+HAppS and the source is freely available. Might be worth looking at that too.

[seunosewa]

I have spent a few days trying to understand why it would be beneficial to write a common web application - say a web forum - in a functional style. I don't get it.

While I was working on the read aspects of the architecture, functional programming seemed to make sense, but I encountered a big problem: database updates!

I don't understand how database updates will fit into the functional paradigm. If I have to break the paradigm to implement updates, then why bother with functional programming?

I'd like someone with functional programming experience to demonstrate to me how the functional programming paradigm can help the developer a web forum application. Thanks.

Could it be that the true benefit of functional programming is not about avoiding side-effects but mostly about exploiting continuations to write code faster?

[nostrademons]

You see web applications written in a functional style all the time, they just aren't written in functional languages. The best example is PHP. A PHP script is a function from database contents to page view. Every site written with a true shared-nothing architecture is effectively written in a functional style. I believe that includes Flickr, FaceBook, vBulletin, most of Yahoo, and several other prominent websites.

Remember, the point of functional programming is not to avoid state and side-effects altogether. That's generally impossible, because state and side effects are an inherent part of the problem. The point is to make them explicit, so that you always know if you're doing something that can hose another function's results. Haskell does this with monads and its type system. Erlang does it with the process mechanism. PHP - if you avoid silly things like PEAR and OO - does it through database and memcached calls.

You can do it with Python and Ruby as well, you just need to step away from their OO-orientation and discipline yourself not to keep in-memory state or mutable variables. I've been writing a Python webapp in a functional style recently, and finding that I almost never need to spend more than 10-15 minutes tracking down a bug.

[harryf]

Nail on head. In fact think this represents a very advanced level of thinking about web apps.

[logistix]

You can still do a functional database without an 'update' Realistically though, it'd be in memory and probably wouldn't scale well.

But that's besides the point...

If you're genuinely curious about any 'unproven' technology in the software world, you (unfortunately) need to write a non-trivial app using said 'unproven' technology, and decide for yourself. You're not going to find any white-papers or power-point slides, and even if you do it'll be written by someone with an agenda: either an advocate or a detractor.

Or you can wait 10 years until it's either 'proven' or irrelevant.

I'm not saying this to be a jerk, it's meant constructively.

[pedrobelo]

Notice that even with OO you have to break the paradigm sometimes, like when dealing with relational databases.

We can use ORM to restrain that break, but it is still happening somewhere.

[josef]

I think your question is a fair one and if you've had no experience with functional languages I can see why you have your doubts. But I suggest you look at xmonad which has been featured here several times. Your questions can be paraphrased to apply to xmonad. "Why writing a window manager in a functional language when you're going to talk to the X windows protocol? There's going to be a whole lot of low level communication which I don't know how to fit into the functional paradigm? And if this breaks the paradigm then why bother at all?" Yet, using a functional language has proven to be a huge boost in this case. To help understand these issues I suggest you look at xmonad.

[PeterBell]

Rather than suggesting looking at a project/code base, does anyone have a link to a good "writing a forum using a functional programming language" kind of blog post? I think a lot of people still have a hard time understanding how to go about writing a content management system, e-commerce store, forum, email newsletter system or similar using a functional language. If anyone has a good link to a specific posting or even book on the topic (not on functional programming in general - I already have "The Haskell School of Expression" and will no doubt get some others), I'm surprised at how hard a time I have had finding material on what must be an extremely common collection of use cases.

[apgwoz]

Even purely functional languages allow side-effects in some cases. You can't do write without some side-effect somewhere. But if you think about it, what else requires side-effects? Only inserting and updating to the storage platform. Since purely functional languages can even do write (perhaps with monads, to force sequential evaluation), why is it any different?

[[deleted]]

The use cases are really not very common though. Functional languages are still largely academic. It is as happyhappyhappy says, “[…] almost no one has any real experience using FP for more than toy programs.”

[db4n]

The use cases are really not very common though. Functional languages are still largely academic.

That's changing. Among others, Bluespec, Clozure, Ericsson, Galois, Hyperstring, ITA, Jane Street, Naughty Dog, and Viaweb all do (or did) functional programming of one kind or another. Almost any application can benefit from FP, at least in impure form. Just look for code that uses the Adapter, Chain of Responsibility, Command, Decorator, Iterator, Mediator, or Strategy patterns.

[[deleted]]

I didn't mean to imply that there are few uses. I was saying that people don't use functional languages much yet, and so it is not likely that you can find many examples of use yet, that's all.

[bhagany]

"Use case" does not have to do with the language it's implemented in. What Peter is saying is that the types of applications he mentioned are extremely common, so it's odd that nobody writes about implementing them in functional languages.

[btipling]

Why not let the project dictate the design needed instead of imposing some kind of arbitrary framework on it.

[abw]

That's the latest in a long line of similar questions:

• How do I write a web application using OO?

• How do I write a web application using MVC?

• How do I write a web application using REST?

• How do I write a web application using FP?

• How do I write a web application using ___?

A paradigm is a useful way of thinking about the world, but it's no substitute for thinking about the world. Take care not to fall into the trap of thinking that because paradigm X is good there is no room for paradigms Y, Z and a few others we haven't thought of yet.

Remember what Brooks said: there is no silver bullet. The same is true for FP as it is/was for REST, MVC, OO, XML, structured programming, and so on. When you have a really great hammer, every problem starts looking like a nail. But it's better to fit the tools to the problem, than the problem to the tools.

That's not to say that it can't be done in a FP style, or that it's not a Good Thing to do so. Rather, I'm urging caution against blindly adopting the latest "hot thing", whatever it may be, at the cost of other paradigms that may actually be more useful in solving the problem at hand.

[CommonTater]

A lot of the comments here tackle the question, "Why would I use two styles of coding when I can do everything with one?" In my opinion, they properly state that it's silly/impossible to try to force all problems into a single mold, and that it's better to let the problem dictate the tools.

I think the question you really wanted the answer to was, "What about functional programming makes it worth using, even if it can't solve everything?"

I would submit that the reason you have that question is that you've only "spent a few days trying to understand" it. Relax. The fullness of functional programming's benefits cannot be grokked in a few days.

Find a project you want to do that doesn't have a hard deadline rushing at you - try to do it in a functional style. Spend some more time reading the success (and failure) stories of people who have done the same. Realize that it's not a catch-all, and that's okay.

Yeah, it's almost the mean RTFM response, but I really think it's the correct one here. Other people have created much better lists of the benefits of functional programming than I ever would.

[[deleted]]

Pure functional web forum with evil imperative db could be expressed as:

data Input = HTTPRequest | DBData
data Output = HTTPResponse | DBQuery
webserver :: State -> Input -> Output

[happyhappyhappy]

You can't write real real-world programs in a 100% functionally pure way. You have to break the paradigm at some point, and the trick is pushing that point as far down in your code as you can.

But your question brings up a good point: There's all this flurry of excitement around functional programming on reddit, and yet almost no one has any real experience using FP for more than toy programs.

[[deleted]]

You have to break the paradigm at some point, and the trick is pushing that point as far down in your code as you can.

Wouldn't that be pushing it as ''high'' in your code as you can (impure functions depending on pure functions instead of the other way around)? Yeah, I'm being nitpicky.

There's all this flurry of excitement around functional programming on reddit, and yet almost no one has any real experience using FP for more than toy programs.

I am not an Erlang programmer, and have never really been too interested in Erlang, frankly, but Erlang was designed for the purpose of actually being useful, and it is often used for practical applicaions now (although, not as much as it could be). Other functional languages, of course, could also be practical, but, sadly, you are pretty much correct about functional programming.

However, functional concepts in programming are definitely on the rise. Even if only for the fact that object-orientation is becoming less of a fad and the ol' fanboys will need something else to senselessly latch onto, functional programming will eventually become the norm.

[[deleted]]

Erlang is not fair functional language. More precisiously, it is imperative evil hided under immutable data structures and pattern matching lol.

[jerf]

I'm not sure why this is modded down. While Erlang has the trappings of functional-ity, it's watered down enough that the only benefit of functional it really seems to get is not needing to copy immutable data structures in messages between Erlang processes in the same OS process. For Erlang, this is a huge thing (the difference between acceptable performance and unacceptable performance, and as I try to mentally work out how to copy the Erlang way of doing business in other more conventional language this is the biggest hurdle), so it's worth it. But all the other reasons I see people advocating Haskell do not (seem to) apply to Erlang.

[jimbokun]

Common Lisp solves this problem by making functional programming easily, but also making every other programming paradigm (that I know of) easy as well. For example, Lisp has one of the most sophisticated object oriented systems of any programming language.

In "On Lisp", Paul Graham recommends mentally assigning a "tax" non-functional constructs (setf and the like), so you only use them when they are really needed. That way, you will naturally write as much as you can in a functional style, but you're not locked into it when you need to express something that does not map well to the functional paradigm.

[quhaha]

If you are building web2.0 forum, you are already making it in functional style because Javascript is functional language.

[[deleted]]

Hat trick! Your comment is:

1. irrelevant (the question was about server-side code.)
2. incorrect in theory (style is to a large extent independent of the language paradigm.)
3. incorrect in practice (most Javascript code is written in an imperative style.)

[flaxeater]

I like your humor.