diff --git a/examples/defer-with-error.rs b/examples/defer-with-error.rs new file mode 100644 index 000000000..26d56d77c --- /dev/null +++ b/examples/defer-with-error.rs @@ -0,0 +1,70 @@ +// Go's defer in Rust, with error value return. + +use std::rc::Rc; +use std::sync::RwLock; + +struct Defer { + f: F +} + +impl Drop for Defer { + fn drop(&mut self) { + (self.f)() + } +} + +// Only added this for Go-syntax familiarity ;-) +fn defer(f: F) -> Defer { + 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 = Rc>>; + +/////////////////// +// 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 = 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!