12.7 Constructors and Initializers

src/com/craftinginterpreters/lox/Interpreter.java

@@ -72,11 +72,11 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         Object right = evaluate(expr.right);
 
         switch (expr.operator.type) {
-            case BANG:
-                return !isTruthy(right);
-            case MINUS:
-                checkNumberOperand(expr.operator, right);
-                return -(double) right;
+        case BANG:
+            return !isTruthy(right);
+        case MINUS:
+            checkNumberOperand(expr.operator, right);
+            return -(double) right;
         }
 
         // Unreachable.
@@ -185,7 +185,7 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
 
         Map<String, LoxFunction> methods = new HashMap<>();
         for (Stmt.Function method : stmt.methods) {
-            LoxFunction function = new LoxFunction(method, environment);
+            LoxFunction function = new LoxFunction(method, environment, method.name.lexeme.equals("init"));
             methods.put(method.name.lexeme, function);
         }
 
@@ -202,7 +202,7 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
 
     @Override
     public Void visitFunctionStmt(Stmt.Function stmt) {
-        LoxFunction function = new LoxFunction(stmt, environment);
+        LoxFunction function = new LoxFunction(stmt, environment, false);
         environment.define(stmt.name.lexeme, function);
         return null;
     }
@@ -274,41 +274,41 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         Object right = evaluate(expr.right);
 
         switch (expr.operator.type) {
-            case GREATER:
-                checkNumberOperands(expr.operator, left, right);
-                return (double) left > (double) right;
-            case GREATER_EQUAL:
-                checkNumberOperands(expr.operator, left, right);
-                return (double) left >= (double) right;
-            case LESS:
-                checkNumberOperands(expr.operator, left, right);
-                return (double) left < (double) right;
-            case LESS_EQUAL:
-                checkNumberOperands(expr.operator, left, right);
-                return (double) left <= (double) right;
-            case BANG_EQUAL:
-                return !isEqual(left, right);
-            case EQUAL_EQUAL:
-                return isEqual(left, right);
-            case MINUS:
-                checkNumberOperands(expr.operator, left, right);
-                return (double) left - (double) right;
-            case PLUS:
-                if (left instanceof Double && right instanceof Double) {
-                    return (double) left + (double) right;
-                }
+        case GREATER:
+            checkNumberOperands(expr.operator, left, right);
+            return (double) left > (double) right;
+        case GREATER_EQUAL:
+            checkNumberOperands(expr.operator, left, right);
+            return (double) left >= (double) right;
+        case LESS:
+            checkNumberOperands(expr.operator, left, right);
+            return (double) left < (double) right;
+        case LESS_EQUAL:
+            checkNumberOperands(expr.operator, left, right);
+            return (double) left <= (double) right;
+        case BANG_EQUAL:
+            return !isEqual(left, right);
+        case EQUAL_EQUAL:
+            return isEqual(left, right);
+        case MINUS:
+            checkNumberOperands(expr.operator, left, right);
+            return (double) left - (double) right;
+        case PLUS:
+            if (left instanceof Double && right instanceof Double) {
+                return (double) left + (double) right;
+            }
 
-                if (left instanceof String && right instanceof String) {
-                    return (String) left + (String) right;
-                }
+            if (left instanceof String && right instanceof String) {
+                return (String) left + (String) right;
+            }
 
-                throw new RuntimeError(expr.operator, "Operands must be two numbers or two strings.");
-            case SLASH:
-                checkNumberOperands(expr.operator, left, right);
-                return (double) left / (double) right;
-            case STAR:
-                checkNumberOperands(expr.operator, left, right);
-                return (double) left * (double) right;
+            throw new RuntimeError(expr.operator, "Operands must be two numbers or two strings.");
+        case SLASH:
+            checkNumberOperands(expr.operator, left, right);
+            return (double) left / (double) right;
+        case STAR:
+            checkNumberOperands(expr.operator, left, right);
+            return (double) left * (double) right;
         }
 
         // Unreachable.

src/com/craftinginterpreters/lox/LoxClass.java

@@ -28,11 +28,17 @@ class LoxClass implements LoxCallable {
     @Override
     public Object call(Interpreter interpreter, List<Object> arguments) {
         LoxInstance instance = new LoxInstance(this);
+        LoxFunction initializer = findMethod("init");
+        if (initializer != null) {
+            initializer.bind(instance).call(interpreter, arguments);
+        }
         return instance;
     }
 
     @Override
     public int arity() {
-        return 0;
+        LoxFunction initializer = findMethod("init");
+        if (initializer == null) return 0;
+        return initializer.arity();
     }
 }

src/com/craftinginterpreters/lox/LoxFunction.java

@@ -5,8 +5,10 @@ import java.util.List;
 class LoxFunction implements LoxCallable {
     private final Stmt.Function declaration;
     private final Environment closure;
+    private final boolean isInitializer;
 
-    LoxFunction(Stmt.Function declaration, Environment closure) {
+    LoxFunction(Stmt.Function declaration, Environment closure, boolean isInitializer) {
+        this.isInitializer = isInitializer;
         this.closure = closure;
         this.declaration = declaration;
     }
@@ -14,7 +16,7 @@ class LoxFunction implements LoxCallable {
     LoxFunction bind(LoxInstance instance) {
         Environment environment = new Environment(closure);
         environment.define("this", instance);
-        return new LoxFunction(declaration, environment);
+        return new LoxFunction(declaration, environment, isInitializer);
     }
 
     @Override
@@ -33,8 +35,13 @@ class LoxFunction implements LoxCallable {
         try {
             interpreter.executeBlock(declaration.body, environment);
         } catch (Return returnValue) {
+            if (isInitializer)
+                return closure.getAt(0, "this");
             return returnValue.value;
         }
+
+        if (isInitializer)
+            return closure.getAt(0, "this");
         return null;
     }
 

src/com/craftinginterpreters/lox/Resolver.java

@@ -15,12 +15,11 @@ class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
     }
 
     private enum FunctionType {
-        NONE, FUNCTION, METHOD
+        NONE, FUNCTION, INITIALIZER, METHOD
     }
 
     private enum ClassType {
-        NONE,
-        CLASS
+        NONE, CLASS
     }
 
     private ClassType currentClass = ClassType.NONE;
@@ -52,6 +51,9 @@ class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
 
         for (Stmt.Function method : stmt.methods) {
             FunctionType declaration = FunctionType.METHOD;
+            if (method.name.lexeme.equals("init")) {
+                declaration = FunctionType.INITIALIZER;
+            }
             resolveFunction(method, declaration);
         }
 
@@ -98,6 +100,9 @@ class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
         }
 
         if (stmt.value != null) {
+            if (currentFunction == FunctionType.INITIALIZER) {
+                Lox.error(stmt.keyword, "Can't return a value from an initializer.");
+            }
             resolve(stmt.value);
         }
 
@@ -173,8 +178,7 @@ class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
     @Override
     public Void visitThisExpr(Expr.This expr) {
         if (currentClass == ClassType.NONE) {
-            Lox.error(expr.keyword,
-                      "Can't use 'this' outside of a class.");
+            Lox.error(expr.keyword, "Can't use 'this' outside of a class.");
             return null;
         }