Slow down and re-read.
>> You get: the knowledge that your function's output will only depend on its input.
>> You pay: you gotta stop using those for-loops and [i]ndexes, and start using maps, folds, filters etc.
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.
