The most valuable property of functional programming is not immutability, monads, or writing everything as an arrow function.
It is referential transparency.
An expression is referentially transparent when it can be replaced by its value without changing the behavior of the program. That sounds mathematical because it is. It is also one of the most practical constraints we can impose on production software.
Referential transparency reduces what a reader must know. It makes behavior local. It turns dependencies, errors, and effects into things that can be seen and composed instead of reconstructed from runtime state.
That helps a person reviewing a pull request. It helps the next person debugging an incident. It also helps an AI coding agent operating inside a finite context window. Humans and models have very different minds, but they share one limitation: both struggle when the meaning of a line depends on invisible history.
