What is difference between functional and imperati

Question

Most of the mainstream languages, including object-oriented programming (OOP) languages such as C#, Visual Basic, C++, and Java were designed to primarily support imperative (procedural) programming, whereas Haskell/gofer like languages are purely functional. Can anybody elaborate on what is the difference between these two ways of programming?

I know it depends on user requirements to choose the way of programming but why is it recommended to learn functional programming languages?

Answer 1

Most modern languages are in varying degree both imperative and functional but to better understand functional programming, it will be best to take an example of pure functional language like Haskell in contrast of imperative code in not so functional language like java/c#. I believe it is always easy to explain by example, so below is one.

Functional programming: calculate factorial of n i.e n! i.e n x (n-1) x (n-2) x ...x 2 X 1

-- | Haskell comment goes like
-- | below 2 lines is code to calculate factorial and 3rd is it's execution  

factorial 0 = 1
factorial n = n * factorial (n - 1)
factorial 3

-- | for brevity let's call factorial as f; And x => y shows order execution left to right
-- | above executes as := f(3) as 3 x f(2) => f(2) as 2 x f(1) => f(1) as 1 x f(0) => f(0) as 1  
-- | 3 x (2 x (1 x (1)) = 6

Notice that Haskel allows function overloading to the level of argument value. Now below is example of imperative code in increasing degree of imperativeness :

//somewhat functional way
function factorial(n) {
  if(n < 1) {
     return 1;
  }
  return n * n-1;   
}
factorial(3);

//somewhat more imperative way
function imperativeFactor(n) {
  int f = 1
  for(int i = 1; i <= n; i++) {
     f = f * i
  }
  return f;
}

This read can be a good reference to understand that how imperative code focus more on how part, state of machine ( i in for loop), order of execution, flow control.

The later example can be seen as java/c# lang code roughly and first part as limitation of the language itself in contrast of Haskell to overload the function by value ( zero ) and hence can be said it is not purist functional language, on the other hand you can say it support functional prog. to some extent.

Disclosure: none of the above code is tested/executed but hopefully should be good enough to convey the concept; also I would appreciate comments for any such correction :)

Answer 2

Functional Programming is a form of declarative programming, which describe the logic of computation and the order of execution is completely de-emphasized.

Problem: I want to change this creature from a horse to a giraffe.

• Lengthen neck
• Lengthen legs
• Apply spots
• Give the creature a black tongue
• Remove horse tail

Each item can be run in any order to produce the same result.

Imperative Programming is procedural. State and order is important.

Problem: I want to park my car.

1. Note the initial state of the garage door
2. Stop car in driveway
3. If the garage door is closed, open garage door, remember new state; otherwise continue
4. Pull car into garage
5. Close garage door

Each step must be done in order to arrive at desired result. Pulling into the garage while the garage door is closed would result in a broken garage door.