Subprogram-valued Aspect Declarations

Author: Denis Mazzucato

features/rfc-oop-attributes.rst

@@ -5,68 +5,357 @@
 Summary
 =======
 
+This RFC introduces a shorthand notation to declare subprogram-valued aspects.
+
 Motivation
 ==========
 
+Currently, to specify a subprogram-valued aspect for a type, one needs to first
+declare a subprogram, that most of the times is never referred to elsewhere,
+and then link it to the type via an aspect specification or attribute
+definition clause. This pattern creates unnecessary boilerplate code,
+disconnects the aspect specification from the denoted subprogram declaration,
+and introduces potential name resolution issues.
+
 Guide-level explanation
 =======================
 
-Today, specifying a subprogram attribute of a type requires to declare a
-subprogram and then to use an aspect or an attribute to link it to a type, e.g.
+Subprogram-valued Aspect Declarations
+-------------------------------------
+
+Currently, specifying a subprogram-valued aspect for a type requires to first
+declare the subprogram and then apply the corresponding aspect specification or
+attribute definition clause. For example:
 
 .. code-block:: ada
 
    type T is null record;
 
    for T'Write use My_Write;
 
-   procedure My_Write(
-      Stream : not null access Ada.Streams.Root_Stream_Type'Class;
+   procedure My_Write
+     (Stream : not null access Ada.Streams.Root_Stream_Type'Class;
+      Item : in T);
+
+Or via an aspect specification:
+
+.. code-block:: ada
+
+   type T is null record with Write => My_Write;
+
+   procedure My_Write
+     (Stream : not null access Ada.Streams.Root_Stream_Type'Class;
       Item : in T);
 
-This proposal allows to define directly the procedure as being an attribute of
-the type:
+This proposal introduces **subprogram-valued aspect declarations** that allow
+to specify a subprogram as an aspect value directly within the subprogram
+declaration.
+For example, the above can be rewritten as:
 
 .. code-block:: ada
 
    type T is null record;
 
-   procedure T'Write(
-      Stream : not null access Ada.Streams.Root_Stream_Type'Class;
+   procedure T'Write
+     (Stream : not null access Ada.Streams.Root_Stream_Type'Class;
       Item : in T);
 
-In addition, this can be used as a scoped attribute, possibly with re-ordering
-of parameters to ensure that the first parameter is conceptually the one
-"owning" the primitive, e.g.:
+The code above both declares the subprogram ``T'Write`` and specifies it as the
+``Write`` aspect of type ``T``.
+Intuitively, this shorthand notation translates to the following aspect
+specification:
+
+.. code-block:: ada
+
+   type T is null record with Write => @'Write;
+
+Where "@" denotes the enclosing type name, cf. ``T`` in this case. When such
+subprogram is inherited, "@" will also denote the derived type name.
+
+A Derived Map Example
+^^^^^^^^^^^^^^^^^^^^^
+
+With the subprogram-valued aspect declarations, we could write a hierarchy of a
+map as follows:
+
+.. code-block:: ada
+
+   package P is
+      type T is tagged null record;
+
+      --  T has exactly two keys: Natural and Boolean
+      function T'Constant_Indexing (X : T; I : Natural) return Natural;
+      function T'Constant_Indexing (X : T; B : Boolean) return Natural;
+   end P;
+
+.. code-block:: ada
+
+   with P;
+
+   package Q is
+      type T2 is new P.T with null record;
+
+      --  The behavior of the Natural key is overridden
+      overriding -- Optional
+      function T2'Constant_Indexing (X : T2; I : Natural) return Natural;
+
+      --  The Boolean key is implicitly inherited from P.T
+      --  function T2'Constant_Indexing (X : T2; B : Boolean) return Natural;
+
+      --  Additionally, T2 supports a Float key
+      function T2'Constant_Indexing (X : T2; F : Float) return Float;
+   end Q;
+
+The aspect specification on ``T`` is ``... => @'Constant_Indexing``, where "@"
+denotes both ``T`` and ``T2``.
+As you can see from the example above, the usual rules of inheritance apply.
+
+There is no clash between subprogram-valued aspect declarations for the same
+aspect name, in the same scope *but* for different types, since the aspect
+specification is qualified by "@", which denotes the enclosing type name.
+
+.. code-block:: ada
+
+   type A is tagged null record;
+   procedure A'Constant_Indexing (X : A; I : Natural) return Natural;
+
+   type B is tagged null record;
+   procedure B'Constant_Indexing (X : B; I : Natural) return Natural;
+
+Which intuitively translates to:
+
+- ``type A ... with Constant_Indexing => @'Constant_Indexing``
+  where "@" denotes only ``A``, and
+
+- ``type B ... with Constant_Indexing => @'Constant_Indexing``
+  where "@" denotes only ``B``.
+
+As expected, ``A'Constant_Indexing`` and ``B'Constant_Indexing`` are not
+homonyms.
+
+Primitive or Nonprimitive?
+--------------------------
+
+Subprogram-valued aspect declarations can be used to define both primitive and
+nonprimitive operations of a type.
+For instance, we could define a container type as:
+
+.. code-block:: ada
+
+   type Container is tagged record
+      ...
+   end record;
+   type Cursor is new Positive;
+
+   function Container'First (C : Container) return Cursor;
+   function Container'Next (C : Container; Pos : Cursor) return Cursor;
+   function Container'Has_Element (C : Container; Pos : Cursor) return Boolean;
+   function Container'Element (C : Container; Pos : Cursor) return Integer;
+
+All the subprogram-valued aspect
+declarations of ``Container`` are primitive operations.
+The usual rules of primitive operations apply.
+
+Scoped Records
+--------------
+
+In relation to the scoped records and class types proposed in the *RFC OOP
+primitives*,
+subprogram-valued aspect declarations can be used accordingly:
 
 .. code-block:: ada
 
    type T is tagged record
 
-      procedure T'Write(
-         Stream : not null access Ada.Streams.Root_Stream_Type'Class;
+      procedure T'Write
+        (Stream : not null access Ada.Streams.Root_Stream_Type'Class;
          Item : in T);
 
    end T;
 
-This provide a seamless solution in particular to scope a destructor:
-
+This provide a seamless solution in particular to scope oop primitives:
 
 .. code-block:: ada
 
-   type T is tagged record
+   type T is class record
+      procedure T'Constructor (Self : in out T);
       procedure T'Destructor (Self : in out T);
+
+      procedure T'Write
+        (Stream : not null access Ada.Streams.Root_Stream_Type'Class;
+         Item : in T);
    end record;
 
+This approach is quite natural and consistent with the rest of the proposal:
+all aspect specifications are gathered together within the scoped environment.
+
+Unfortunately, the subprogram-valued aspect declarations inside a scoped class
+may clash with the class type rules where each subprogram is required to be
+primitive and following the rules of First_Controlling_Parameter
+(e.g., see ``T'Write`` above
+where ``Item`` is the second formal, not the first). The rule's motivation
+comes, for instance,
+from the fact that class primitives are called with the prefix notation, and
+that no primitive operation should be defined afterwards.
+
+To solve this issue, we allow (only) subprogram-valued aspect declarations
+inside scoped environments to be nonprimitive and not following the
+First_Controlling_Parameter rule. Since, these subprograms are never called
+with the prefix notation, this relaxation is safe.
+
+Hierarchy with Write, Read, Output, and Put_Image
+-------------------------------------------------
+
+Here it follows an example of usage of subprogram-valued aspect declarations
+with a simple hierarchy involving a combination of ``Write``, ``Read``,
+``Output``, and
+``Put_Image`` aspects.
+
+.. code-block:: ada
+
+   with Ada.Strings.Text_Buffers; use Ada.Strings.Text_Buffers;
+   with Ada.Streams; use Ada.Streams;
+
+   package Geometry is
+      type Shape is tagged record
+         X, Y : Integer;
+      end record;
+
+      procedure Shape'Read
+        (Stream : not null access Root_Stream_Type'Class;
+         Item   : out Shape);
+      procedure Shape'Output
+        (Stream : not null access Root_Stream_Type'Class;
+         Item   : in Shape);
+      procedure Shape'Put_Image
+        (Buffer : in out Root_Buffer_Type'Class;
+         Item   : Shape);
+
+      type Circle is new Shape with record
+         Radius : Integer;
+      end record;
+
+      overriding
+      procedure Circle'Read
+        (Stream : not null access Root_Stream_Type'Class;
+         Item   : out Circle);
+      procedure Circle'Write
+        (Stream : not null access Root_Stream_Type'Class;
+         Item   : in Circle);
+      overriding
+      procedure Circle'Put_Image
+        (Buffer : in out Root_Buffer_Type'Class;
+         Item   : Circle);
+   end Geometry;
+
+   ...
+
+   with Ada.Strings.Text_Buffers; use Ada.Strings.Text_Buffers;
+   with Ada.Streams; use Ada.Streams;
+   with Ada.Text_IO; use Ada.Text_IO;
+
+   package body Geometry is
+      procedure Shape'Read
+        (Stream : not null access Root_Stream_Type'Class;
+         Item   : out Shape) is
+      begin
+         Put_Line ("Shape'Read called");
+         Integer'Read (Stream, Item.X);
+         Integer'Read (Stream, Item.Y);
+      end Shape'Read;
+
+      procedure Shape'Output
+        (Stream : not null access Root_Stream_Type'Class;
+         Item   : in Shape) is
+      begin
+         Put_Line ("Shape'Output called");
+         Integer'Output (Stream, Item.X);
+         Integer'Output (Stream, Item.Y);
+      end Shape'Output;
+
+      procedure Shape'Put_Image
+        (Buffer : in out Root_Buffer_Type'Class;
+         Item   : Shape) is
+      begin
+         Put_Line ("Shape'Put_Image called");
+         Integer'Put_Image (Buffer, Item.X);
+         Integer'Put_Image (Buffer, Item.Y);
+      end Shape'Put_Image;
+
+      overriding
+      procedure Circle'Read
+        (Stream : not null access Root_Stream_Type'Class;
+         Item   : out Circle) is
+      begin
+         Put_Line ("Circle'Read called");
+         Integer'Read (Stream, Item.Radius);
+         Shape'Read (Stream, Shape (Item));
+      end Circle'Read;
+
+      procedure Circle'Write
+        (Stream : not null access Root_Stream_Type'Class;
+         Item   : in Circle) is
+      begin
+         Put_Line ("Circle'Write called");
+         Integer'Write (Stream, Item.Radius);
+         Shape'Output (Stream, Shape (Item)); -- Mixing Write and Output here
+      end Circle'Write;
+
+      overriding
+      procedure Circle'Put_Image
+        (Buffer : in out Root_Buffer_Type'Class;
+         Item   : Circle) is
+      begin
+         Put_Line (Shape(Item)'Image);
+         Put_Line ("Circle'Put_Image called");
+         Integer'Put_Image (Buffer, Item.Radius);
+      end Circle'Put_Image;
+   end Geometry;
+
 Reference-level explanation
 ===========================
 
+This proposal introduces syntactic sugar for the declaration and specification
+of subprogram-valued aspects. It is fully backward compatible with existing Ada
+code.
+
+When a subprogram-valued aspect declaration is encountered, it is equivalent
+to an aspect specification where the aspect value is "@'<Aspect_Name>", where
+"@" denotes the the name of the enclosing type and any of its derived types.
+
 Rationale and alternatives
 ==========================
 
+While the notation "@'<Aspect_Name>" is not a syntactic Ada construct and it is
+used above in this proposal at an intuitive level for the aspect
+specifications, a syntactic ``@'<Aspect_Name>`` used in the subprogram name
+could further shorten our notation. This second use of ``@`` denotes the
+enclosing type name; no need to account for derived types here.
+The big advantage is when the type name is long; for instance, we could write:
+
+.. code-block:: ada
+
+   type Indexable_Container_With_A_Long_Name is tagged record
+      function @'Constant_Indexing
+        (Container : aliased @;
+         Position  : Offset) return Constant_Reference_Type;
+      function @'Constant_Indexing
+        (Container : aliased @;
+         Position  : Custom_Offset) return Constant_Reference_Type;
+      function @'Variable_Indexing
+        (Container : aliased in out @;
+         Position  : Offset) return Reference_Type;
+   end record;
+
+This use of ``@`` transcends the current proposal and should be discussed in a
+separate RFC.
+
 Drawbacks
 =========
 
+The IDE support for this shorthand notation might be complex as it would need
+to connect aspect calls with their (potential) declarations.
+
 Prior art
 =========
 
@@ -76,4 +365,3 @@ Unresolved questions
 Future possibilities
 ====================
 
-