Rust RAII is better than the Haskell bracket pattern

Re:Rust RAII is better than the Haskell bracket pattern by steveklabnik on Tuesday October 09, 2018 @11:13AM (#18176572)

> the whole point of Rust is effortless correctness and safety.

Rust has never been about proving correctness. Yes, correctness is a goal, but it is subservient to other goals, depending on details.

Furthermore, it's not clear that this can really be implemented in a reasonable way, see https://news.ycombinator.com/item?id=18175838

> it seems sometimes that Rust management would rather focus on cool new language enhancements

In this comment, you're complaining that we haven't implemented a "cool new language enhancement." This is at odds with your desire stated here.

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Re:Rust RAII is better than the Haskell bracket pattern by electrograv on Tuesday October 09, 2018 @12:57PM (#18177633)

Apologies if it sounded like I demanded a “cool new language enhancement” solution to this issue. On the contrary; if it were just documented that either fs::File [1] or the io::Write trait [2] could silently lose data with no error codes when dropped, that would be one such sufficient solution.

Perhaps I misread this documentation; if so, I don’t think I’d be alone here. I don’t see any particular mention that dropping an fs::File could lead to data loss, and I had generally assumed major edge cases like ‘data loss from a file system library’ would be documented.

[1] https://doc.rust-lang.org/std/fs/struct.File.html

[2] https://doc.rust-lang.org/std/io/trait.Write.html

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Re:Rust RAII is better than the Haskell bracket pattern by steveklabnik on Tuesday October 09, 2018 @01:04PM (#18177708)

It falls out of general principle; destructors may not be called. That said, I would happily accept a PR to make this explicit. Even an issue would be nice!

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Re:Rust RAII is better than the Haskell bracket pattern by Animats on Tuesday October 09, 2018 @01:24PM (#18177916)

Furthermore, it's not clear that this can really be implemented in a reasonable way

Which is why RAII in a language without exceptions is inappropriate for a resource which has a status on closeout.

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Re:Rust RAII is better than the Haskell bracket pattern by pcwalton on Tuesday October 09, 2018 @01:28PM (#18177970)

The alternative to closing the file in the destructor is leaking OS resources in the event of unexpected control flow destroying the file object. I fail to see how that is preferable.

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Re:Rust RAII is better than the Haskell bracket pattern by masklinn on Tuesday October 09, 2018 @02:14PM (#18178409)

A linear resource would require that the resource is explicitly released on all codepaths no?

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Re:Rust RAII is better than the Haskell bracket pattern by pcwalton on Tuesday October 09, 2018 @02:21PM (#18178474)

Not in Rust, and not in any language where you can put resources in objects with shared ownership (i.e. any remotely popular general purpose language).

Throwing exceptions isn't a particularly good solution either, for the same reason. Exceptions are hard to reliably handle when you can't easily reason about where they will be thrown from.

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