//! 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::{Ref, RefCell, RefMut},
collections::HashSet,
rc::Rc,
};
use codemap::Span;
use crate::{
errors::{Error, ErrorKind},
upvalues::Upvalues,
value::Closure,
vm::VM,
Value,
};
use super::{Lambda, TotalDisplay};
/// Internal representation of the different states of a thunk.
///
/// Upvalues must be finalised before leaving the initial state
/// (Suspended or RecursiveClosure). The [`value()`] function may
/// not be called until the thunk is in the final state (Evaluated).
#[derive(Clone, Debug)]
enum ThunkRepr {
/// Thunk is closed over some values, suspended and awaiting
/// execution.
Suspended {
lambda: Rc<Lambda>,
upvalues: Upvalues,
span: Span,
},
/// Thunk currently under-evaluation; encountering a blackhole
/// value means that infinite recursion has occured.
Blackhole,
/// Fully evaluated thunk.
Evaluated(Value),
}
/// A thunk is created for any value which requires non-strict
/// evaluation due to self-reference or lazy semantics (or both).
/// Every reference cycle involving `Value`s will contain at least
/// one `Thunk`.
#[derive(Clone, Debug)]
pub struct Thunk(Rc<RefCell<ThunkRepr>>);
impl Thunk {
pub fn new_closure(lambda: Rc<Lambda>) -> Self {
Thunk(Rc::new(RefCell::new(ThunkRepr::Evaluated(Value::Closure(
Closure {
upvalues: Rc::new(Upvalues::with_capacity(lambda.upvalue_count)),
lambda: lambda.clone(),
#[cfg(debug_assertions)]
is_finalised: false,
},
)))))
}
pub fn new_suspended(lambda: Rc<Lambda>, span: Span) -> Self {
Thunk(Rc::new(RefCell::new(ThunkRepr::Suspended {
upvalues: Upvalues::with_capacity(lambda.upvalue_count),
lambda: lambda.clone(),
span,
})))
}
/// Evaluate the content of a thunk, potentially repeatedly, until a
/// non-thunk value is returned.
///
/// This will change the existing thunk (and thus all references to it,
/// providing memoization) through interior mutability. In case of nested
/// thunks, the intermediate thunk representations are replaced.
pub fn force(&self, vm: &mut VM) -> Result<(), ErrorKind> {
loop {
let mut thunk_mut = self.0.borrow_mut();
match *thunk_mut {
ThunkRepr::Evaluated(Value::Thunk(ref inner_thunk)) => {
let inner_repr = inner_thunk.0.borrow().clone();
*thunk_mut = inner_repr;
}
ThunkRepr::Evaluated(_) => return Ok(()),
ThunkRepr::Blackhole => return Err(ErrorKind::InfiniteRecursion),
ThunkRepr::Suspended { .. } => {
if let ThunkRepr::Suspended {
lambda,
upvalues,
span,
} = std::mem::replace(&mut *thunk_mut, ThunkRepr::Blackhole)
{
drop(thunk_mut);
vm.enter_frame(lambda, upvalues, 0)
.map_err(|e| ErrorKind::ThunkForce(Box::new(Error { span, ..e })))?;
(*self.0.borrow_mut()) = ThunkRepr::Evaluated(vm.pop())
}
}
}
}
}
pub fn finalise(&self, stack: &[Value]) {
self.upvalues_mut().resolve_deferred_upvalues(stack);
#[cfg(debug_assertions)]
{
let inner: &mut ThunkRepr = &mut self.0.as_ref().borrow_mut();
if let ThunkRepr::Evaluated(Value::Closure(c)) = inner {
c.is_finalised = true;
}
}
}
pub fn is_evaluated(&self) -> bool {
matches!(*self.0.borrow(), ThunkRepr::Evaluated(_))
}
/// Returns a reference to the inner evaluated value of a thunk.
/// It is an error to call this on a thunk that has not been
/// forced, or is not otherwise known to be fully evaluated.
// Note: Due to the interior mutability of thunks this is
// difficult to represent in the type system without impacting the
// API too much.
pub fn value(&self) -> Ref<Value> {
Ref::map(self.0.borrow(), |thunk| match thunk {
ThunkRepr::Evaluated(value) => {
#[cfg(debug_assertions)]
if matches!(
value,
Value::Closure(Closure {
is_finalised: false,
..
})
) {
panic!("Thunk::value called on an unfinalised closure");
}
return value;
}
ThunkRepr::Blackhole => panic!("Thunk::value called on a black-holed thunk"),
ThunkRepr::Suspended { .. } => panic!("Thunk::value called on a suspended thunk"),
})
}
pub fn upvalues(&self) -> Ref<'_, Upvalues> {
Ref::map(self.0.borrow(), |thunk| match thunk {
ThunkRepr::Suspended { upvalues, .. } => upvalues,
ThunkRepr::Evaluated(Value::Closure(Closure { upvalues, .. })) => upvalues,
_ => panic!("upvalues() on non-suspended thunk"),
})
}
pub fn upvalues_mut(&self) -> RefMut<'_, Upvalues> {
RefMut::map(self.0.borrow_mut(), |thunk| match thunk {
ThunkRepr::Suspended { upvalues, .. } => upvalues,
ThunkRepr::Evaluated(Value::Closure(Closure {
upvalues,
#[cfg(debug_assertions)]
is_finalised,
..
})) => {
#[cfg(debug_assertions)]
if *is_finalised {
panic!("Thunk::upvalues_mut() called on a finalised closure");
}
Rc::get_mut(upvalues)
.expect("upvalues_mut() was called on a thunk which already had multiple references to it")
}
thunk => panic!("upvalues() on non-suspended thunk: {thunk:?}"),
})
}
/// Do not use this without first reading and understanding
/// `tvix/docs/value-pointer-equality.md`.
pub(crate) fn ptr_eq(&self, other: &Self) -> bool {
Rc::ptr_eq(&self.0, &other.0)
}
}
impl TotalDisplay for Thunk {
fn total_fmt(&self, f: &mut std::fmt::Formatter<'_>, set: &mut ThunkSet) -> std::fmt::Result {
if !set.insert(self) {
return f.write_str("<CYCLE>");
}
match self.0.try_borrow() {
Ok(repr) => match &*repr {
ThunkRepr::Evaluated(v) => v.total_fmt(f, set),
_ => f.write_str("internal[thunk]"),
},
_ => f.write_str("internal[thunk]"),
}
}
}
/// A wrapper type for tracking which thunks have already been seen in a
/// context. This is necessary for cycle detection.
///
/// The inner `HashSet` is not available on the outside, as it would be
/// potentially unsafe to interact with the pointers in the set.
#[derive(Default)]
pub struct ThunkSet(HashSet<*mut ThunkRepr>);
impl ThunkSet {
/// Check whether the given thunk has already been seen. Will mark the thunk
/// as seen otherwise.
pub fn insert(&mut self, thunk: &Thunk) -> bool {
let ptr: *mut ThunkRepr = thunk.0.as_ptr();
self.0.insert(ptr)
}
}
