docs(tvix/eval): upvalues.rs: define "upvalue", comment with_stack

This commit adds a comment for the benefit of non-Lua-users
explaining what an upvalue is.  It also adds a comment explaining
what `with_stack` does, including a brief reminder of nix's
slightly-unusual-but-well-justified handling of this construct.

Signed-off-by: Adam Joseph <adam@westernsemico.com>
Change-Id: If6d0e133292451af906e1c728e34010f99be8c7c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7007
Reviewed-by: j4m3s <james.landrein@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
This commit is contained in:
Adam Joseph 2022-10-13 13:41:30 -07:00 committed by clbot
parent 13a5e7dd5b
commit 4b01e594d5

View file

@ -2,6 +2,10 @@
//! relevant to both thunks (delayed computations for lazy-evaluation)
//! as well as closures (lambdas that capture variables from the
//! surrounding scope).
//!
//! The upvalues of a scope are whatever data are needed at runtime
//! in order to resolve each free variable in the scope to a value.
//! "Upvalue" is a term taken from Lua.
use std::{
cell::{Ref, RefMut},
@ -10,26 +14,43 @@ use std::{
use crate::{opcode::UpvalueIdx, Value};
/// Structure for carrying upvalues inside of thunks & closures. The
/// implementation of this struct encapsulates the logic for capturing
/// and accessing upvalues.
/// Structure for carrying upvalues of an UpvalueCarrier. The
/// implementation of this struct encapsulates the logic for
/// capturing and accessing upvalues.
///
/// Nix's `with` cannot be used to shadow an enclosing binding --
/// like Rust's `use xyz::*` construct, but unlike Javascript's
/// `with (xyz)`. This means that Nix has two kinds of identifiers,
/// which can be distinguished at compile time:
///
/// - Static identifiers, which are bound in some enclosing scope by
/// `let`, `name:` or `{name}:`
/// - Dynamic identifiers, which are not bound in any enclosing
/// scope
#[derive(Clone, Debug, PartialEq)]
pub struct Upvalues {
upvalues: Vec<Value>,
/// The upvalues of static identifiers. Each static identifier
/// is assigned an integer identifier at compile time, which is
/// an index into this Vec.
static_upvalues: Vec<Value>,
/// The upvalues of dynamic identifiers, if any exist. This
/// consists of the value passed to each enclosing `with val;`,
/// from outermost to innermost.
with_stack: Option<Vec<Value>>,
}
impl Upvalues {
pub fn with_capacity(count: usize) -> Self {
Upvalues {
upvalues: Vec::with_capacity(count),
static_upvalues: Vec::with_capacity(count),
with_stack: None,
}
}
/// Push an upvalue at the end of the upvalue list.
pub fn push(&mut self, value: Value) {
self.upvalues.push(value);
self.static_upvalues.push(value);
}
/// Set the captured with stack.
@ -53,7 +74,7 @@ impl Index<UpvalueIdx> for Upvalues {
type Output = Value;
fn index(&self, index: UpvalueIdx) -> &Self::Output {
&self.upvalues[index.0]
&self.static_upvalues[index.0]
}
}
@ -69,13 +90,13 @@ pub trait UpvalueCarrier {
/// Read an upvalue at the given index.
fn upvalue(&self, idx: UpvalueIdx) -> Ref<'_, Value> {
Ref::map(self.upvalues(), |v| &v.upvalues[idx.0])
Ref::map(self.upvalues(), |v| &v.static_upvalues[idx.0])
}
/// Resolve deferred upvalues from the provided stack slice,
/// mutating them in the internal upvalue slots.
fn resolve_deferred_upvalues(&self, stack: &[Value]) {
for upvalue in self.upvalues_mut().upvalues.iter_mut() {
for upvalue in self.upvalues_mut().static_upvalues.iter_mut() {
if let Value::DeferredUpvalue(idx) = upvalue {
*upvalue = stack[idx.0].clone();
}