FP is a cult.
It's a programming paradigm based on simple, well-understood mathematical framework. It's also the starting point for most research in programming language theory.
“A closure can capture constants and variables from the surrounding context in which it’s defined. The closure can then refer to and modify the values of those constants and variables from within its body, even if the original scope that defined the constants and variables no longer exists.”
https://docs.swift.org/swift-book/LanguageGuide/Closures.htm...
What makes you think Swift only supports functional programming?
> I would have used Haskell doc reference
And if you would have, you would have disproved your own point. In Haskell, closures cannot mutate state, because mutable state is not part of the language semantics. It's modeled explicitly via monads.