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I'm mostly a C/C++ programmer, which means that the majority of my experience is with procedural and object-oriented paradigms. However, as many C++ programmers are aware, C++ has shifted in emphasis over the years to a functional-esque style, culminating finally in the addition of lambdas and closures in C++0x.

Regardless, while I have considerable experience coding in a functional style using C++, I have very little experience with actual functional languages such as Lisp, Haskell, etc.

I've recently began studying these languages, because the idea of "no side-effects" in purely functional languages has always intrigued me, especially with regards to its applications to concurrency and distributed computing.

However, coming from a C++ background I'm confused as to how this "no side-effects" philsophy works with asynchronous programming. By asynchronous programming I mean any framework/API/coding style which dispatches user-provided event handlers to handle events which occur asynchronously (outside the flow of the program.) This includes asynchronous libraries such as Boost.ASIO, or even just plain old C signal handlers or Java GUI event handlers.

The one thing all of these have in common is that the nature of asynchronous programming seems to require the creation of side-effects (state), in order for the main flow of the program to become aware that an asynchronous event handler has been invoked. Typically, in a framework like Boost.ASIO, an event handler changes the state of an object, so that the effect of the event is propagated beyond the life-time of the event handler function. Really, what else can an event handler do? It can't "return" a value to the call point, because there is no call point. The event handler is not part of the main flow of the program, so the only way it can have any effect on the actual program is to change some state (or else longjmp to another execution point).

So it seems that asynchronous programming is all about asynchronously producing side-effects. This seems completely at odds with the goals of functional programming. How are these two paradigms reconciled (in practice) in functional languages?

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Wow, I was just about to write a question likes this and did not know how to put it and then saw this in the suggestions! –  Amogh Talpallikar May 8 '13 at 10:01
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4 Answers 4

All of your logic is sound, except that I think your understanding of functional programming is a bit too extreme. In the real world functional programming, just like object-oriented, or imperative programming is about mindset and how you approach the problem. You can still write programs in the spirit of functional programming while modifying application state.

In fact, you have to modify application state to actually do anything. The Haskell guys will tell you their programs are 'pure' because they wrap all of their state changes in a monad. However, their programs still do interact with the outside world. (Otherwise what is the point!)

Functional programming emphasis "no side effects" when it makes sense. However, to do real-world programming, like you said, you do need to modify the state of the world. (For example, responding to events, writing to disk, and so on.)

For more information on asynchronous programming in functional languages, I strongly urge you to look into F#'s Asynchronous Workflows programming model. It allows you to write functional programs while hiding all the messy details of thread transition within a library. (In a manner very similar to Haskell style monads.)

If the 'body' of the thread simply computes a value, then spawning multiple threads and having them compute values in parallel is still within the functional paradigm.

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Also: looking at Erlang helps. The language is very simple, pure (all data is immutable) , and is all about asynchronous processing. –  9000 Jan 22 '11 at 2:34
    
Basically after understanding the benefits of functional approach and changing state only when it matters will automatically make sure that even if you work in say something like Java, you know when to modify state and how to keep such things in control. –  Amogh Talpallikar Oct 4 '13 at 12:40
    
Disagree - a fact that program is composed from 'pure' functions doesn't mean that it don't iteract with outside world, it's mean, that every function in a program for one set of arguments will always return same result, and this is (purity) a big deal, because, from practical view - such program will be less buggy, more 'testable', successful execution of functions could be proved mathematically. –  Gill Bates Nov 10 '13 at 17:05
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This is a fascinating question. The most interesting take on it is, in my view, the approach adopted in Clojure and explained in this video:

http://www.infoq.com/presentations/Value-Identity-State-Rich-Hickey

Basically the "solution" proposed is as follows:

  • You write most of your code as classic "pure" functions with immutable data structures and no side effects
  • Side effects are isolated via the use of managed references which control change subject to software transactional memory rules (i.e. all your updates to mutable state take place within a proper isolated transaction)
  • If you take this view of the world, you can see asynchronous "events" as triggers for a transactional update of mutable state where the update is itself a pure function.

I've probably not expressed the idea as clearly as others have done, but I hope this gives the general idea - basically it's using a concurrent STM system to provide the "bridge" between pure functional programming and asynchronous event handling.

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I'm confused as to how this "no side-effects" philsophy works with asynchronous programming. By asynchronous programming I mean ...

That would be the point, then.

A sound, no side-effect style is incompatible with frameworks that depend on state. Find a new framework.

Python's WSGI standard, for example, allows us to build no side-effect applications.

The idea is that the various "state changes" are reflected by an environment of values that can be built incrementally. Each request is a pipeline of transformations.

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"allows build no side-effect applications" I think there's a word missing in there somewhere. –  Christopher Mahan Jan 22 '11 at 2:15
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One note: a functional language is pure, but its runtime is not.

For example, the Haskell runtimes involve queues, thread multiplexing, garbage collection, etc... all of which is not pure.

A good example is laziness. Haskell supports lazy evaluation (that is the default, actually). You create a lazy value by preparing an operation, you can then create multiple copies of this value, and it's still "lazy" as long as it's not required. When the result is needed, or if the runtime finds some time, the value is actually computed, and the state of the lazy object changes to reflects that it is no longer required to perform the computation (once more) to get its result. It is now available through all references, so the state of the object has changed, even though it is a pure language.

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