When imperative style fits better?

Question

From the Programming in Scala (second edition), bottom of the p.98:

A balanced attitude for Scala programmers

Prefer vals, immutable objects, and methods without side effects. Reach for them first. Use vars, mutable objects, and methods with side effects when you have a specific need and justification for them.

It is explained on previous pages why to prefer vals, immutable objects, and methods without side effects so this sentence makes perfect sense.

But second sentence:"Use vars, mutable objects, and methods with side effects when you have a specific need and justification for them." is not explained so well.

So my question is:

What is justification or specific need to use vars, mutable objects and methods with side effect?

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P.s.: It would be great if someone could provide some examples for each of those (besides explanation).

Answer 1

In many cases functional programming increases the level of abstraction and hence makes your code more concise and easier/faster to write and understand. But there are situations where the resulting bytecode cannot be as optimized (fast) as for an imperative solution.

Currently (Scala 2.9.1) one good example is summing up ranges:

(1 to 1000000).foldLeft(0)(_ + _)

Versus:

var x = 1
var sum = 0
while (x <= 1000000) {
  sum += x
  x += 1
}

If you profile these you will notice a significant difference in execution speed. So sometimes performance is a really good justification.