Sure, everything as immutable does make certain guarantees about the code a lot easier. Also certain algorithms become very nice.
Purity is pretty awesome. I really like conceptual Haskell (actual real Haskell tends to become too abstract for my brain to remember important details when I come back to a project...).
The (sad) fact is that mutating a bit of state is actually often much faster, or a lot cleaner to read / understand than implementing a fully pure functional version. And by limiting access to state, you make sure you don't accidentally introduce dependencies.
Using closures like this, can end up being very similar to object-oriented code (in the best-case, strictly limited sense) whereby each area of concern in the code can only modify what you expect it to be able to modify, and you can be sure nowhere else in the code can access it.
Purity is pretty awesome. I really like conceptual Haskell (actual real Haskell tends to become too abstract for my brain to remember important details when I come back to a project...).
The (sad) fact is that mutating a bit of state is actually often much faster, or a lot cleaner to read / understand than implementing a fully pure functional version. And by limiting access to state, you make sure you don't accidentally introduce dependencies.
Using closures like this, can end up being very similar to object-oriented code (in the best-case, strictly limited sense) whereby each area of concern in the code can only modify what you expect it to be able to modify, and you can be sure nowhere else in the code can access it.