This is a major insight. Defining a local function isn't a big deal you can always just copy and pasta it out to global scope.

Any time you merge state with function you can no longer move the function. This is the same problem as OOP. Closures can't be modular the same way methods in objects can't be modular.

The smallest unit of testable module is the combinator. John Carmack literally mentioned he does pure functional programming now which basically everyone in this entire thread is completely ignoring.