Re: Reprise: 'in out' parameters for functions

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

Wojtek Narczynski wrote:

>> I take it Ada forces abstraction in version in cases?
>> Could you name an example unit to look at?
> 
> 'Inversion' not 'in version' incase it were more than a typo. When you
> implement a semaphore over a protected object, or in general when you
> need to lock / unlock by yourself. Classic example:
> 
> polyorb-tasking-profiles-full_tasking-mutexes.adb
> 
> Or try to implement tree crabbing. Hell, try to implement a list
> updateable concurrently from multiple tasks. You _will_ end up
> implementing a semaphore over a protected, over a semaphore.
> 
> Or the simplest possible: two protected counters, try to get the sum
> atomically.

This has nothing to do with abstraction inversion. The problem here is that
Ada does not have multiple protected actions. Same problem with tasks,
there cannot be rendezvous with multiple tasks. It is much work to solve
that. Especially to convince people that prefix notation is inherently bad.

>> : type system unable
>> : to express physical units,
>> 
>> Still, you might have
>> 
>>    type Quantity is abstract tagged private;
>> 
>>    function in_meters (x: Quantity) return Units.meter;
>>    function in_yards (x: Quantity) return Units.yard;
>>    --  etc...
> 
> Sure, let us continue:
> 
> function in_kilograms (x: Quantity) return Units.yard;
> -- Blows at runtime
> 
> This just cannot be done right in Ada.

This is wrong:

http://home.t-online.de/home/Christ-Usch.Grein/Ada/Dimension.html

The type system is capable to express dimensioned units. The major problem
here is not the type system, but inability to get rid of statically known
discriminants, which makes an implementation inefficient.

>> : But the problem real problem IMO is that
>> : the development of the language has stagnated.
>> 
>> What's missing?
> 
> From the language? For example parameters for exceptions.

How they could have parameters? There are only two ways for dealing with
exceptions. Either 1) all of them are of one [specific] type (maybe an
implicit one) or 2) they are of different types (maybe rooted in the same
root type [class-wide approach]). Ada has 1), which excludes parameters. If
you think that 2) would be better for Ada then solve the following:

generic
package Crazy is
   type Virtual is new Exception with ...;
      -- Note that Virtual exists in an instance of Crazy only.
      -- It is finalized together with the instance.
   procedure Show_Me (Object : Virtual);
end Crazy;

package body Crazy is
   Local : Integer := 0;

   procedure Show_Me (Object : Virtual) is
   begin
      Put_Line (Integer'Image (Local));
   end Show_Me;
begin
   raise Virtual (your fine parameters);
end Crazy;

declare
   package Catch_It is new Crazy;
begin
   null;
exception
   when Error : others =>
      -- What sort of type has Error?
      -- Can I dispatch to Show_Me?
      -- What it will print?

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de