Add `#module_exec` and `#class_exec` in `mruby-class-ext` gem.

mrbgems/mruby-class-ext/src/class.c

@@ -15,6 +15,42 @@ mrb_mod_singleton_class_p(mrb_state *mrb, mrb_value self)
   return mrb_bool_value(mrb_type(self) == MRB_TT_SCLASS);
 }
 
+/*
+ *  call-seq:
+ *     module_exec(arg...) {|var...| block } -> obj
+ *     class_exec(arg...) {|var...| block } -> obj
+ *
+ * Evaluates the given block in the context of the
+ * class/module. The method defined in the block will belong
+ * to the receiver. Any arguments passed to the method will be
+ * passed to the block. This can be used if the block needs to
+ * access instance variables.
+ *
+ *     class Thing
+ *     end
+ *     Thing.class_exec{
+ *       def hello() "Hello there!" end
+ *     }
+ *     puts Thing.new.hello()
+ */
+
+static mrb_value
+mrb_mod_module_exec(mrb_state *mrb, mrb_value self)
+{
+  const mrb_value *argv;
+  mrb_int argc;
+  mrb_value blk;
+
+  mrb_get_args(mrb, "*&", &argv, &argc, &blk);
+
+  if (mrb_nil_p(blk)) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "no block given");
+  }
+
+  mrb->c->ci->target_class = mrb_class_ptr(self);
+  return mrb_yield_cont(mrb, blk, self, argc, argv);
+}
+
 void
 mrb_mruby_class_ext_gem_init(mrb_state *mrb)
 {
@@ -22,6 +58,8 @@ mrb_mruby_class_ext_gem_init(mrb_state *mrb)
 
   mrb_define_method(mrb, mod, "name", mrb_mod_name, MRB_ARGS_NONE());
   mrb_define_method(mrb, mod, "singleton_class?", mrb_mod_singleton_class_p, MRB_ARGS_NONE());
+  mrb_define_method(mrb, mod, "module_exec", mrb_mod_module_exec, MRB_ARGS_ANY()|MRB_ARGS_BLOCK());
+  mrb_define_method(mrb, mod, "class_exec", mrb_mod_module_exec, MRB_ARGS_ANY()|MRB_ARGS_BLOCK());
 }
 
 void

mrbgems/mruby-class-ext/test/module.rb

@@ -32,3 +32,24 @@ assert 'Module#singleton_class?' do
   assert_false cls.singleton_class?
   assert_true scl.singleton_class?
 end
+
+assert 'Module#module_eval' do
+  mod = Module.new
+  mod.class_exec(1,2,3) do |a,b,c|
+    assert_equal([1,2,3], [a,b,c])
+    def hi
+      "hi"
+    end
+  end
+  cls = Class.new
+  cls.class_exec(42) do |x|
+    assert_equal(42, x)
+    include mod
+    def hello
+      "hello"
+    end
+  end
+  obj = cls.new
+  assert_equal("hi", obj.hi)
+  assert_equal("hello", obj.hello)
+end