merge(defer.rs): Integrate at //fun/defer_rs
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fun/defer_rs/.gitignore
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fun/defer_rs/.gitignore
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/target/
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**/*.rs.bk
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Cargo.lock
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fun/defer_rs/Cargo.toml
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fun/defer_rs/Cargo.toml
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[package]
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name = "defer"
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version = "0.1.0"
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authors = ["Vincent Ambo <tazjin@gmail.com>"]
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[dependencies]
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fun/defer_rs/README.md
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fun/defer_rs/README.md
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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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fun/defer_rs/examples/defer-with-error.rs
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fun/defer_rs/examples/defer-with-error.rs
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// Go's defer in Rust, with error value return.
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use std::rc::Rc;
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use std::sync::RwLock;
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struct Defer<F: Fn()> {
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f: F
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}
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impl <F: Fn()> Drop for Defer<F> {
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fn drop(&mut self) {
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(self.f)()
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}
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}
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// Only added this for Go-syntax familiarity ;-)
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fn defer<F: Fn()>(f: F) -> Defer<F> {
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Defer { f }
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}
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// Convenience type synonym. This is a reference-counted smart pointer to
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// a shareable, mutable variable.
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// Rust does not allow willy-nilly mutation of shared variables, so explicit
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// write-locking must be performed.
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type ErrorHandle<T> = Rc<RwLock<Option<T>>>;
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///////////////////
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// Usage example //
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///////////////////
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#[derive(Debug)] // Debug trait for some default way to print the type.
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enum Error { DropError }
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fn main() {
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// Create a place to store the error.
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let drop_err: ErrorHandle<Error> = Default::default(); // create empty error
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// Introduce an arbitrary scope block (so that we still have control after
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// the defer runs):
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{
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let mut i = 1;
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// Rc types are safe to clone and share for multiple ownership.
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let err_handle = drop_err.clone();
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// Call defer and let the closure own the cloned handle to the error:
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let token = defer(move || {
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// do something!
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println!("Value is: {}", i);
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// ... oh no, it went wrong!
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*err_handle.write().unwrap() = Some(Error::DropError);
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});
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i += 1;
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println!("Value is: {}", i);
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// token goes out of scope here - drop() is called.
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}
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match *drop_err.read().unwrap() {
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Some(ref err) => println!("Oh no, an error occured: {:?}!", err),
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None => println!("Phew, everything went well.")
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};
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}
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// Prints:
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// Value is: 2
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// Value is: 1
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// Oh no, an error occured: DropError!
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31
fun/defer_rs/examples/defer.rs
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fun/defer_rs/examples/defer.rs
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// Go's defer in Rust!
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struct Defer<F: Fn()> {
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f: F
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}
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impl <F: Fn()> Drop for Defer<F> {
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fn drop(&mut self) {
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(self.f)()
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}
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}
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// Only added this for Go-syntax familiarity ;-)
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fn defer<F: Fn()>(f: F) -> Defer<F> {
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Defer { f }
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}
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fn main() {
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let mut i = 1;
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// Calling it "token" ... could be something else. The lifetime of this
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// controls when the action is run.
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let _token = defer(move || println!("Value is: {}", i));
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i += 1;
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println!("Value is: {}", i);
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}
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// Prints:
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// Value is: 2
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// Value is: 1
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40
fun/defer_rs/examples/undefer.rs
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fun/defer_rs/examples/undefer.rs
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// Go's defer in Rust, with a little twist!
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struct Defer<F: Fn()> {
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f: F
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}
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impl <F: Fn()> Drop for Defer<F> {
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fn drop(&mut self) {
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(self.f)()
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}
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}
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// Only added this for Go-syntax familiarity ;-)
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fn defer<F: Fn()>(f: F) -> Defer<F> {
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Defer { f }
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}
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// Changed your mind about the defer?
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// (Note: This leaks the closure! Don't actually do this!)
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fn undefer<F: Fn()>(token: Defer<F>) {
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use std::mem;
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mem::forget(token);
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}
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fn main() {
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let mut i = 1;
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// Calling it "token" ... could be something else. The lifetime of this
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// controls when the action is run.
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let token = defer(move || println!("Value is: {}", i));
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i += 1;
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println!("Value is: {}", i);
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// Oh, now I changed my mind about the previous defer:
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undefer(token);
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}
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// Prints:
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// Value is: 2
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