Re: List container strawman

["Nick Roberts" <nickroberts@adaos.worldonline.co.uk>]

I'm sorry, I'm throw one of classic spanners in the works here, but I'm
afraid I think you've got your approach horribly horribly wrong!

For a start, all this mularchy about insertion and deletion is just plain
silly! It really is. You don't need them, and shouldn't implement them. Yes,
really. Let me show you why.

Suppose you want to have a list of names (strings), and you want to
normalise them as follows:

(a) convert them all to uppercase;
(b) all beginning with "MACxy", where x is a letter, to have "MCxy" inserted
afterwards;
(c) all beginning with "?" to be deleted.

I'm going to frame the answer in terms of my own proposal, please forgive me
for that. The essential answer is that you don't keep a list of the strings,
but you keep a list of access values to them. You then simply build a new
list (of access values), and then delete the old one:

   with Ada.Characters.Handling; use Ada.Characters.Handling;
   ...
   type Name_Ref is access [all] String;
   package Name_Ref_Iteration is new Iteration(Name_Ref);

   procedure Normalize_Names (
      From: in out Name_Ref_Iteration.Sequence_Recorder'Class;
      Into: Name_Ref_Iteration.Sequence_Recorder'Class) is
      Ref: Name_Ref; N: Natural;
   begin
      Rewrite(Into); -- possibly redundant (possibly poor design, actually)
      Restart(From); -- ditto
      while not End_of_Data(From) loop
         Read(From,Ref);
         if Ref /= null and then Ref.all'Length > 0 and then
Ref(Ref.all'First) /= '?' then
            To_Upper(Ref.all);
            Write(Into,Ref);
            N := Ref.all.First-1; -- in case string doesn't start at 1
            if Ref.all'Length >= 5 and then Ref(N+1..N+3) = "MAC" and then
Is_Letter(Ref(N+4)) then
               Write(Into, new String'("MC"&Ref(N+4..Ref.all'Last)));
            end if;
         else
            Ref := null; -- or maybe use Unchecked_Deallocation
         end if;
      end loop;
      Rewrite(From); -- erases it (again, possibly wrong here)
      Restart(Into); -- again possibly redundant
   end Normalize_Names;

Note how this procedure can be passed parameters of ANY 'sequence recorder'
container type (instantiated on Name_Ref), not just lists. Supposing we
wanted to use a list container type:

   package Name_Ref_Lists is new xyz.Lists(Name_Ref_Iteration);
   subtype Name_List is Name_Ref_Lists.List_Type;
   L1, L2: Name_List;
   ... set up L1 with names
   Normalize_Names(L1,L2);

A note about the Restarts and Rewrites: they are probably better left
outside procedures such as this, in fact. I have quietly deleted oddities
like null pointers: a real program should probably raise exceptions.

Trust me, guys, this approach is majorly better in every way (easier to use,
easier to read & understand, easier to implement, more memory efficient,
more speed efficient) than messing around with inserts and deletes, for the
vast majority of situations.

I know I can seem pompous, overbearing, etc., etc., especially about this
subject, but it pains me to see you going so far astray, and spending so
much time arguing about features that you don't need anyway!

It also gives me another opportunity to demonstrate the idea of having a set
of abstract container types, upon which operations (such as Normalize_Names)
can be hung, thus freeing you of: (1) having to worry about which container
type to use when writing the procedure; (2) the procedure shackling you to
one particular container type. Do you turn away from this Utopia? ;-)

--
Best wishes,
Nick Roberts