What is difference between functional and imperative programming languages?

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

Answer 1

Definition: An imperative language uses a sequence of statements to determine how to reach a certain goal. These statements are said to change the state of the program as each one is executed in turn.

Examples: Java is an imperative language. For example, a program can be created to add a series of numbers:

 int total = 0;
 int number1 = 5;
 int number2 = 10;
 int number3 = 15;
 total = number1 + number2 + number3; 

Each statement changes the state of the program, from assigning values to each variable to the final addition of those values. Using a sequence of five statements the program is explicitly told how to add the numbers 5, 10 and 15 together.

Functional languages: The functional programming paradigm was explicitly created to support a pure functional approach to problem solving. Functional programming is a form of declarative programming.

Advantages of Pure Functions: The primary reason to implement functional transformations as pure functions is that pure functions are composable: that is, self-contained and stateless. These characteristics bring a number of benefits, including the following: Increased readability and maintainability. This is because each function is designed to accomplish a specific task given its arguments. The function does not rely on any external state.

Easier reiterative development. Because the code is easier to refactor, changes to design are often easier to implement. For example, suppose you write a complicated transformation, and then realize that some code is repeated several times in the transformation. If you refactor through a pure method, you can call your pure method at will without worrying about side effects.

Easier testing and debugging. Because pure functions can more easily be tested in isolation, you can write test code that calls the pure function with typical values, valid edge cases, and invalid edge cases.

For OOP People or Imperative languages:

Object-oriented languages are good when you have a fixed set of operations on things and as your code evolves, you primarily add new things. This can be accomplished by adding new classes which implement existing methods and the existing classes are left alone.

Functional languages are good when you have a fixed set of things and as your code evolves, you primarily add new operations on existing things. This can be accomplished by adding new functions which compute with existing data types and the existing functions are left alone.

Cons:

It depends on the user requirements to choose the way of programming, so there is harm only when users don’t choose the proper way.

When evolution goes the wrong way, you have problems:

• Adding a new operation to an object-oriented program may require editing many class definitions to add a new method
• Adding a new kind of thing to a functional program may require editing many function definitions to add a new case.

Answer 2

I think it's possible to express functional programming in an imperative fashion:

• Using a lot of state check of objects and if... else/ switch statements
• Some timeout/ wait mechanism to take care of asynchornousness

There are huge problems with such approach:

• Rules/ procedures are repeated
• Statefulness leaves chances for side-effects/ mistakes

Functional programming, treating functions/ methods like objects and embracing statelessness, was born to solve those problems I believe.

Example of usages: frontend applications like Android, iOS or web apps' logics incl. communication with backend.

Other challenges when simulating functional programming with imperative/ procedural code:

• Race condition
• Complex combination and sequence of events. For example, user tries to send money in a banking app. Step 1) Do all of the following in parallel, only proceed if all is good a) Check if user is still good (fraud, AML) b) check if user has enough balance c) Check if recipient is valid and good (fraud, AML) etc. Step 2) perform the transfer operation Step 3) Show update on user's balance and/ or some kind of tracking. With RxJava for example, the code is concise and sensible. Without it, I can imagine there'd be a lot of code, messy and error prone code

I also believe that at the end of the day, functional code will get translated into assembly or machine code which is imperative/ procedural by the compilers. However, unless you write assembly, as humans writing code with high level/ human-readable language, functional programming is the more appropriate way of expression for the listed scenarios

Answer 3

Imperative programming style was practiced in web development from 2005 all the way to 2013.

With imperative programming, we wrote out code that listed exactly what our application should do, step by step.

The functional programming style produces abstraction through clever ways of combining functions.

There is mention of declarative programming in the answers and regarding that I will say that declarative programming lists out some rules that we are to follow. We then provide what we refer to as some initial state to our application and we let those rules kind of define how the application behaves.

Now, these quick descriptions probably don’t make a lot of sense, so lets walk through the differences between imperative and declarative programming by walking through an analogy.

Imagine that we are not building software, but instead we bake pies for a living. Perhaps we are bad bakers and don’t know how to bake a delicious pie the way we should.

So our boss gives us a list of directions, what we know as a recipe.

The recipe will tell us how to make a pie. One recipe is written in an imperative style like so:

1. Mix 1 cup of flour
2. Add 1 egg
3. Add 1 cup of sugar
4. Pour the mixture into a pan
5. Put the pan in the oven for 30 minutes and 350 degrees F.

The declarative recipe would do the following:

1 cup of flour, 1 egg, 1 cup of sugar - initial State

Rules

1. If everything mixed, place in pan.
2. If everything unmixed, place in bowl.
3. If everything in pan, place in oven.

So imperative approaches are characterized by step by step approaches. You start with step one and go to step 2 and so on.

You eventually end up with some end product. So making this pie, we take these ingredients mix them, put it in a pan and in the oven and you got your end product.

In a declarative world, its different.In the declarative recipe we would separate our recipe into two separate parts, start with one part that lists the initial state of the recipe, like the variables. So our variables here are the quantities of our ingredients and their type.

We take the initial state or initial ingredients and apply some rules to them.

So we take the initial state and pass them through these rules over and over again until we get a ready to eat rhubarb strawberry pie or whatever.

So in a declarative approach, we have to know how to properly structure these rules.

So the rules we might want to examine our ingredients or state, if mixed, put them in a pan.

With our initial state, that doesn’t match because we haven’t yet mixed our ingredients.

So rule 2 says, if they not mixed then mix them in a bowl. Okay yeah this rule applies.

Now we have a bowl of mixed ingredients as our state.

Now we apply that new state to our rules again.

So rule 1 says if ingredients are mixed place them in a pan, okay yeah now rule 1 does apply, lets do it.

Now we have this new state where the ingredients are mixed and in a pan. Rule 1 is no longer relevant, rule 2 does not apply.

Rule 3 says if the ingredients are in a pan, place them in the oven, great that rule is what applies to this new state, lets do it.

And we end up with a delicious hot apple pie or whatever.

Now, if you are like me, you may be thinking, why are we not still doing imperative programming. This makes sense.

Well, for simple flows yes, but most web applications have more complex flows that cannot be properly captured by imperative programming design.

In a declarative approach, we may have some initial ingredients or initial state like textInput=“”, a single variable.

Maybe text input starts off as an empty string.

We take this initial state and apply it to a set of rules defined in your application.

1. If a user enters text, update text input. Well, right now that doesn’t apply.

2. If template is rendered, calculate the widget.

3. If textInput is updated, re render the template.

Well, none of this applies so the program will just wait around for an event to happen.

So at some point a user updates the text input and then we might apply rule number 1.

We may update that to “abcd”

So we just updated our text and textInput updates, rule number 2 does not apply, rule number 3 says if text input is update, which just occurred, then re render the template and then we go back to rule 2 thats says if template is rendered, calculate the widget, okay lets calculate the widget.

In general, as programmers, we want to strive for more declarative programming designs.

Imperative seems more clear and obvious, but a declarative approach scales very nicely for larger applications.

Answer 4

• Imperative Languages:

• Efficient execution

• Complex semantics

• Complex syntax

• Concurrency is programmer designed

• Complex testing, has no referential transparency, has side effects

• Has state

• Functional Languages:

• Simple semantics

• Simple syntax

• Less efficient execution

• Programs can automatically be made concurrent

• Simple testing, has referential transparency, has no side effects

• Has no state