Programmers use mutable variables, but, the practice causes trouble.

One use of mutable variables is to create flags.

Wouldn't it be nice if there were a declarative way to express flags?

I think that parsers do this.

I discuss the issues in https://guitarvydas.github.io/2021/04/28/Parsing-vs-FP-Pattern-Matching.html

[Q: What else, if anything, are mutable variables used for?]