Chapter 8: Statements and State

jlox/src/main/java/com/craftinginterpreters/lox/AstPrinter.java

@@ -1,5 +1,7 @@
 package com.craftinginterpreters.lox;
 
+import com.craftinginterpreters.lox.Expr.Variable;
+
 class AstPrinter implements Expr.Visitor<String> {
   String print(Expr expr) {
     return expr.accept(this);
@@ -52,4 +54,10 @@ class AstPrinter implements Expr.Visitor<String> {
     System.out.println(new AstPrinter().print(expression));
   }
 
+  @Override
+  public String visitVariableExpr(Variable expr) {
+    // TODO Auto-generated method stub
+    return null;
+  }
+
 }

jlox/src/main/java/com/craftinginterpreters/lox/Environment.java

@@ -0,0 +1,47 @@
+package com.craftinginterpreters.lox;
+
+import java.util.HashMap;
+import java.util.Map;
+
+class Environment {
+  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 + "'.");
+  }
+
+  void assign(Token name, Object value) {
+    if (values.containsKey(name.lexeme)) {
+      values.put(name.lexeme, value);
+      return;
+    }
+
+    if (enclosing != null) {
+      enclosing.assign(name, value);
+      return;
+    }
+
+    throw new RuntimeError(name, "Undefined variable '" + name.lexeme + "'.");
+  }
+
+  void define(String name, Object value) {
+    values.put(name, value);
+  }
+}

jlox/src/main/java/com/craftinginterpreters/lox/Expr.java

@@ -4,10 +4,26 @@ import java.util.List;
 
 abstract class Expr {
   interface Visitor<R> {
+    R visitAssignExpr(Assign expr);
     R visitBinaryExpr(Binary expr);
     R visitGroupingExpr(Grouping expr);
     R visitLiteralExpr(Literal expr);
     R visitUnaryExpr(Unary expr);
+    R visitVariableExpr(Variable expr);
+  }
+  static class Assign extends Expr {
+    Assign(Token name, Expr value) {
+      this.name = name;
+      this.value = value;
+    }
+
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitAssignExpr(this);
+    }
+
+    final Token name;
+    final Expr value;
   }
   static class Binary extends Expr {
     Binary(Expr left, Token operator, Expr right) {
@@ -63,6 +79,18 @@ abstract class Expr {
     final Token operator;
     final Expr right;
   }
+  static class Variable extends Expr {
+    Variable(Token name) {
+      this.name = name;
+    }
+
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitVariableExpr(this);
+    }
+
+    final Token name;
+  }
 
   abstract <R> R accept(Visitor<R> visitor);
 }

jlox/src/main/java/com/craftinginterpreters/lox/Interpreter.java

@@ -1,21 +1,95 @@
 package com.craftinginterpreters.lox;
 
+import java.util.List;
+
+import com.craftinginterpreters.lox.Expr.Assign;
 import com.craftinginterpreters.lox.Expr.Binary;
 import com.craftinginterpreters.lox.Expr.Grouping;
 import com.craftinginterpreters.lox.Expr.Literal;
 import com.craftinginterpreters.lox.Expr.Unary;
+import com.craftinginterpreters.lox.Expr.Variable;
+import com.craftinginterpreters.lox.Stmt.Block;
+import com.craftinginterpreters.lox.Stmt.Expression;
+import com.craftinginterpreters.lox.Stmt.Print;
+import com.craftinginterpreters.lox.Stmt.Var;
 
-class Interpreter implements Expr.Visitor<Object> {
+class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
+  private Environment environment = new Environment();
 
-  void interpret(Expr expression) {
+  void interpret(List<Stmt> statements) {
     try {
-      Object value = evaluate(expression);
-      System.out.println(stringify(value));
+      for (Stmt statement : statements) {
+        execute(statement);
+      }
     } catch (RuntimeError error) {
       Lox.runtimeError(error);
     }
   }
 
+  private Object evaluate(Expr expr) {
+    return expr.accept(this);
+  }
+
+  private void execute(Stmt stmt) {
+    stmt.accept(this);
+  }
+
+  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(Block stmt) {
+    executeBlock(stmt.statements, new Environment(environment));
+    return null;
+  }
+
+  @Override
+  public Void visitExpressionStmt(Expression stmt) {
+    evaluate(stmt.expression);
+    return null;
+  }
+
+  @Override
+  public Void visitPrintStmt(Print stmt) {
+    Object value = evaluate(stmt.expression);
+    System.out.println(stringify(value));
+    return null;
+  }
+
+  @Override
+  public Object visitVariableExpr(Variable expr) {
+    return environment.get(expr.name);
+  }
+
+  @Override
+  public Void visitVarStmt(Var stmt) {
+    Object value = null;
+    if (stmt.initializer != null) {
+      value = evaluate(stmt.initializer);
+    }
+
+    environment.define(stmt.name.lexeme, value);
+    return null;
+  }
+
+  @Override
+  public Object visitAssignExpr(Assign expr) {
+    Object value = evaluate(expr.value);
+    environment.assign(expr.name, value);
+    return value;
+  }
+
   @Override
   public Object visitBinaryExpr(Binary expr) {
     Object left = evaluate(expr.left);
@@ -137,9 +211,4 @@ class Interpreter implements Expr.Visitor<Object> {
 
     return object.toString();
   }
-
-  private Object evaluate(Expr expr) {
-    return expr.accept(this);
-  }
-
 }

jlox/src/main/java/com/craftinginterpreters/lox/Lox.java

@@ -55,11 +55,11 @@ public class Lox {
         List<Token> tokens = scanner.scanTokens();
 
         Parser parser = new Parser(tokens);
-        Expr expression = parser.parse();
+        List<Stmt> statements = parser.parse();
 
         if (hadError) return;
 
-        interpreter.interpret(expression);
+        interpreter.interpret(statements);
     }
 
     static void error(int line, String message) {

jlox/src/main/java/com/craftinginterpreters/lox/Parser.java

@@ -1,5 +1,6 @@
 package com.craftinginterpreters.lox;
 
+import java.util.ArrayList;
 import java.util.List;
 
 class Parser {
@@ -13,16 +14,96 @@ class Parser {
     this.tokens = tokens;
   }
 
-  Expr parse() {
+  List<Stmt> parse() {
+    List<Stmt> statements = new ArrayList<>();
+
+    while (!isAtEnd()) {
+      statements.add(declaration());
+    }
+
+    return statements;
+  }
+
+  private Expr expression() {
+    return assignment();
+  }
+
+  private Stmt declaration() {
     try {
-      return expression();
+      if (match(TokenType.VAR)) {
+        return varDeclaration();
+      }
+
+      return statement();
     } catch (ParseError error) {
+      synchronize();
       return null;
     }
   }
 
-  private Expr expression() {
-    return equality();
+  private Stmt statement() {
+    if (match(TokenType.PRINT)) {
+      return printStatement();
+    }
+
+    if (match(TokenType.LEFT_BRACE)) {
+      return new Stmt.Block(block());
+    }
+
+    return expressionStatement();
+  }
+
+  private Stmt printStatement() {
+    Expr value = expression();
+    consume(TokenType.SEMICOLON, "Expect ';' after value.");
+    return new Stmt.Print(value);
+  }
+
+  private Stmt varDeclaration() {
+    Token name = consume(TokenType.IDENTIFIER, "Expect variable name.");
+
+    Expr initializer = null;
+    if (match(TokenType.EQUAL)) {
+      initializer = expression();
+    }
+
+    consume(TokenType.SEMICOLON, "Expect ';' after variable declaration.");
+    return new Stmt.Var(name, initializer);
+  }
+
+  private Stmt expressionStatement() {
+    Expr expr = expression();
+    consume(TokenType.SEMICOLON, "Expect ';' after expression.");
+    return new Stmt.Expression(expr);
+  }
+
+  private List<Stmt> block() {
+    List<Stmt> statements = new ArrayList<>();
+
+    while (!check(TokenType.RIGHT_BRACE) && !isAtEnd()) {
+      statements.add(declaration());
+    }
+
+    consume(TokenType.RIGHT_BRACE, "Expect '}' after block.");
+    return statements;
+  }
+
+  private Expr assignment() {
+    Expr expr = equality();
+
+    if (match(TokenType.EQUAL)) {
+      Token equals = previous();
+      Expr value = assignment();
+
+      if (expr instanceof Expr.Variable) {
+        Token name = ((Expr.Variable)expr).name;
+        return new Expr.Assign(name, value);
+      }
+
+      error(equals, "Invalid assigment target.");
+    }
+
+    return expr;
   }
 
   private Expr equality() {
@@ -95,6 +176,10 @@ class Parser {
       return new Expr.Literal(previous().literal);
     }
 
+    if (match(TokenType.IDENTIFIER)) {
+      return new Expr.Variable(previous());
+    }
+
     if (match(TokenType.LEFT_PAREN)) {
       Expr expr = expression();
       consume(TokenType.RIGHT_PAREN, "Expect ')' after expression.");

jlox/src/main/java/com/craftinginterpreters/lox/Stmt.java

@@ -0,0 +1,64 @@
+package com.craftinginterpreters.lox;
+
+import java.util.List;
+
+abstract class Stmt {
+  interface Visitor<R> {
+    R visitBlockStmt(Block stmt);
+    R visitExpressionStmt(Expression stmt);
+    R visitPrintStmt(Print stmt);
+    R visitVarStmt(Var stmt);
+  }
+  static class Block extends Stmt {
+    Block(List<Stmt> statements) {
+      this.statements = statements;
+    }
+
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitBlockStmt(this);
+    }
+
+    final List<Stmt> statements;
+  }
+  static class Expression extends Stmt {
+    Expression(Expr expression) {
+      this.expression = expression;
+    }
+
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitExpressionStmt(this);
+    }
+
+    final Expr expression;
+  }
+  static class Print extends Stmt {
+    Print(Expr expression) {
+      this.expression = expression;
+    }
+
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitPrintStmt(this);
+    }
+
+    final Expr expression;
+  }
+  static class Var extends Stmt {
+    Var(Token name, Expr initializer) {
+      this.name = name;
+      this.initializer = initializer;
+    }
+
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitVarStmt(this);
+    }
+
+    final Token name;
+    final Expr initializer;
+  }
+
+  abstract <R> R accept(Visitor<R> visitor);
+}

jlox/src/main/java/com/craftinginterpreters/tool/GenerateAst.java

@@ -13,10 +13,18 @@ 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"));
+                "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 {

tests/test1.lox

@@ -0,0 +1,19 @@
+var a = "global a";
+var b = "global b";
+var c = "global c";
+{
+  var a = "outer a";
+  var b = "outer b";
+  {
+    var a = "inner a";
+    print a;
+    print b;
+    print c;
+  }
+  print a;
+  print b;
+  print c;
+}
+print a;
+print b;
+print c;