53 lines
2.4 KiB
Markdown
53 lines
2.4 KiB
Markdown
defer in Rust
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=============
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After a Hacker News discussion about implementing Go's `defer` keyword in C++,
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I stumbled upon [this comment](https://news.ycombinator.com/item?id=15523589)
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and more specifically this response to it by "Occivink":
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> There's plenty of one-time cases where you don't want to declare an entire
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> class but still enjoy scope-based functions.
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Specificall the "don't want to declare an entire class" suggests that languages
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like C++ have high friction for explaining your desired invariant (cleanup is
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run when `$thing` is destroyed) to the compiler.
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It seems like most languages either hand-wave this away (*cough* Java *cough*)
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or use what seems like a workaround (`defer`).
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Rust has the so-called `Drop` trait, which is a typeclass that contains a single
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method with no return value that is run when a variable is dropped (i.e. goes out
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of scope).
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This works fine for most general cases - i.e. closing file handlers - but can
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get complicated if other use-cases of `defer` are considered:
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* returning an error-value by mutating a reference in the enclosing scope (oh boy)
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* deferring a decision about when/whether to run cleanup to the caller
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While thinking about how to do this with the `Drop` trait I realised that `defer`
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can actually be trivially implemented in Rust, using `Drop`.
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A simple implementation of `defer` can be seen in [defer.rs](examples/defer.rs),
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an implementation using shared mutable state for error returns is in the file
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[defer-with-error.rs](examples/defer-with-error.rs) and an implementation that
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allows cleanup to be *cancelled* (don't _actually_ do this, it leaks a pointer)
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is in [undefer.rs](examples/undefer.rs).
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Whether any of this is actually useful is not up to me to decide. I haven't
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actually had a real-life need for this.
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You can run the examples with `cargo run --example defer`, etc.
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## Notes
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* `Drop` is not guaranteed to run in case of panics or program aborts, if you
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need support for that check out [scopeguard](https://github.com/bluss/scopeguard)
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* `undefer` could be implemented safely by, for example, carrying a boolean that
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by default causes execution to happen but can be flipped to disable it
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## Further reading:
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* [The Pain Of Real Linear Types in Rust](https://gankro.github.io/blah/linear-rust/)
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* [Go's defer](https://tour.golang.org/flowcontrol/12)
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* [Rust's Drop](https://doc.rust-lang.org/std/ops/trait.Drop.html)
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