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Date: Tue, 4 Aug 2015 16:56:25 -0700 (PDT)
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Subject: Mutating elements of constant list using a container element iterator
From: martinbbjerregaard@gmail.com
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Xref: number.nntp.giganews.com comp.lang.ada:194667
Date: 2015-08-04T16:56:25-07:00
List-Id: <comp.lang.ada>

Mutating elements of a constant container in a "for ... of ... loop" is possible with the "standard" container types but not the indefinite or bounded containers:

with Ada.Containers.Doubly_Linked_Lists;
with Ada.Containers.Indefinite_Doubly_Linked_Lists;

procedure Strange_Behavior is
   
   type Test is tagged
      record
         Value : Integer;
      end record;
   
   package Test_Lists is new Ada.Containers.Doubly_Linked_Lists (Element_Type => Test);
   package Test_Indefinite_Lists is new Ada.Containers.Indefinite_Doubly_Linked_Lists (Element_Type => Test);
   
   function Make_List return Test_Lists.List is
   begin
      return Result : Test_Lists.List := Test_Lists.Empty_List do
         for I in 0 .. 100 loop
            Result.Append (Test'(Value => I));
         end loop;
      end return;
   end Make_List;
   
   function Make_Indefinite_List return Test_Indefinite_Lists.List is
   begin
      return Result : Test_Indefinite_Lists.List := Test_Indefinite_Lists.Empty_List do
         for I in 0 .. 100 loop
            Result.Append (Test'(Value => I));
         end loop;
      end return;
   end Make_Indefinite_List;
   
   V1 : constant Test_Lists.List := Make_List;
   V2 : constant Test_Indefinite_Lists.List := Make_Indefinite_List;
   
begin
   
   for Item of V1 loop
      Item.Value := 0; -- no error
   end loop;
   
   for Item of V2 loop
      Item.Value := 0; -- error: "left hand side of assignment must be a variable"
   end loop;
   
end Strange_Behavior;

http://www.ada-auth.org/standards/12rat/html/Rat12-8-3.html says:
"If we write
for E of The_List loop
   ...   -- do something to Element E
end loop;
then we can change the element E unless The_List has been declared as constant."

Is this a bug?