# Strict vs Lazy Evaluation

By

Travis Horn

Strict and lazy are two programming paradigms.

In strictly evaluated programming languages, expressions are evaluated as soon as they are defined.

let numbers = []
let i

for (i = 1; i < 9999; i = i + 1) {
	numbers.push(i)
}

numbers.slice(0, 5)
// Returns [1,2,3,4,5]

The code above fills an array with 9999 numbers. Even though we ultimately only ask for the first 5.

In lazily evaluated programming languages, expressions are only evaluated when they're needed.

numbers = [1..9999]
take 5 numbers
-- Returns [1,2,3,4,5]

Even though the list of numbers starts at 1 and goes up to 9999, the computer doesn't actually need to calculate up to 9999. When we ask for the first 5, it calculates and returns those 5 only.

In fact, we don't have to stop at 9999. We could tell Haskell that numbers represents all integers from 1 to infinity.

numbers = [1..]
take 5 numbers
-- Returns [1,2,3,4,5]

Haskell uses lazy evaluation.