statements and block scoping in java

java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/AstPrinter.java

@@ -1,43 +0,0 @@
-package ch.vanwa.lox_interpreter;
-
-class AstPrinter implements Expr.Visitor<String> {
-  String print(Expr expr) {
-    return expr.accept(this);
-  }
-
-  @Override
-  public String visitBinaryExpr(Expr.Binary expr) {
-    return parenthesize(expr.operator.lexeme(),
-        expr.left, expr.right);
-  }
-
-  @Override
-  public String visitGroupingExpr(Expr.Grouping expr) {
-    return parenthesize("group", expr.expression);
-  }
-
-  @Override
-  public String visitLiteralExpr(Expr.Literal expr) {
-    if (expr.value == null)
-      return "nil";
-    return expr.value.toString();
-  }
-
-  @Override
-  public String visitUnaryExpr(Expr.Unary expr) {
-    return parenthesize(expr.operator.lexeme(), expr.right);
-  }
-
-  private String parenthesize(String name, Expr... exprs) {
-    StringBuilder builder = new StringBuilder();
-
-    builder.append("(").append(name);
-    for (Expr expr : exprs) {
-      builder.append(" ");
-      builder.append(expr.accept(this));
-    }
-    builder.append(")");
-
-    return builder.toString();
-  }
-}

java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/Environment.java

@@ -0,0 +1,79 @@
+package ch.vanwa.lox_interpreter;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Environment mapping variable names to their values.
+ */
+class Environment {
+  /**
+   * Enclosing parent environment, can be null (only used for the outermost global
+   * environment).
+   */
+  final Environment enclosing;
+
+  /**
+   * Mapping of variable names to their values.
+   */
+  private final Map<String, Object> values = new HashMap<>();
+
+  /**
+   * Initialise environment without an enclosing environment.
+   */
+  Environment() {
+    enclosing = null;
+  }
+
+  /**
+   * Initialise environment with an enclosing environment.
+   */
+  Environment(Environment enclosing) {
+    this.enclosing = enclosing;
+  }
+
+  /**
+   * Get the value of a variable. Throw an error if that variable is not known in
+   * this environment or in any of the enclosing ones.
+   *
+   * @param name Token with the variable lexeme used as variable name.
+   */
+  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() + "'.");
+  }
+
+  /**
+   * Assign a value to a variable. Throw an error if that variable is not known in
+   * this environment or in any of the enclosing ones.
+   */
+  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() + "'.");
+  }
+
+  /**
+   * Define a new variable in this environment.
+   */
+  void define(String name, Object value) {
+    values.put(name, value);
+  }
+}

java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/Expr.java

@@ -4,6 +4,8 @@ import java.util.List;
 
 abstract class Expr {
   interface Visitor<R> {
+    R visitAssignExpr(Assign expr);
+
     R visitBinaryExpr(Binary expr);
 
     R visitGroupingExpr(Grouping expr);
@@ -11,6 +13,23 @@ abstract class 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 {
@@ -71,5 +90,18 @@ abstract class Expr {
     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);
 }

java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/Interpreter.java

@@ -1,13 +1,19 @@
 package ch.vanwa.lox_interpreter;
 
+import java.util.List;
+
 /**
  * Evaluate Lox expressions.
  */
-class Interpreter implements Expr.Visitor<Object> {
-  void interpret(Expr expression) {
+class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
+
+  private Environment environment = new Environment();
+
+  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);
     }
@@ -27,6 +33,65 @@ class Interpreter implements Expr.Visitor<Object> {
     return expr.accept(this);
   }
 
+  private void execute(Stmt stmt) {
+    stmt.accept(this);
+  }
+
+  /**
+   * Execute all statements within a block while using lexical scoping for
+   * variables.
+   */
+  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);
+    return null;
+  }
+
+  @Override
+  public Void visitPrintStmt(Stmt.Print stmt) {
+    Object value = evaluate(stmt.expression);
+    System.out.println(stringify(value));
+    return null;
+  }
+
+  @Override
+  public Void visitVarStmt(Stmt.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(Expr.Assign expr) {
+    Object value = evaluate(expr.value);
+    environment.assign(expr.name, value);
+    return value;
+  }
+
   @Override
   public Object visitBinaryExpr(Expr.Binary expr) {
     Object left = evaluate(expr.left);
@@ -90,6 +155,11 @@ class Interpreter implements Expr.Visitor<Object> {
     return null;
   }
 
+  @Override
+  public Object visitVariableExpr(Expr.Variable expr) {
+    return environment.get(expr.name);
+  }
+
   /**
    * Throw a runtime error if the operand is not an instance of Double.
    */

java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/Lox.java

@@ -69,12 +69,13 @@ public class Lox {
     var scanner = new Scanner(source);
     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);
   }
 
   /**

java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/Parser.java

@@ -1,5 +1,6 @@
 package ch.vanwa.lox_interpreter;
 
+import java.util.ArrayList;
 import java.util.List;
 
 import static ch.vanwa.lox_interpreter.TokenType.*;
@@ -41,16 +42,94 @@ class Parser {
    *
    * @return Expression signifying the whole program or null, if parsing fails.
    */
-  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(VAR)) {
+        return varDeclaration();
+      }
+
+      return statement();
     } catch (ParseError error) {
+      synchronize();
       return null;
     }
   }
 
-  private Expr expression() {
-    return equality();
+  private Stmt statement() {
+    if (match(PRINT)) {
+      return printStatement();
+    }
+    if (match(LEFT_BRACE)) {
+      return new Stmt.Block(block());
+    }
+
+    return expressionStatement();
+  }
+
+  private Stmt printStatement() {
+    Expr value = expression();
+    consume(SEMICOLON, "Expect ';' after value.");
+    return new Stmt.Print(value);
+  }
+
+  private Stmt varDeclaration() {
+    Token name = consume(IDENTIFIER, "Expect variable name.");
+
+    Expr initializer = null;
+    if (match(EQUAL)) {
+      initializer = expression();
+    }
+
+    consume(SEMICOLON, "Expect ';' after variable declaration.");
+    return new Stmt.Var(name, initializer);
+  }
+
+  private Stmt expressionStatement() {
+    Expr expr = expression();
+    consume(SEMICOLON, "Expect ';' after expression.");
+    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();
+
+    if (match(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 assignment target.");
+    }
+
+    return expr;
   }
 
   private Expr equality() {
@@ -123,6 +202,10 @@ class Parser {
       return new Expr.Literal(previous().literal());
     }
 
+    if (match(IDENTIFIER)) {
+      return new Expr.Variable(previous());
+    }
+
     if (match(LEFT_PAREN)) {
       Expr expr = expression();
       consume(RIGHT_PAREN, "Expect ')' after expression.");

java/lox-interpreter/src/main/java/ch/vanwa/lox_interpreter/Stmt.java

@@ -0,0 +1,71 @@
+package ch.vanwa.lox_interpreter;
+
+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);
+}

java/lox-interpreter/src/main/java/ch/vanwa/tool/GenerateAst.java

@@ -14,10 +14,18 @@ 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(