chapter 13

examples/inheritance.lox

@@ -0,0 +1,9 @@
+class Doughnut {
+  fn cook() {
+    print("Fry until golden brown.");
+  }
+}
+
+class BostonCream < Doughnut {}
+
+BostonCream().cook();

examples/super.lox

@@ -0,0 +1,14 @@
+class Doughnut {
+  fn cook() {
+    print("Fry until golden brown.");
+  }
+}
+
+class BostonCream < Doughnut {
+  fn cook() {
+    super.cook();
+    print("Pipe full of custard and coat with chocolate.");
+  }
+}
+
+BostonCream().cook();

rlox/classes.rb

@@ -34,9 +34,10 @@ class LoxInstance
 end
 
 class LoxClass
-  def initialize(name, methods)
+  def initialize(name, methods, superclass)
     @name = name
     @methods = methods
+    @super = superclass
   end
 
   def to_s()
@@ -78,5 +79,9 @@ class LoxClass
     if @methods.has_key? name.lexeme
       return @methods[name.lexeme].bind(instance)
     end
+
+    if @super
+      return @super.find_method(instance, name)
+    end
   end
 end

rlox/executor.rb

@@ -2,9 +2,9 @@ require './rlox/environment'
 require './rlox/parse'
 require './rlox/scan'
 
-Env = Struct.new(:map, :scopes, :vars, :fn, :cls) do
+Env = Struct.new(:map, :scopes, :vars, :fn, :cls, :parent) do
   def child()
-    Env.new(map, scopes, Environment.new(vars), fn)
+    Env.new(map, scopes, Environment.new(vars), fn, cls, self)
   end
 
   def lookup(expr, name)
@@ -19,7 +19,7 @@ end
 
 class Executor
   def initialize()
-    @env = Env.new({}, [], Environment.global, nil, nil)
+    @env = Env.new({}, [], Environment.global, nil, nil, nil)
     @scanner = Scanner.new
     @parser = Parser.new
   end

rlox/expression.rb

@@ -319,13 +319,27 @@ Set = Struct.new(:object, :name, :value) do
     val
   end
 end
-Klass = Struct.new(:name, :methods) do
+Klass = Struct.new(:name, :methods, :superclass) do
   def resolve(env)
     enclosing = env.cls
     env.cls = :class
     declare(env.scopes, name)
+
+    if superclass != nil
+      env.cls = :subclass
+      superclass.resolve(env)
+    end
+
     define(env.scopes, name)
 
+    if superclass
+      begin_scope(env.scopes)
+
+      dummy = Token.new(:id, "super", nil, 0)
+      declare(env.scopes, dummy)
+      define(env.scopes, dummy)
+    end
+
     begin_scope(env.scopes)
     dummy = Token.new(:id, "self", nil, 0)
     declare(env.scopes, dummy)
@@ -336,14 +350,33 @@ Klass = Struct.new(:name, :methods) do
     }
 
     end_scope(env.scopes)
+
+    if superclass
+      end_scope(env.scopes)
+    end
+
     env.cls = enclosing
   end
 
   def eval(env)
+    supcls = nil
+    if superclass
+      supcls = superclass.eval(env)
+
+      if not supcls.is_a? LoxClass
+        raise ExecError.new(name.line, "Superclass must be a class.")
+      end
+    end
+
     if name
       env.vars.define(name.lexeme, nil)
     end
 
+    if superclass
+      env = env.child
+      env.vars.define("super", supcls)
+    end
+
     methods_dict = {}
     methods.each { | method |
       fn = Callable.new(method.name.lexeme, method.params, method.body, env) {
@@ -357,7 +390,11 @@ Klass = Struct.new(:name, :methods) do
       methods_dict[method.name.lexeme] = fn
     }
 
-    klass = LoxClass.new(name ? name.lexeme : nil, methods_dict)
+    klass = LoxClass.new(name ? name.lexeme : nil, methods_dict, supcls)
+
+    if superclass
+      env = env.parent
+    end
 
     if name
       env.vars.assign(name, klass)
@@ -378,6 +415,37 @@ Self = Struct.new(:keyword) do
     env.lookup(self, keyword)
   end
 end
+Super = Struct.new(:keyword, :method) do
+  def resolve(env)
+    if not env.cls
+      raise VarError.new(keyword.line, "Cannot use 'super' outside of a class.")
+    elsif env.cls != :subclass
+      raise VarError.new(
+        keyword.line,
+        "Cannot use 'super' in a class without superclass."
+      )
+    end
+
+    resolve_local(env, self, keyword)
+  end
+
+  def eval(env)
+    distance = env.map[self]
+    superclass = env.vars.get_at(keyword, distance)
+    dummy = Token.new(:id, "self", nil, 0)
+    obj = env.vars.get_at(dummy, distance-1)
+    m = superclass.find_method(obj, method)
+
+    if not m
+      raise ExecError.new(
+        method.line,
+        "Undefined 'super' property '#{method.lexeme}'."
+      )
+    end
+
+    m
+  end
+end
 
 Expression = Struct.new(:expression) do
   def resolve(env)

rlox/parse.rb

@@ -226,6 +226,11 @@ class Parser
       Literal.new(nil)
     elsif match(:self)
       Self.new(previous)
+    elsif match(:super)
+      kw = previous
+      consume(:dot, "Expect '.' after 'super'.")
+      method = consume(:id, "Expect superclass method name.")
+      Super.new(kw, method)
     elsif match(:number, :string)
       Literal.new(previous.literal)
     elsif match(:left_paren)
@@ -426,6 +431,13 @@ class Parser
     consume(:class, "Expect 'class' keyword.")
     name = consume(:id, "Expect class name.")
 
+    superclass = nil
+
+    if match(:lt)
+      consume(:id, "Expect superclass name.")
+      superclass = Var.new(previous)
+    end
+
     consume(:left_brace, "Expect '{' before body of class '#{name.lexeme}.'")
 
     methods = []
@@ -436,7 +448,7 @@ class Parser
 
     consume(:right_brace, "Expect '}' after class body.")
 
-    Klass.new(name, methods)
+    Klass.new(name, methods, superclass)
   end
 
   def declaration()