13.3 Calling Superclass Methods

src/com/craftinginterpreters/lox/Environment.java

@@ -4,7 +4,7 @@ import java.util.HashMap;
 import java.util.Map;
 
 class Environment {
-    private final Environment enclosing;
+    final Environment enclosing;
     private final Map<String, Object> values = new HashMap<>();
 
     Environment() {

src/com/craftinginterpreters/lox/Interpreter.java

@@ -62,6 +62,22 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         return value;
     }
 
+    @Override
+    public Object visitSuperExpr(Expr.Super expr) {
+        int distance = locals.get(expr);
+        LoxClass superclass = (LoxClass) environment.getAt(distance, "super");
+
+        LoxInstance object = (LoxInstance) environment.getAt(distance - 1, "this");
+
+        LoxFunction method = superclass.findMethod(expr.method.lexeme);
+
+        if (method == null) {
+            throw new RuntimeError(expr.method, "Undefined property '" + expr.method.lexeme + "'.");
+        }
+
+        return method.bind(object);
+    }
+
     @Override
     public Object visitThisExpr(Expr.This expr) {
         return lookUpVariable(expr.keyword, expr);
@@ -190,6 +206,11 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         }
         environment.define(stmt.name.lexeme, null);
 
+        if (stmt.superclass != null) {
+            environment = new Environment(environment);
+            environment.define("super", superclass);
+        }
+
         Map<String, LoxFunction> methods = new HashMap<>();
         for (Stmt.Function method : stmt.methods) {
             LoxFunction function = new LoxFunction(method, environment, method.name.lexeme.equals("init"));
@@ -197,6 +218,11 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         }
 
         LoxClass klass = new LoxClass(stmt.name.lexeme, (LoxClass) superclass, methods);
+
+        if (superclass != null) {
+            environment = environment.enclosing;
+        }
+
         environment.assign(stmt.name, klass);
         return null;
     }

src/com/craftinginterpreters/lox/Parser.java

@@ -362,6 +362,13 @@ class Parser {
             return new Expr.Literal(previous().literal);
         }
 
+        if (match(SUPER)) {
+            Token keyword = previous();
+            consume(DOT, "Expect '.' after 'super'.");
+            Token method = consume(IDENTIFIER, "Expect superclass method name.");
+            return new Expr.Super(keyword, method);
+        }
+
         if (match(THIS))
             return new Expr.This(previous());
 

src/com/craftinginterpreters/lox/Resolver.java

@@ -19,7 +19,7 @@ class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
     }
 
     private enum ClassType {
-        NONE, CLASS
+        NONE, CLASS, SUBCLASS
     }
 
     private ClassType currentClass = ClassType.NONE;
@@ -51,9 +51,16 @@ class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
         }
 
         if (stmt.superclass != null) {
+            currentClass = ClassType.SUBCLASS;
+
             resolve(stmt.superclass);
         }
 
+        if (stmt.superclass != null) {
+            beginScope();
+            scopes.peek().put("super", true);
+        }
+
         beginScope();
         scopes.peek().put("this", true);
 
@@ -67,6 +74,10 @@ class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
 
         endScope();
 
+        if (stmt.superclass != null) {
+            endScope();
+        }
+
         currentClass = enclosingClass;
         return null;
     }
@@ -183,6 +194,18 @@ class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
         return null;
     }
 
+    @Override
+    public Void visitSuperExpr(Expr.Super expr) {
+        if (currentClass == ClassType.NONE) {
+            Lox.error(expr.keyword, "Can't use 'super' outside of a class.");
+        } else if (currentClass != ClassType.SUBCLASS) {
+            Lox.error(expr.keyword, "Can't use 'super' in a class with no superclass.");
+        }
+
+        resolveLocal(expr, expr.keyword);
+        return null;
+    }
+
     @Override
     public Void visitThisExpr(Expr.This expr) {
         if (currentClass == ClassType.NONE) {

src/com/craftinginterpreters/tool/GenerateAst.java

@@ -19,7 +19,8 @@ public class GenerateAst {
                         "Call : Expr callee, Token paren, List<Expr> arguments", "Get : Expr object, Token name",
                         "Grouping : Expr expression", "Literal : Object value",
                         "Logical : Expr left, Token operator, Expr right", "Set : Expr object, Token name, Expr value",
-                        "This : Token keyword", "Unary : Token operator, Expr right", "Variable : Token name"));
+                        "Super : Token keyword, Token method", "This : Token keyword",
+                        "Unary : Token operator, Expr right", "Variable : Token name"));
         defineAst(outputDir, "Stmt",
                 Arrays.asList("Block : List<Stmt> statements",
                         "Class : Token name, Expr.Variable superclass, List<Stmt.Function> methods",