Chapter 8: Add blocks and scopes

src/com/craftinginterpreters/lox/AstPrinter.java

@@ -1,5 +1,7 @@
 package com.craftinginterpreters.lox;
 
+import java.util.List;
+
 /**
  * AstPrinter
  */
@@ -55,6 +57,11 @@ class AstPrinter implements Expr.Visitor<String>, Stmt.Visitor<String> {
         return expression.expression.accept(this);
     }
 
+    @Override
+    public String visitBlockStmt(Stmt.Block block) {
+        return parenthesize("", block.statements);
+    }
+
     private String parenthesize(String name, Expr... exprs) {
         StringBuilder builder = new StringBuilder();
 
@@ -68,6 +75,19 @@ class AstPrinter implements Expr.Visitor<String>, Stmt.Visitor<String> {
         return builder.toString();
     }
 
+    private String parenthesize(String name, List<Stmt> statements) {
+        StringBuilder builder = new StringBuilder();
+
+        builder.append("(").append(name);
+        for (Stmt statement : statements) {
+            builder.append(" ");
+            builder.append(statement.accept(this));
+        }
+        builder.append(")");
+
+        return builder.toString();
+    }
+
     public static void main(String[] args) {
         Expr expression = new Expr.Binary(
                 new Expr.Unary(new Token(TokenType.MINUS, "-", null, 1), new Expr.Literal(123)),

src/com/craftinginterpreters/lox/Environment.java

@@ -4,13 +4,25 @@ import java.util.HashMap;
 import java.util.Map;
 
 class Environment {
+    private final Environment enclosing;
     private final Map<String, Object> values = new HashMap<>();
 
+    Environment() {
+        enclosing = null;
+    }
+
+    Environment(Environment enclosing) {
+        this.enclosing = enclosing;
+    }
+
     Object get(Token name) {
         if (values.containsKey(name.lexeme)) {
             return values.get(name.lexeme);
         }
 
+        if (enclosing != null)
+            return enclosing.get(name);
+
         throw new RuntimeError(name, "Undefined variable '" + name.lexeme + "'.");
     }
 
@@ -20,6 +32,11 @@ class Environment {
             return;
         }
 
+        if (enclosing != null) {
+            enclosing.assign(name, value);
+            return;
+        }
+
         throw new RuntimeError(name, "Undefined variable '" + name.lexeme + "'.");
     }
 

src/com/craftinginterpreters/lox/Interpreter.java

@@ -89,6 +89,26 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         stmt.accept(this);
     }
 
+    private void executeBlock(List<Stmt> statements, Environment environment) {
+        Environment previous = this.environment;
+
+        try {
+            this.environment = environment;
+
+            for (Stmt statement : statements) {
+                execute(statement);
+            }
+        } finally {
+            this.environment = previous;
+        }
+    }
+
+    @Override
+    public Void visitBlockStmt(Stmt.Block stmt) {
+        executeBlock(stmt.statements, new Environment(environment));
+        return null;
+    }
+
     @Override
     public Void visitExpressionStmt(Stmt.Expression stmt) {
         evaluate(stmt.expression);

src/com/craftinginterpreters/lox/Parser.java

@@ -45,6 +45,8 @@ class Parser {
     private Stmt statement() {
         if (match(PRINT))
             return printStatement();
+        if (match(LEFT_BRACE))
+            return new Stmt.Block(block());
 
         return expressionStatement();
     }
@@ -73,6 +75,17 @@ class Parser {
         return new Stmt.Expression(expr);
     }
 
+    private List<Stmt> block() {
+        List<Stmt> statements = new ArrayList<>();
+
+        while (!check(RIGHT_BRACE) && !isAtEnd()) {
+            statements.add(declaration());
+        }
+
+        consume(RIGHT_BRACE, "Expect '}' after block.");
+        return statements;
+    }
+
     private Expr assignment() {
         Expr expr = equality();
 

src/com/craftinginterpreters/tool/GenerateAst.java

@@ -15,10 +15,11 @@ public class GenerateAst {
         String outputDir = args[0];
 
         defineAst(outputDir, "Expr",
-                  Arrays.asList("Assign : Token name, Expr value", "Binary : Expr left, Token operator, Expr right", "Grouping : Expr expression",
-                        "Literal : Object value", "Unary : Token operator, Expr right", "Variable : Token name"));
-        defineAst(outputDir, "Stmt", Arrays.asList("Expression : Expr expression", "Print : Expr expression",
-                "Var : Token name, Expr initializer"));
+                Arrays.asList("Assign : Token name, Expr value", "Binary : Expr left, Token operator, Expr right",
+                        "Grouping : Expr expression", "Literal : Object value", "Unary : Token operator, Expr right",
+                        "Variable : Token name"));
+        defineAst(outputDir, "Stmt", Arrays.asList("Block : List<Stmt> statements", "Expression : Expr expression",
+                "Print : Expr expression", "Var : Token name, Expr initializer"));
     }
 
     private static void defineAst(String outputDir, String baseName, List<String> types) throws IOException {