# Imperative vs Declarative

By

Travis Horn

Imperative and declarative are two programming paradigms. A paradigm is a way to classify programming languages based on their features.

Imperative programming consists of commands for the computer to perform. They tell the computer how to change its state.

function sum (array) {
	let i
	let sum = 0
	
	for (i = 0; i < array.length; i = i + 1) {
		sum = sum + array[i]
	}
	
	return sum
}

sum ([1,2,3,4,5])
// Returns 15

The code above does not give a definition of what a sum actually is. Instead it is describing an algorithm that can produce a sum.

Declarative programming expresses the logic without describing its control flow. It tells the computer what it must accomplish rather than describing how to do it.

sum [] = 0
sum (x:xs) = x + sum xs

sum [1,2,3,4,5]
-- Returns 15

Haskell is a declarative programming language.