[BringTheTanks]

"Imagine you've implemented world peace. Now let's try to drop couple of nukes in the middle of this..."

Anyway, the point of functional programming is to have as many "immovable parts" as possible, because mutation stands in for parts in motion. And everyone in engineering knows parts that move are usually the first point of failure.

You might need a moving part here or there in a usable design eventually (not always) but you should do you darn best to have them isolated and as few as possible.

[cousin_it]

Aha! Moving parts are failure-prone because of friction. And friction happens when something's changing under you. Which means mutation. Nice analogy ;-)