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From: madmats@elcgl.epfl.ch
Newsgroups: comp.lang.ada
Subject: Re: Hiding Concurrency
Message-ID: <1991Apr19.031148.1@elcgl.epfl.ch>
Date: 19 Apr 91 02:11:48 GMT
References: <1991Apr15.211346.15760@cs.odu.edu> <jls.671951147@rutabaga>
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Followup-To: comp.lang.c++
Organization: Ecole Polytechnique Federale de Lausanne
List-Id: <comp.lang.ada>

In article <jls.671951147@rutabaga>, jls@rutabaga.Rational.COM (Jim Showalter) writes:
> Why did you use semaphores instead of tasking for this problem?
> If you had used tasking, your concurrency problems would have
> gone away--that is, after all, what tasking is FOR.

Because tasks can only be accessed through entries, and composite ADTs often
require iterators (implemented as generic procedures in non-concurrent ADTs).

For instance, try to convert this to a concurrent ADT without using semaphores:

generic
   type Item_Type is private;
   with function "<" (Left, Right : Item_Type) return Boolean is <>;
package Sets is

   type Set is limited private;

   procedure Insert (Item : in Item_Type;
                     Into : in out Set);

   procedure Remove (Item : in Item_Type;
                     From : in out Set);

   generic
      with procedure Action (Item : in Item_Type);
   procedure Enumerate (The_Set : in Set);

end Sets;

You will have much trouble specifying Enumerate as a task entry (not to
speak of implementation, as Enumerate will be recursive if the Sets
are implemented as binary trees).

Mats