Functional programming DOES NOT PERMIT mutability or side-effects. I think the word you don't realize is missing there is "Pure". Pure functional programming,

Functional programming DOES NOT PERMIT mutability or side-effects. I think the word you don't realize is missing there is "Pure". Pure functional programming, and pure functional style, is everything you think functional programming and functional style are. Purity is precisely the technical requirement that constrains a function to not have side-effects and to not mutate data. If, as you say, functional programming did not permit mutability, the term "pure functional programming" would have been meaningless. But functional programming is not, and has never been, solely pure functional programming; and despite the positive connotations that the word "pure" commonly evokes, functional programming with mutable data is not some kind of inferior version of pure functional programming, a poor distant relative fresh from the provinces come to beg favors from a wealthy merchant. No. This belief commonly stems from ignorance of the history of functional languages coupled with wide-eyed admiration of an especially elegant pure functional idiom recently learned. Some people believe that the purer the better (this belief has only gained popularity relatively recently, in fact), others think that mutable state is just fine where it helps rather than hinders. Both kinds of people happily write functional programs using functional programming languages.