private "subtype"s

private "subtype"s

[Georg Bauhaus]

A similar question has been partially answerd a while
ago by M. Heaney, in the sense that something
like the following code is not possible. But why?

package Whoops is  -- illegal!

   subtype Constr is private;
   --  [[Constr]] is constrained, but the constraints need
   -- not be known outside

private
   subtype Constr is Natural range 0..99;
end Whoops;

Georg Bauhaus

Re: private "subtype"s

[Ted Dennison]

In article <8rsubg$ka7$1@news-hrz.uni-duisburg.de>,
  sb463ba@l1-hrz.uni-duisburg.de (Georg Bauhaus) wrote:
> package Whoops is  -- illegal!
>
>    subtype Constr is private;
>    --  [[Constr]] is constrained, but the constraints need
>    -- not be known outside
>
> private
>    subtype Constr is Natural range 0..99;
> end Whoops;

That makes a little bit of sense. After all, if the client doesn't know
that Constr is a subtype of Natural, then what should the compiler do if
the client tries to assign a Constr to an Integer? The most probable
answer is "report an error". But if that's the case, Constr is really no
better to the client than a derived type. If fact, its worse. So why not
just use a derived type and be done with it?

--
T.E.D.

http://www.telepath.com/~dennison/Ted/TED.html
Day 1 of Free Europe


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Before you buy.

Re: private "subtype"s

[Georg Bauhaus]

Ted Dennison:
: That makes a little bit of sense.

At least :-)  I started thinking it doesn't
and only made things awkwardly complex. So
would I be better off using some tagged records
for the following example?

package Society is

   type Person is
     (Dumb, Boring, Average, Skilled, Smart, Clever, Genius, Freaky);
   --  (rather incomplete)

end Society;

generic
   How_Many: Positive;
   --  counting the crowd
package Society.Club  is
-- sub-
   type Correct_People is private;
   --  a selection of more distinguished persons

   type Participants is
     array(1..How_Many) of Correct_People;

   procedure Party(P: Participants);
   procedure Celebrate(P: Participants) renames Party;
   --  stay PC

private
-- sub-
   type Correct_People is new Person
     range Smart..Genius;

end Society.Club;

generic
   How_Many: Positive;
   --  counting the crowd
package Society.Joe_Bloggs  is
-- sub-
   type Cheap_Flat_People is private;
   --  a selection of persons

   type Participants is
     array(1..How_Many) of Cheap_Flat_People;

   procedure Party(P: Participants);
   procedure Get_Drunk(P: Participants) renames Party;
   --  well, maybe

private
-- sub-
   type Cheap_Flat_People is new Person
     range Dumb..Average;
   --  if you can believe this

end Society.Joe_Bloggs;


The admittedly vague idea in the code
is to have someone like Groucho Marx enter the
Club and not having to be converted to another
type of person...
 But with "-- sub-"types below it looks like
requiring constraints to trigger similar things
like types can do. Yes, so there...