Add block scoping of variables

build.rs

@@ -59,6 +59,7 @@ fn do_ast_codegen<W: Write>(mut output: W) {
 
     let statement_types = BTreeMap::from([
         ("Expression", "expression: Expr"),
+        ("Block", "statements: Vec<Stmt>"),
         ("Print", "expression: Expr"),
         ("Var", "name: Token, initializer: Option<Expr>"),
     ]);

src/eval.rs

@@ -1,8 +1,4 @@
-use std::{
-    fmt::{self, Display, Formatter},
-    rc::Rc,
-    string::ParseError,
-};
+use std::fmt::{self, Display, Formatter};
 
 use crate::{
     ast::{Expr, ExprVisitor, LiteralValue, Stmt, StmtVisitor},
@@ -174,7 +170,7 @@ impl ExprVisitor<Result<EvaluatedValue, ScriptError>> for InterpreterRunner<'_>
     fn visit_variable(&mut self, name: &Token) -> Result<EvaluatedValue, ScriptError> {
         self.interpreter
             .env
-            .get(name.lexeme())
+            .get_value(name.lexeme())
             .ok_or_else(|| ScriptError {
                 message: format!("Undefined variable: {}", name.lexeme()),
                 location: String::new(),
@@ -200,10 +196,23 @@ impl StmtVisitor<Result<(), ScriptError>> for InterpreterRunner<'_> {
             .as_ref()
             .map_or(Ok(EvaluatedValue::Nil), |expr| self.visit_expr(expr))?;
 
-        self.interpreter.env.set(name.lexeme(), initialized_value);
+        self.interpreter
+            .env
+            .set_value(name.lexeme(), initialized_value);
 
         Ok(())
     }
+
+    fn visit_block(&mut self, statements: &Vec<Stmt>) -> Result<(), ScriptError> {
+        self.interpreter.env.enter_scope();
+        let result = statements
+            .iter()
+            .try_for_each(|statement| self.visit_stmt(statement));
+
+        self.interpreter.env.exit_scope();
+
+        result
+    }
 }
 
 fn convert_arithmetic_operands(

src/eval/environment.rs

@@ -1,22 +1,187 @@
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 
 use super::value::EvaluatedValue;
 
-#[derive(Debug, Default)]
+#[derive(Debug)]
 pub struct Environment {
+    scopes: BTreeMap<u32, Scope>,
+}
+
+#[derive(Debug)]
+struct Scope {
+    parent: Option<u32>,
     values: HashMap<String, EvaluatedValue>,
 }
 
+impl Default for Environment {
+    fn default() -> Self {
+        let mut scopes = BTreeMap::new();
+        scopes.insert(Self::ROOT_KEY, Scope::new_global_scope());
+        Self { scopes }
+    }
+}
+
 impl Environment {
+    const ROOT_KEY: u32 = 0;
+
     pub fn new() -> Self {
         Self::default()
     }
 
-    pub fn get(&self, name: &str) -> Option<&EvaluatedValue> {
+    #[must_use]
+    pub fn root_scope(&self) -> &Scope {
+        self.scopes
+            .get(&Self::ROOT_KEY)
+            .expect("no root environment defined")
+    }
+
+    #[must_use]
+    pub fn root_scope_mut(&mut self) -> &mut Scope {
+        self.scopes
+            .get_mut(&Self::ROOT_KEY)
+            .expect("no root environment defined")
+    }
+
+    pub fn enter_scope(&mut self) {
+        let parent_key = self.current_scope_key();
+        let child_key = self.next_scope_key();
+
+        let env = Scope::new_child_scope(parent_key);
+        let insert_result = self.scopes.insert(child_key, env);
+        assert!(
+            insert_result.is_none(),
+            "collision when inserting environment {child_key}"
+        );
+    }
+
+    pub fn exit_scope(&mut self) {
+        let to_remove_key = self.current_scope_key();
+        assert!(to_remove_key != Self::ROOT_KEY, "cannot leave global scope");
+
+        self.scopes.remove(&to_remove_key);
+    }
+
+    pub fn get_value(&self, name: &str) -> Option<&EvaluatedValue> {
+        let mut cursor = Some(self.current_scope());
+        while let Some(scope) = cursor {
+            let value = scope.get(name);
+            if value.is_some() {
+                return value;
+            }
+
+            cursor = scope.parent.and_then(|key| self.scopes.get(&key));
+        }
+
+        None
+    }
+
+    pub fn set_value(&mut self, name: &str, value: EvaluatedValue) {
+        self.current_scope_mut().set(name, value);
+    }
+
+    fn current_scope(&self) -> &Scope {
+        // this can't fail, given we know where these scope ids come from
+        self.scopes.get(&self.current_scope_key()).unwrap()
+    }
+
+    fn current_scope_mut(&mut self) -> &mut Scope {
+        // this can't fail, given we know where these scope ids come from
+        self.scopes.get_mut(&self.current_scope_key()).unwrap()
+    }
+
+    fn current_scope_key(&self) -> u32 {
+        self.scopes
+            .last_key_value()
+            .map(|(&k, _v)| k)
+            // this should *NEVER* happen
+            .expect("no root scope defined")
+    }
+
+    fn next_scope_key(&self) -> u32 {
+        let parent = self.current_scope_key();
+
+        parent.checked_add(1).expect("too many nested scopes")
+    }
+}
+
+impl Scope {
+    fn new_global_scope() -> Self {
+        Self {
+            values: HashMap::new(),
+            parent: None,
+        }
+    }
+
+    fn new_child_scope(parent: u32) -> Self {
+        Self {
+            values: HashMap::new(),
+            parent: Some(parent),
+        }
+    }
+
+    fn get(&self, name: &str) -> Option<&EvaluatedValue> {
         self.values.get(name)
     }
 
-    pub fn set<S: Into<String>>(&mut self, name: S, value: EvaluatedValue) {
+    fn set<S: Into<String>>(&mut self, name: S, value: EvaluatedValue) {
         self.values.insert(name.into(), value);
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_can_get_value_from_root_scope() {
+        let mut global = Environment::new();
+        global.set_value("foo", EvaluatedValue::Number(42_f64));
+
+        assert_eq!(
+            Some(&EvaluatedValue::Number(42_f64)),
+            global.get_value(r"foo")
+        );
+    }
+
+    #[test]
+    fn test_can_get_value_from_parent_scope() {
+        let mut global = Environment::new();
+        global.set_value("foo", EvaluatedValue::Number(42_f64));
+
+        // Make two child scopes, just so they exist
+        global.enter_scope();
+        global.enter_scope();
+
+        assert_eq!(
+            Some(&EvaluatedValue::Number(42_f64)),
+            global.get_value(r"foo")
+        );
+    }
+
+    #[test]
+    fn test_insert_value_into_non_root_scope() {
+        let mut global = Environment::new();
+
+        global.enter_scope();
+        global.set_value("foo", EvaluatedValue::Number(42_f64));
+
+        global.enter_scope();
+
+        assert_eq!(
+            Some(&EvaluatedValue::Number(42_f64)),
+            global.get_value(r"foo")
+        );
+    }
+
+    #[test]
+    fn test_leaving_scope_removes_value() {
+        let mut global = Environment::new();
+
+        global.enter_scope();
+        global.set_value("foo", EvaluatedValue::Number(42_f64));
+
+        global.exit_scope();
+
+        assert_eq!(None, global.get_value(r"foo"));
+    }
+}

src/parse.rs

@@ -1,5 +1,7 @@
 use std::iter::Peekable;
 
+use itertools::Itertools;
+
 use crate::{
     ast::{Expr, LiteralValue, Stmt},
     lex::{Token, TokenKind},
@@ -120,6 +122,8 @@ fn parse_var_declaration<I: Iterator<Item = Token>>(
 fn parse_statement<I: Iterator<Item = Token>>(iter: &mut Peekable<I>) -> Result<Stmt, ParseError> {
     if match_token_kind!(iter, TokenKind::Print).is_ok() {
         parse_statement_containing_expression(iter, |expression| Stmt::Print { expression })
+    } else if match_token_kind!(iter, TokenKind::LeftBrace).is_ok() {
+        parse_block_statement(iter)
     } else {
         parse_statement_containing_expression(iter, |expression| Stmt::Expression { expression })
     }
@@ -138,6 +142,18 @@ fn parse_statement_containing_expression<I: Iterator<Item = Token>, F: Fn(Expr)
         })
 }
 
+fn parse_block_statement<I: Iterator<Item = Token>>(
+    iter: &mut Peekable<I>,
+) -> Result<Stmt, ParseError> {
+    let mut statements = vec![];
+    while match_token_kind!(iter, TokenKind::RightBrace).is_err() {
+        let statement = parse_declaration(iter)?;
+        statements.push(statement);
+    }
+
+    Ok(Stmt::Block { statements })
+}
+
 fn parse_expression<I: Iterator<Item = Token>>(iter: &mut Peekable<I>) -> Result<Expr, ParseError> {
     parse_equality(iter)
 }