//! 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,
fmt::Debug,
rc::Rc,
};
use crate::{
errors::ErrorKind,
spans::LightSpan,
upvalues::Upvalues,
value::Closure,
vm::generators::{self, GenCo},
Value,
};
use super::{Lambda, TotalDisplay};
use codemap::Span;
/// Internal representation of a suspended native thunk.
struct SuspendedNative(Box<dyn Fn() -> Result<Value, ErrorKind>>);
impl Debug for SuspendedNative {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(f, "SuspendedNative({:p})", self.0)
}
}
/// 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(Debug)]
enum ThunkRepr {
/// Thunk is closed over some values, suspended and awaiting
/// execution.
Suspended {
lambda: Rc<Lambda>,
upvalues: Rc<Upvalues>,
light_span: LightSpan,
},
/// Thunk is a suspended native computation.
Native(SuspendedNative),
/// Thunk currently under-evaluation; encountering a blackhole
/// value means that infinite recursion has occured.
Blackhole {
/// Span at which the thunk was first forced.
forced_at: LightSpan,
/// Span at which the thunk was originally suspended.
suspended_at: Option<LightSpan>,
/// Span of the first instruction of the actual code inside
/// the thunk.
content_span: Option<Span>,
},
/// Fully evaluated thunk.
Evaluated(Value),
}
impl ThunkRepr {
fn debug_repr(&self) -> String {
match self {
ThunkRepr::Evaluated(v) => format!("thunk(val|{})", v),
ThunkRepr::Blackhole { .. } => "thunk(blackhole)".to_string(),
ThunkRepr::Native(_) => "thunk(native)".to_string(),
ThunkRepr::Suspended { lambda, .. } => format!("thunk({:p})", *lambda),
}
}
}
/// 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(
Rc::new(Closure {
upvalues: Rc::new(Upvalues::with_capacity(lambda.upvalue_count)),
lambda: lambda.clone(),
}),
)))))
}
pub fn new_suspended(lambda: Rc<Lambda>, light_span: LightSpan) -> Self {
Thunk(Rc::new(RefCell::new(ThunkRepr::Suspended {
upvalues: Rc::new(Upvalues::with_capacity(lambda.upvalue_count)),
lambda: lambda.clone(),
light_span,
})))
}
pub fn new_suspended_native(native: Box<dyn Fn() -> Result<Value, ErrorKind>>) -> Self {
Thunk(Rc::new(RefCell::new(ThunkRepr::Native(SuspendedNative(
native,
)))))
}
fn prepare_blackhole(&self, forced_at: LightSpan) -> ThunkRepr {
match &*self.0.borrow() {
ThunkRepr::Suspended {
light_span, lambda, ..
} => ThunkRepr::Blackhole {
forced_at,
suspended_at: Some(light_span.clone()),
content_span: Some(lambda.chunk.first_span()),
},
_ => ThunkRepr::Blackhole {
forced_at,
suspended_at: None,
content_span: None,
},
}
}
// TODO(amjoseph): de-asyncify this
pub async fn force(self, co: GenCo, span: LightSpan) -> Result<Value, ErrorKind> {
// If the current thunk is already fully evaluated, return its evaluated
// value. The VM will continue running the code that landed us here.
if self.is_forced() {
return Ok(self.value().clone());
}
// Begin evaluation of this thunk by marking it as a blackhole, meaning
// that any other forcing frame encountering this thunk before its
// evaluation is completed detected an evaluation cycle.
let inner = self.0.replace(self.prepare_blackhole(span));
match inner {
// If there was already a blackhole in the thunk, this is an
// evaluation cycle.
ThunkRepr::Blackhole {
forced_at,
suspended_at,
content_span,
} => Err(ErrorKind::InfiniteRecursion {
first_force: forced_at.span(),
suspended_at: suspended_at.map(|s| s.span()),
content_span,
}),
// If there is a native function stored in the thunk, evaluate it
// and replace this thunk's representation with the result.
ThunkRepr::Native(native) => {
let value = native.0()?;
// Force the returned value again, in case the native call
// returned a thunk.
let value = generators::request_force(&co, value).await;
self.0.replace(ThunkRepr::Evaluated(value.clone()));
Ok(value)
}
// When encountering a suspended thunk, request that the VM enters
// it and produces the result.
ThunkRepr::Suspended {
lambda,
upvalues,
light_span,
} => {
let value =
generators::request_enter_lambda(&co, lambda, upvalues, light_span).await;
// This may have returned another thunk, so we need to request
// that the VM forces this value, too.
let value = generators::request_force(&co, value).await;
self.0.replace(ThunkRepr::Evaluated(value.clone()));
Ok(value)
}
// If an inner value is found, force it and then update. This is
// most likely an inner thunk, as `Thunk:is_forced` returned false.
ThunkRepr::Evaluated(val) => {
let value = generators::request_force(&co, val).await;
self.0.replace(ThunkRepr::Evaluated(value.clone()));
Ok(value)
}
}
}
pub fn finalise(&self, stack: &[Value]) {
self.upvalues_mut().resolve_deferred_upvalues(stack);
}
pub fn is_evaluated(&self) -> bool {
matches!(*self.0.borrow(), ThunkRepr::Evaluated(_))
}
pub fn is_suspended(&self) -> bool {
matches!(
*self.0.borrow(),
ThunkRepr::Suspended { .. } | ThunkRepr::Native(_)
)
}
/// Returns true if forcing this thunk will not change it.
pub fn is_forced(&self) -> bool {
match *self.0.borrow() {
ThunkRepr::Evaluated(Value::Thunk(_)) => false,
ThunkRepr::Evaluated(_) => true,
_ => false,
}
}
/// 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) => value,
ThunkRepr::Blackhole { .. } => panic!("Thunk::value called on a black-holed thunk"),
ThunkRepr::Suspended { .. } | ThunkRepr::Native(_) => {
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.as_ref(),
ThunkRepr::Evaluated(Value::Closure(c)) => &c.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, .. } => Rc::get_mut(upvalues).unwrap(),
ThunkRepr::Evaluated(Value::Closure(c)) => Rc::get_mut(
&mut Rc::get_mut(c).unwrap().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 {
if Rc::ptr_eq(&self.0, &other.0) {
return true;
}
match &*self.0.borrow() {
ThunkRepr::Evaluated(Value::Closure(c1)) => match &*other.0.borrow() {
ThunkRepr::Evaluated(Value::Closure(c2)) => Rc::ptr_eq(c1, c2),
_ => false,
},
_ => false,
}
}
/// Helper function to format thunks in observer output.
pub(crate) fn debug_repr(&self) -> String {
self.0.borrow().debug_repr()
}
}
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.borrow() {
ThunkRepr::Evaluated(v) => v.total_fmt(f, set),
ThunkRepr::Suspended { .. } | ThunkRepr::Native(_) => f.write_str("<CODE>"),
other => write!(f, "internal[{}]", other.debug_repr()),
}
}
}
/// 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<*const 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: *const ThunkRepr = thunk.0.as_ptr();
self.0.insert(ptr)
}
}
#[derive(Default, Clone)]
pub struct SharedThunkSet(Rc<RefCell<ThunkSet>>);
impl SharedThunkSet {
/// Check whether the given thunk has already been seen. Will mark the thunk
/// as seen otherwise.
pub fn insert(&self, thunk: &Thunk) -> bool {
self.0.borrow_mut().insert(thunk)
}
}
