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//! This module implements the runtime representation of Thunks.
//!
//! Thunks are a special kind of Nix value, similar to a 0-argument
//! closure that yields some value. Thunks are used to implement the
//! lazy evaluation behaviour of Nix:
//!
//! Whenever the compiler determines that an expression should be
//! evaluated lazily, it creates a thunk instead of compiling the
//! expression value directly. At any point in the runtime where the
//! actual value of a thunk is required, it is "forced", meaning that
//! the encompassing computation takes place and the thunk takes on
//! its new value.
//!
//! Thunks have interior mutability to be able to memoise their
//! computation. Once a thunk is evaluated, its internal
//! representation becomes the result of the expression. It is legal
//! for the runtime to replace a thunk object directly with its value
//! object, but when forcing a thunk, the runtime *must* mutate the
//! memoisable slot.
use std::{cell::RefCell, rc::Rc};
use crate::Value;
use super::Lambda;
/// Internal representation of the different states of a thunk.
#[derive(Debug)]
enum ThunkRepr {
/// Thunk is suspended and awaiting execution.
Suspended { lambda: Lambda },
/// Thunk is closed over some values, suspended and awaiting
/// execution.
ClosedSuspended {
lambda: Lambda,
upvalues: Vec<Value>,
},
/// Thunk currently under-evaluation; encountering a blackhole
/// value means that infinite recursion has occured.
Blackhole,
/// Fully evaluated thunk.
Evaluated(Value),
}
#[derive(Clone, Debug)]
pub struct Thunk(Rc<RefCell<ThunkRepr>>);
impl Thunk {
pub fn new(lambda: Lambda) -> Self {
Thunk(Rc::new(RefCell::new(ThunkRepr::Suspended { lambda })))
}
}
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