I definitely search when I write programs. Do you write a program from start to finish and it works, or do you iterate and debug and sometimes find you've made a misstep & need to change your approach?

Python's optional typing wasn't capable of spotting my issue, that was my point. Note I didn't say it was a better choice. I said, "strictly less complex." I wouldn't make a claim that Rust is "better." That only has meaning relative to a set of requirements. (Furthermore, I mentioned Python was a better choice for my project.)

Tree shaped lifetimes are a very robust and easy to work with set of lifetimes. Of course you can add more degrees of freedom and write programs with graph lifetimes. That's fine, but it's pretty clearly more complex. I view allowing arbitrary lifetimes as something I don't want unless it's, well, absolutely necessary is too strong, but unless there's a compelling reason. It's not something I want to spend my complexity budget on, because it makes it more difficult to reason about and debug a program.

How do you measure the complexity you're talking about here?

What if a program using graph shaped lifetimes was 10 times shorter than a program that had to use tree shaped lifetimes? Would you still say that the tree shaped program is less complex?

With Python for instance, it's just really difficult to say, because you may have arbitrary exceptions at any point. When writing long lived services, this is very significant; you need to handle all the exceptions that may be raised by you can't actually determine what that set is. I've had to read the source code of my dependencies on more than one occasion to figure this out, and still, you never get all of them before production.

With Rust this is much more manageable. Technically you could have an arbitrary panic at each step, but it's much less common to catch and handle panics than it is exceptions, and recoverable errors are handed via return values. Relative to Python, it's a breeze to figure out when errors may happen and what to do about them.

Similarly with my example before, what type are the keys in a Python dictionary? Probably the ones you meant to put there, but who really knows. What types are in the keys of a Rust dictionary? The ones you put there explicitly, full stop. There's no way for that to get screwed up or change from under you.

But I'm still not sure whether this idea is sufficient to explain away the situation where you have to write a far longer program in order to work around those restrictions.

For instance, safe Rust cannot express linked lists in the usual way, i.e using pointers. So in order to write a linked list in safe Rust, you would have to reimplement some of the functionality that pointers provide using array indices (or similar).

This program would be very long indeed, plus it would cause many of the same safety issues that safe Rust was supposed to avoid. Essentially, what this program would do is create an unsafe memory management system in safe Rust.

I believe that the dominant factor for complexity of a program is its length. If you have two programs that produce the same output for the same input and one is 10 tokens long while the other one is 10,000 tokens long then the first program will always be less complex.

I say "I believe" because I'm a bit out of my depth here. These claims (mine and yours) clearly need formal proofs and some formal definition of complexity.

But the thing is, you don't write C programs in Rust and you don't generally use doubly linked lists. (Linked lists turn out to be a very niche data structure that doesn't work well with modern CPUs anyway, but I digress.) You'd probably use a Vec or a BTree from the stdlib anywhere you're thinking of using a linked list.

So I don't think it's really the case that programs are significantly longer in Rust. Rust is more explicit, so you'll end up moving some things that existed as comments or documentation or just in your own head into code - that's a win that doesn't increase complexity, only exposes it. That program may look larger, but it's only because you can see it better.

It really depends on what those 10 tokens are doing. If I have a token that creates a new universe, seeds it with life, and creates the conditions for life to develop an optimal linked list - it might solve the problem in one step, but our atomic unit here is absolutely massive.

Similarly, if I compile a program to assembly I'll generally get many more tokens. But I can't really buy that I've increased the complexity of the program here.

I'm pretty satisfied with this understanding but I understand your desire for greater rigor.

The usefulness of linked lists is entirely beside the point. They just serve as an example for situations where additional restrictions can cause a program to be longer or more difficult to understand.

>It really depends on what those 10 tokens are doing. If I have a token that creates a new universe, seeds it with life, and creates the conditions for life to develop an optimal linked list - it might solve the problem in one step, but our atomic unit here is absolutely massive.

This example shows why I think that your claims lack a definition of complexity. You're now saying that the output of a program determines its complexity. I don't think that's a useful measure of complexity because it doesn't allow us to compare the complexity of two programs that produce the same output.

Rust's tree-shaped lifetimes are a big benefit for me, as someone who values not wasting RAM.

(Seriously. Why did I have to upgrade my computer to 32GiB of RAM when I haven't appreciably changed what I use it for since I bought it wth 4GiB of RAM and sized that amount based on intent to run a VirtualBox VM or two? ...JavaScript bloat. ...though I will admit that I also upgraded the CPU, RAM, and motherboard in the transition from 4GiB to 16GiB... but that was because a RAM socket went bad.)

I think of borrow-checker errors as "Wait a minute. Can you clarify what you intended here? Did you want multiple ownership? A copy? Was accessing it this far down a mistake?"