[RFC] OOP - Dispatching

considered/rfc-oop-dispatching.rst

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+- Feature Name: Standard OOP model
+- Start Date: May 5th 2020
+- RFC PR:
+- RFC Issue:
+
+Summary
+=======
+
+Motivation
+==========
+
+Guide-level explanation
+=======================
+
+Dispatching and Class Wide Views
+--------------------------------
+
+A new library / partition level pragma is introduced, Default_Dispatching_Calls.
+When active, calls to primitives are dispatching (when the tag can be
+determined statically, the compiler may optimize the call to be static) unless
+explicitelly marked as static (for example, through 'Super). E.g:
+
+.. code-block:: ada
+
+   pragma Default_Dispatching_Calls (On);
+
+   type Root is tagged null record;
+
+   procedure P (Self : in out Root);
+
+   type Child is new Root with null record;
+
+   procedure P (Self : in out Root);
+
+   type A_Root is access all Root;
+
+   procedure P (V : in out Root) is
+   begin
+      V.P2; -- Dispatching
+   end P;
+
+   V : A_Root := new Child;
+
+   V.P; -- dispatching
+
+Default_Dispatching_Calls is On by default on new version of the language.
+
+Reference-level explanation
+===========================
+
+
+Rationale and alternatives
+==========================
+
+It is a potential vulnerability not to call an overriden primitive. This may
+lead to an object to be in an state that has not been anticipated, in particular
+when the role of the overriden primitive is to keep the state of the derived
+object consistant. It's also commonly the case in most OOP languages that
+dispatching is the default expected behavior and non dispatching the exception.
+
+This also fix a common confusion in Ada, where the dispatching parameter of A
+primitive is itself non-dispatching and requires so-called redispatching. The
+following code becomes then much more natural to write:
+
+.. code-block:: ada
+
+   package P is
+      type T1 is tagged record
+         procedure P (Self : in out T1);
+
+         procedure P2 (Self : in out T1);
+      end T1;
+   end P;
+
+   package P is
+      type body T1 is tagged record
+         procedure P (Self : in out T1) is
+         begin
+            Self.P2; -- Dispatching
+         end P;
+      end T1;
+   end P;
+
+
+Drawbacks
+=========
+
+
+Prior art
+=========
+
+
+Unresolved questions
+====================
+
+Future possibilities
+====================