merge(defer.rs): Integrate at //fun/defer_rs

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Vincent Ambo 2019-12-21 00:53:42 +00:00
commit fbdc9b1d60
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/target/
**/*.rs.bk
Cargo.lock

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[package]
name = "defer"
version = "0.1.0"
authors = ["Vincent Ambo <tazjin@gmail.com>"]
[dependencies]

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

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// Go's defer in Rust, with error value return.
use std::rc::Rc;
use std::sync::RwLock;
struct Defer<F: Fn()> {
f: F
}
impl <F: Fn()> Drop for Defer<F> {
fn drop(&mut self) {
(self.f)()
}
}
// Only added this for Go-syntax familiarity ;-)
fn defer<F: Fn()>(f: F) -> Defer<F> {
Defer { f }
}
// Convenience type synonym. This is a reference-counted smart pointer to
// a shareable, mutable variable.
// Rust does not allow willy-nilly mutation of shared variables, so explicit
// write-locking must be performed.
type ErrorHandle<T> = Rc<RwLock<Option<T>>>;
///////////////////
// Usage example //
///////////////////
#[derive(Debug)] // Debug trait for some default way to print the type.
enum Error { DropError }
fn main() {
// Create a place to store the error.
let drop_err: ErrorHandle<Error> = Default::default(); // create empty error
// Introduce an arbitrary scope block (so that we still have control after
// the defer runs):
{
let mut i = 1;
// Rc types are safe to clone and share for multiple ownership.
let err_handle = drop_err.clone();
// Call defer and let the closure own the cloned handle to the error:
let token = defer(move || {
// do something!
println!("Value is: {}", i);
// ... oh no, it went wrong!
*err_handle.write().unwrap() = Some(Error::DropError);
});
i += 1;
println!("Value is: {}", i);
// token goes out of scope here - drop() is called.
}
match *drop_err.read().unwrap() {
Some(ref err) => println!("Oh no, an error occured: {:?}!", err),
None => println!("Phew, everything went well.")
};
}
// Prints:
// Value is: 2
// Value is: 1
// Oh no, an error occured: DropError!

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// Go's defer in Rust!
struct Defer<F: Fn()> {
f: F
}
impl <F: Fn()> Drop for Defer<F> {
fn drop(&mut self) {
(self.f)()
}
}
// Only added this for Go-syntax familiarity ;-)
fn defer<F: Fn()>(f: F) -> Defer<F> {
Defer { f }
}
fn main() {
let mut i = 1;
// Calling it "token" ... could be something else. The lifetime of this
// controls when the action is run.
let _token = defer(move || println!("Value is: {}", i));
i += 1;
println!("Value is: {}", i);
}
// Prints:
// Value is: 2
// Value is: 1

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// Go's defer in Rust, with a little twist!
struct Defer<F: Fn()> {
f: F
}
impl <F: Fn()> Drop for Defer<F> {
fn drop(&mut self) {
(self.f)()
}
}
// Only added this for Go-syntax familiarity ;-)
fn defer<F: Fn()>(f: F) -> Defer<F> {
Defer { f }
}
// Changed your mind about the defer?
// (Note: This leaks the closure! Don't actually do this!)
fn undefer<F: Fn()>(token: Defer<F>) {
use std::mem;
mem::forget(token);
}
fn main() {
let mut i = 1;
// Calling it "token" ... could be something else. The lifetime of this
// controls when the action is run.
let token = defer(move || println!("Value is: {}", i));
i += 1;
println!("Value is: {}", i);
// Oh, now I changed my mind about the previous defer:
undefer(token);
}
// Prints:
// Value is: 2