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Comment by username90 | Hacker News Reader

The major problem with functional programming is that many idioms requires a garbage collector. For example having pure operations on binary trees by just replacing dirty nodes leaving all old references intact wouldn't be possible in rust.

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Regarding binary trees: You can always go back to using old-fashioned arrays and indices. A few simple contiguous arrays of nodes (e.g. a key array and value array) is often all you need, where nodes simply refer to each other via int indices into these arrays. You may be surprised that this often yields some of the best-performing data structures, sometimes better than those using raw pointers and non-contiguous heap allocations.

And, this works in Rust just as it does in C. But at this point, you lose out on the benefits of Rust's static type system and borrow-checker:

1. The compiler will no longer be able to correctness-check the validity of these 'int' style references to other memory.

2. If something does go wrong and you read the array with a bad integer index, Rust will just 'panic' and crash the application. Unlike C, there will be no risk of memory errors or related security vulnerabilities. But on the other hand, each read being bounds-checked will make such Rust code slightly slower than is possible with C/C++. (Though I think you can use 'unsafe' blocks if you want to hyper-optimize akin to C.)

But regarding GC langauges, I generally agree; they're almost always more trouble than they're worth in any performance-sensitive context. You often end up using approaches like this to optimize around the GC (e.g. int indexes into pre-allocated arrays) which ultimately means you're coding C-style in a GC language anyway, which defeats the whole point of GC's productivity-enhancing benefit -- at that point, why not just go all the way and use C or Zig or Rust etc.?

It isn't entirely impossible. Erlang's processes (actors, not actual OS processes; they are like mini-green threads / fibers) very often just throw away all their data after they are done with their work, and that data is often entirely stored entirely in the stack, without any GC needed -- even if the language is GC-ed. Also, all GC is per actor so there's no stop-the-world GC pauses.

But the scenario you outline, yep, it does seem impossible with a borrow checker.

And don't get me wrong. I am gradually learning Rust and I love it. But I can see myself being a little grumpy with the procedural style and often quirky syntax. Oh well, can't have it all, right?

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