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//! Lexical environments (scope chains) for the tree-walker over the new model
//! (`ROADMAP.md` §3 → Phase D migration, the function/closure piece).
//!
//! A `Scope` is a reference-counted frame of `name → `[`NanBox`] bindings with
//! a link to its enclosing scope. A function **closes over** the scope it was
//! defined in by keeping an `Rc` to it, so its captured variables stay live and
//! shared after the defining call returns — the property a flat stack of scopes
//! cannot provide. Resolution walks the parent chain inner-first.
//!
//! Bindings hold only [`NanBox`] values (heap references are handles), so a
//! scope is `'static` and the GC can trace a closure's captured handles via
//! `Scope::for_each_handle`.
//!
//! [`NanBox`]: crate::nanbox::NanBox
//!
//! Pure, safe `alloc`-only Rust.
use crate::heap::Handle;
use crate::nanbox::NanBox;
use alloc::collections::BTreeMap;
use alloc::rc::Rc;
use alloc::string::String;
use core::cell::RefCell;
/// A reference-counted lexical scope: its own bindings plus a link to the
/// enclosing scope. Cloning shares the same frame (closures capture by sharing).
#[derive(Clone)]
pub struct Scope(Rc<RefCell<ScopeData>>);
struct ScopeData {
vars: BTreeMap<String, NanBox>,
/// Names declared `const` in this frame (reassignment is a TypeError).
consts: alloc::collections::BTreeSet<String>,
parent: Option<Scope>,
}
impl Scope {
/// A new root scope (no parent).
#[must_use]
pub fn root() -> Self {
Scope(Rc::new(RefCell::new(ScopeData {
vars: BTreeMap::new(),
consts: alloc::collections::BTreeSet::new(),
parent: None,
})))
}
/// A new child scope nested inside `self`.
#[must_use]
pub fn child(&self) -> Self {
Scope(Rc::new(RefCell::new(ScopeData {
vars: BTreeMap::new(),
consts: alloc::collections::BTreeSet::new(),
parent: Some(self.clone()),
})))
}
/// Declares (or redeclares) `name` in *this* scope.
pub fn declare(&self, name: &str, value: NanBox) {
self.0.borrow_mut().vars.insert(String::from(name), value);
}
/// Declares `name` as a `const` binding in *this* scope (reassignment fails).
pub fn declare_const(&self, name: &str, value: NanBox) {
let mut data = self.0.borrow_mut();
data.vars.insert(String::from(name), value);
data.consts.insert(String::from(name));
}
/// Whether the nearest binding of `name` was declared `const`.
#[must_use]
pub fn is_const(&self, name: &str) -> bool {
let data = self.0.borrow();
if data.vars.contains_key(name) {
return data.consts.contains(name);
}
data.parent.as_ref().is_some_and(|p| p.is_const(name))
}
/// Whether `name` is bound in *this* scope (not the enclosing chain).
#[must_use]
pub fn has_local(&self, name: &str) -> bool {
self.0.borrow().vars.contains_key(name)
}
/// Looks up `name`, walking outward through enclosing scopes.
#[must_use]
pub fn get(&self, name: &str) -> Option<NanBox> {
let data = self.0.borrow();
if let Some(v) = data.vars.get(name) {
return Some(*v);
}
data.parent.as_ref().and_then(|p| p.get(name))
}
/// Assigns to the nearest existing binding of `name`; returns `false` if it
/// is not declared anywhere in the chain.
pub fn set(&self, name: &str, value: NanBox) -> bool {
let mut data = self.0.borrow_mut();
if let Some(slot) = data.vars.get_mut(name) {
*slot = value;
return true;
}
match &data.parent {
Some(p) => p.set(name, value),
None => false,
}
}
/// This frame's own `(name, value, is_const)` bindings (not the parent
/// chain) — for snapshotting a closure's captured environment.
#[must_use]
pub fn local_bindings(&self) -> alloc::vec::Vec<(String, NanBox, bool)> {
let data = self.0.borrow();
data.vars
.iter()
.map(|(k, v)| (k.clone(), *v, data.consts.contains(k)))
.collect()
}
/// This scope's enclosing scope, if any.
#[must_use]
pub fn parent(&self) -> Option<Scope> {
self.0.borrow().parent.clone()
}
/// Visits every heap [`Handle`] reachable from this scope chain's bindings
/// (for GC tracing of a closure's captured values).
pub fn for_each_handle(&self, visit: &mut dyn FnMut(Handle)) {
let data = self.0.borrow();
for v in data.vars.values() {
if let Some(raw) = v.as_handle() {
visit(Handle::from_raw(raw));
}
}
if let Some(p) = &data.parent {
p.for_each_handle(visit);
}
}
/// Rewrites every handle binding in this scope chain through `forward` — the
/// mutating mirror of [`for_each_handle`](Scope::for_each_handle), for a
/// moving collector. (A shared parent is visited once per referrer, which is
/// idempotent: `forward` maps an already-forwarded handle to itself.)
pub fn relocate_handles(&self, forward: &dyn Fn(Handle) -> Handle) {
let mut data = self.0.borrow_mut();
for v in data.vars.values_mut() {
if let Some(raw) = v.as_handle() {
*v = NanBox::handle(forward(Handle::from_raw(raw)).to_raw());
}
}
if let Some(p) = &data.parent {
p.relocate_handles(forward);
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn declare_and_resolve_inner_first() {
let root = Scope::root();
root.declare("x", NanBox::number(1.0));
root.declare("y", NanBox::number(2.0));
let child = root.child();
child.declare("x", NanBox::number(99.0)); // shadows
assert_eq!(child.get("x").unwrap().as_number(), Some(99.0)); // inner
assert_eq!(child.get("y").unwrap().as_number(), Some(2.0)); // from parent
assert_eq!(root.get("x").unwrap().as_number(), Some(1.0)); // parent unaffected
assert!(child.get("z").is_none());
}
#[test]
fn set_targets_the_nearest_binding() {
let root = Scope::root();
root.declare("x", NanBox::number(1.0));
let child = root.child();
// No local `x`: assignment reaches the parent's binding.
assert!(child.set("x", NanBox::number(5.0)));
assert_eq!(root.get("x").unwrap().as_number(), Some(5.0));
// A local shadow captures the assignment.
child.declare("x", NanBox::number(10.0));
assert!(child.set("x", NanBox::number(20.0)));
assert_eq!(child.get("x").unwrap().as_number(), Some(20.0));
assert_eq!(root.get("x").unwrap().as_number(), Some(5.0)); // parent unchanged
// Assigning an undeclared name fails.
assert!(!child.set("nope", NanBox::number(0.0)));
}
#[test]
fn captured_scope_outlives_and_shares() {
// A child keeps its parent alive and sees the parent's later mutations
// (the closure-capture property).
let captured = {
let outer = Scope::root();
outer.declare("count", NanBox::number(0.0));
let inner = outer.child();
outer.set("count", NanBox::number(7.0)); // mutate after capture
inner // `outer` drops here, but `inner` holds an Rc to it
};
assert_eq!(captured.get("count").unwrap().as_number(), Some(7.0));
}
#[test]
fn for_each_handle_visits_chain() {
let root = Scope::root();
root.declare("a", NanBox::handle(1));
root.declare("n", NanBox::number(5.0)); // not a handle
let child = root.child();
child.declare("b", NanBox::handle(2));
let mut seen = alloc::vec::Vec::new();
child.for_each_handle(&mut |h| seen.push(h.to_raw()));
seen.sort_unstable();
assert_eq!(seen, alloc::vec![1, 2]);
}
}