Split state and scopes, less ref-counting 🔀

1 files changed, 36 insertions, 5 deletions

diff --git a/src/eval/scope.rs b/src/eval/scope.rs
index dd7cc1da..62ee7e40 100644
--- a/src/eval/scope.rs
+++ b/src/eval/scope.rs
@@ -1,27 +1,58 @@
 use std::collections::HashMap;
 use std::fmt::{self, Debug, Formatter};
+use std::iter;
 
 use super::Value;
 
-/// A map from identifiers to values.
+/// A hierarchy of scopes.
+#[derive(Debug, Clone, PartialEq)]
+pub struct Scopes<'a> {
+    /// The active scope.
+    top: Scope,
+    /// The stack of lower scopes.
+    scopes: Vec<Scope>,
+    /// The base scope.
+    base: &'a Scope,
+}
+
+impl<'a> Scopes<'a> {
+    /// Create a new hierarchy of scopes.
+    pub fn new(base: &'a Scope) -> Self {
+        Self { top: Scope::new(), scopes: vec![], base }
+    }
+
+    /// Look up the value of a variable in the scopes.
+    pub fn get(&self, var: &str) -> Option<&Value> {
+        iter::once(&self.top)
+            .chain(&self.scopes)
+            .chain(iter::once(self.base))
+            .find_map(|scope| scope.get(var))
+    }
+
+    /// Define a variable in the active scope.
+    pub fn define(&mut self, var: impl Into<String>, value: impl Into<Value>) {
+        self.top.set(var, value);
+    }
+}
+
+/// A map from variable names to values.
 #[derive(Default, Clone, PartialEq)]
 pub struct Scope {
     values: HashMap<String, Value>,
 }
 
 impl Scope {
-    // Create a new empty scope with a fallback function that is invoked when no
-    // match is found.
+    // Create a new empty scope.
     pub fn new() -> Self {
         Self::default()
     }
 
-    /// Return the value of the given variable.
+    /// Look up the value of a variable.
     pub fn get(&self, var: &str) -> Option<&Value> {
         self.values.get(var)
     }
 
-    /// Store the value for the given variable.
+    /// Store the value for a variable.
     pub fn set(&mut self, var: impl Into<String>, value: impl Into<Value>) {
         self.values.insert(var.into(), value.into());
     }