Re:Rust RAII is better than the Haskell bracket pattern
by simias on Tuesday October 09, 2018 @05:48AM
(#18174522)
>Much as in C++, this is not really allowed: drop runs during panic unwinding, a panic during a panic will hard-abort the entire program.
Right, I didn't really consider that a "drawback" because I'm in the camp that considers that panic! shouldn't unwind but actually abort the process here and there anyway. But you're right that if you rely on the default unwinding behavior panic!ing in destructors is a very bad idea.
Re:Rust RAII is better than the Haskell bracket pattern
by cesarb on Tuesday October 09, 2018 @06:20AM
(#18174659)
> But you're right that if you rely on the default unwinding behavior
You do rely on the default unwinding behavior anyway at least for `cargo test`: the test framework depends on being able to catch the unwinds from `assert_eq!` and similar.
Much as in C++, this is not really allowed: drop runs during panic unwinding, a panic during a panic will hard-abort the entire program.
> I don't know if there's an elegant way to solve this.
I don't really think there is. Maybe opt-in linear types could be added. That would be at the cost of convenience (the compiler would require explicitly closing every file and handling the result, you could not just forget about it and expect it to be closed) but it would fix the issue and would slightly reduce the holding lifetime of resources.
Furthermore, for convenience we could imagine a wrapper pointer converting a linear structure into an affine one.
> How does Python's "with" handle that?
You'll get the exception from `__exit__` chaining to whatever exception triggered it (if any). Exceptions are the normal error-handling mechanism of Python so it's not out of place.