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I've heard a lot of talk about using functional languages such as Haskell as of late. What are some of the big differences, pros and cons of functional programming vs. object-oriented programming?

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One does not reject another. –  mbq Oct 5 '10 at 19:27
@mbq i understand that they aren't mutually exclusive, but I just wanted to try to get a better understanding of the difference of the two approaches. –  GSto Oct 5 '10 at 19:52
Great question. I've been wondering about this too. –  JohnFx Oct 5 '10 at 20:57
The real answer is - theres's no "versus" between them. Check out this question at StackOverflow. –  missingfaktor Oct 18 '10 at 3:03
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4 Answers

I would say that it is more Functional Programming vs Imperative Programming.

The biggest difference is that Imperative programming is about Control flow while Functional programming is about Data flow. Another way to say it is that functional programming only uses expressions while in imperative programming both expressions and statements are used.

For example, in imperative programming variables and loops are common when handling state, while in functional programming the state is handled via parameter passing, which avoids side-effects and assignments.

Imperative pseudo-code for a function for calculate the sum of a list (the sum is kept in a variable):

int sumList(List<int> list) {
    int sum = 0;
    for(int n = 0; n < list.size(); n++) {
        sum = sum + list.get(n);

    return sum;

Functional pseudo-code for the same function (the sum is passed as a parameter):

fun sumList([], sum) = sum
 |  sumList(v::lst, sum) = sumList(lst, v+sum)

I recommend the presentation Taming Effects with Functional Programming by Simon Peyton-Jones for a good introduction to functional concepts.

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You should mention that the functional version is tail-recursive and thus optimized to avoid stack overflows. (Some people might see the recursion and think that functional programming is bad because of it) –  alternative Oct 5 '10 at 22:36
+1 for describing the most important aspect of imperative vs. functional: control flow vs data flow. One thing I have to add is that functional paradigm and OO paradigm is not mutually exclusive; you can use OO paradigm to model how object (data) interacts, and functional paradigm to transform (manipulate) that object. –  Lie Ryan Oct 6 '10 at 10:19
Interestingly, you can model data-as-control and control-as-data as well to intermix. FP can use Arrows and first order functions to pass control flow around and manipulate it like data. OOP uses various design patterns to use objects for altering control flow. –  CodexArcanum Nov 1 '10 at 15:05
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Functional programming is based on a declarative model and has its roots from lambda calculus. It offers a lot of great concepts which can be borrowed from more imperative languages like C++ and C#.

Some examples include referential transparency, lambda functions, first class functions, lazy and eager evaluation, and immutability.

If for nothing else learning functional programming is useful for the concepts that it contains. It will change the way you do programming and think about programming. And I would guess that in the future functional programming will be just as important as object oriented programming has been.

To get started you can chose to use a pure functional language such as Haskell, or you can use a hybrid one like F#.

Most good universities will cover functional programming and if you go to school I'd highly suggest you take that course.

What are some of the big differences, pros and cons of functional programming vs. object-oriented programming?

Well object oriented programming is nice because it allows you to model your complex problem into hierarchies so you can simplify the problem. But it becomes very hard when you start to consider multi threaded programming while using mutable objects. In such cases you need to use heavy use of synchronization objects and it's near impossible to perfect a large application.

That's where functional programming comes in. Because of things like immutability functional programming really simplifies multi threaded programs. It makes it almost trivially easy to parallelize something when you know that given input X to a function it will always output Y. Also you know that a variable (or value in functional programming) can't change mid use from another thread.

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To be clear, Scheme is by no means a pure functional language. –  Jonathan Sterling Oct 6 '10 at 1:11
@Jonathan: Thanks, fixed. –  Brian R. Bondy Oct 6 '10 at 1:22
Your second last paragraph is completely bs. OO doesn't pose any problems in multithreading, mutability does. You seem to be confusing imperative programming with object-oriented programming. Is that the case? –  missingfaktor Oct 16 '10 at 15:02
@missingfaktor: No I am not confusing the concepts. An object typically has accessors, modifiers, data members and member functions. Yes not all objects need to have modifiers and you can implement them as immutable. But if you look at any arbitrary OO program, it will almost certainly have several objects which have modifiers and yet are still used by multi threads. I.e. in an OOP paradigm it's pretty rare to have everything immutable. –  Brian R. Bondy Oct 16 '10 at 22:24
You should read the answers to this question: stackoverflow.com/questions/3949618/fp-and-oo-orthogonal/… –  missingfaktor Oct 17 '10 at 4:28
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Functional programming and object-oriented programming are orthogonal to each other. You can have both in the same language. Examples: Scala, F#, OCaml etc.

Maybe you meant functional vs imperative, as Jonas suggested?

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Do you mean declarative eand imperative are not orthogonal to each other? And I am not refuting the fact that you can do functional in an OO language and vice versa. –  Brian R. Bondy Oct 17 '10 at 13:49
@Brian: Just to reply to your comment: I didn't say anything about the word 'declarative' as it encompasses many other styles than just functional (e.g. logical paradigm, function-level paradigm, tacit paradigm etc.) –  missingfaktor Oct 17 '10 at 13:57
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There is no real versus. They can be perfectly complementary. There are FP languages, which support OOP. But the communities differ in the way they handle modularity.

Users of FP languages tend to achieve modularity through mathematical laws. And prefer proofs to show compliance with their laws.

In imperative OOP users tend to capture the behaviour of the object in test-cases, which can be rerun if the object has changed and achieve in this way modularity.

It is just a small aspect, but I think it is worth mentioning.

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