[tmtvl]

Who paid: programmers switching to FP.

What was the price: two things:

- The programmers must stop using for-loops and [i]ndexes.

- The programmers must start using maps/folds/filters/et cetera.

What was the expected reward: the knowledge that their functions' outputs will only depend on their inputs.

In short: programmers who change their behaviour get the benefit of certainty about specific properties of their programs.