4 ms·

Lazy evaluation, I think?

• dbaupp 11y ago
  That can still be done without a GC, e.g. the following is sketch of lazy evaluation in Rust: use std::mem; enum Lazy<T> { Thunk(Box<Fn() -> T>), EvalInProgress, Forced(T), } impl<T> Lazy<T> { fn new<F: 'static + Fn() -> T>(f: F) -> Lazy<T> { Lazy::Thunk(Box::new(f)) } fn force(&mut self) { *self = match mem::replace(self, Lazy::EvalInProgress) { Lazy::Thunk(f) => Lazy::Forced(f()), Lazy::EvalInProgress => panic!("forcing while evaluating"), Lazy::Forced(x) => Lazy::Forced(x), } } fn get(&mut self) -> &mut T { self.force(); match *self { Lazy::Thunk(_) | Lazy::EvalInProgress => unreachable!(), Lazy::Forced(ref mut x) => x, } } } fn main() { let mut lazy = Lazy::new(|| { println!("evaluating"); 10 }); println!("first: {}", lazy.get()); println!("second: {}", lazy.get()); } Output: evaluating first: 10 second: 10 (There's a pile of tweaks/optimisations that can be made, like making `force` truly a no-op for values that have already been forced and removing the need for the lazy object to be mutable, but the above demonstrates the core idea.)