Environment - Attempt at nesting support

src/interpreter/environment.rs

@@ -7,10 +7,24 @@ use super::{InterpreterResult, Value};
 /// The execution environment.
 #[derive(Debug, Default)]
 pub struct Environment {
+    enclosing: Option<Box<Environment>>,
     values: HashMap<String, Value>,
 }
 
 impl Environment {
+    /// Create an environment enclosed in another.
+    pub fn create_child(parent: Self) -> Self {
+        Self {
+            enclosing: Some(Box::new(parent)),
+            values: HashMap::default(),
+        }
+    }
+
+    /// Restore an environment's parent.
+    pub fn restore_parent(self) -> Self {
+        *self.enclosing.unwrap()
+    }
+
     /// Define a new variable.
     pub fn define(&mut self, name: String, value: Value) {
         self.values.insert(name, value);
@@ -19,10 +33,13 @@ impl Environment {
     /// Get the value of a variable.
     pub fn get(&self, name: &Token) -> InterpreterResult {
         match self.values.get(&name.lexeme as &str) {
-            None => Err(InterpreterError::new(
-                name,
-                &format!("undefined variable '{}'", name.lexeme),
-            )),
+            None => match &self.enclosing {
+                None => Err(InterpreterError::new(
+                    name,
+                    &format!("undefined variable '{}'", name.lexeme),
+                )),
+                Some(parent) => parent.get(name),
+            },
             Some(value) => Ok(value.clone()),
         }
     }
@@ -33,10 +50,13 @@ impl Environment {
             self.values.insert(name.lexeme.clone(), value);
             Ok(Value::Nil)
         } else {
-            Err(InterpreterError::new(
-                name,
-                &format!("undefined variable '{}'", name.lexeme),
-            ))
+            match &mut self.enclosing {
+                None => Err(InterpreterError::new(
+                    name,
+                    &format!("undefined variable '{}'", name.lexeme),
+                )),
+                Some(parent) => parent.assign(name, value),
+            }
         }
     }
 }