A lazily evaluated value store

thunk.rs

use std::cell::Cell;

/// A thunk used for lazy generation and caching of values
struct Thunk<T: Copy> {
    /// Cached value stored in the thunk
    value: Cell<Option<T>>,
    
    /// Generator function to generate a missing value
    generator: fn() -> T
}

impl<T: Copy> Thunk<T> {
    /// Create a new thunk with the provided generator
    pub fn new(generator: fn() -> T) -> Self {
        Self {
            value: Cell::new(None),
            generator: generator
        }
    }
    
    /// Get the value of this thunk, create it from the generator if not already generated
    pub fn value(&self) -> T {
        if self.value.get().is_none() {
            self.value.set(
                Some((self.generator)())
            );
        }
        
        self.value.get()
            .unwrap()
    }
}