add inherited callback to classes

docs/reference.md

@@ -731,6 +731,21 @@ properties and methods from another class.
 
 Here we extend our Inventory class, and limit the amount of items it can carry.
 
+Whenever a class inherits from another, it sends a message to the parent class
+by calling the method `__inherited` on the parent class if it exists.  The
+function recieves two arguments, the class that is being inherited and the
+child class.
+
+    ```moon
+    class Shelf
+      @__inherited: (child) =>
+        print @__name, "was inherited by", child.__name
+
+    -- will print: Shelf was inherited by Cupboard
+    class Cupboard extends Shelf
+      nil
+    ```
+
 ### Super
 
 `super` is a special keyword that can be used in two different ways: It can be

moonscript/transform.lua

@@ -950,6 +950,23 @@ Statement = Transformer({
             return { }
           end
         end)()),
+        build["if"]({
+          cond = {
+            "exp",
+            parent_cls_name,
+            "and",
+            parent_cls_name:chain("__inherited")
+          },
+          ["then"] = {
+            parent_cls_name:chain("__inherited", {
+              "call",
+              {
+                parent_cls_name,
+                cls_name
+              }
+            })
+          }
+        }),
         cls_name
       }
       hoist_declarations(out_body)

moonscript/transform.moon

@@ -486,6 +486,18 @@ Statement = Transformer {
           .group statements
         } else {}
 
+        -- run the inherited callback
+        build["if"] {
+          cond: {"exp",
+            parent_cls_name, "and", parent_cls_name\chain "__inherited"
+          }
+          then: {
+            parent_cls_name\chain "__inherited", {"call", {
+              parent_cls_name, cls_name
+            }}
+          }
+        }
+
         cls_name
       }
 

tests/outputs/class.lua

@@ -37,6 +37,9 @@ Hello = (function()
     end
   })
   _base_0.__class = _class_0
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 local x = Hello(1, 2)
@@ -79,6 +82,9 @@ Simple = (function()
     end
   })
   _base_0.__class = _class_0
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 local Yikes
@@ -112,6 +118,9 @@ Yikes = (function()
     end
   })
   _base_0.__class = _class_0
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 x = Yikes()
@@ -151,6 +160,9 @@ Hi = (function()
     end
   })
   _base_0.__class = _class_0
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 Simple = (function()
@@ -187,6 +199,9 @@ Simple = (function()
     end
   })
   _base_0.__class = _class_0
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 x = Simple()
@@ -227,6 +242,9 @@ Okay = (function()
     end
   })
   _base_0.__class = _class_0
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 local Biggie
@@ -268,6 +286,9 @@ Biggie = (function()
     end
   })
   _base_0.__class = _class_0
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 local Yeah
@@ -307,6 +328,9 @@ Yeah = (function()
     end
   })
   _base_0.__class = _class_0
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 local What
@@ -346,6 +370,9 @@ What = (function()
     end
   })
   _base_0.__class = _class_0
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 Hello = (function()
@@ -391,6 +418,9 @@ Hello = (function()
     end
   })
   _base_0.__class = _class_0
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 do
@@ -442,6 +472,9 @@ CoolSuper = (function()
     end
   })
   _base_0.__class = _class_0
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 x = self.hello
@@ -494,6 +527,9 @@ ClassMan = (function()
   self.hello = 3434
   self.world = 23423
   self.red = function(self) end
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 x = self
@@ -547,6 +583,9 @@ if something then
     hello = "world"
     self.another = "day"
     print("yeah")
+    if _parent_0 and _parent_0.__inherited then
+      _parent_0.__inherited(_parent_0, _class_0)
+    end
     return _class_0
   end)()
 end

tests/outputs/export.lua

@@ -34,6 +34,9 @@ Something = (function()
     end
   })
   _base_0.__class = _class_0
+  if _parent_0 and _parent_0.__inherited then
+    _parent_0.__inherited(_parent_0, _class_0)
+  end
   return _class_0
 end)()
 local What