What is difference between functional and imperative programming languages?
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
Here is the difference:
Imperative:
- Start
- Turn on your shoes size 9 1/2.
- Make room in your pocket to keep an array[7] of keys.
- Put the keys in the room for the keys in the pocket.
- Enter garage.
- Open garage.
- Enter Car.
... and so on and on ...
- Put the milk in the refrigerator.
- Stop.
Declarative, whereof functional is a subcategory:
- Milk is a healthy drink, unless you have problems digesting lactose.
- Usually, one stores milk in a refrigerator.
- A refrigerator is a box that keeps the things in it cool.
- A store is a place where items are sold.
- By "selling" we mean the exchange of things for money.
- Also, the exchange of money for things is called "buying".
... and so on and on ...
- Make sure we have milk in the refrigerator (when we need it - for lazy functional languages).
Summary: In imperative languages you tell the computer how to change bits, bytes and words in it's memory and in what order. In functional ones, we tell the computer what things, actions etc. are. For example, we say that the factorial of 0 is 1, and the factorial of every other natural number is the product of that number and the factorial of its predecessor. We don't say: To compute the factorial of n, reserve a memory region and store 1 there, then multiply the number in that memory region with the numbers 2 to n and store the result at the same place, and at the end, the memory region will contain the factorial.