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From: Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>
Newsgroups: comp.lang.ada
Subject: Re: List container strawman 1.2
Date: 19 Nov 2001 16:08:22 -0500
Organization: NASA Goddard Space Flight Center
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Ted Dennison<dennison@telepath.com> writes:

> In article <3BF3EDE5.FE0ED701@brighton.ac.uk>, John English says...
> (good stuff all)
> >
> >Iterator is nice too but instantiating the generic (and wrapping the
> >operation in an approapriately-shaped procedure first) is probably
> >more hassle than a while I <> Last(L) loop, IMHO...
> 
> That's my feelings on it too. Additionally, I've found you have to use globals
> in the routine to get all of your real work done, which rubs the wrong way
> against about 13 years of training on my part. But there are those here who
> swear by passive iterators.

I handle this in a different way in SAL. See the file
sal-gen-alg-process_all_constant.ads for an example.

The key point is that iterating thru a container is an _algorithm_. It
is instantiated separately from the container (I know, that's too hard
:). In fact, the algorithm can be instantiated in a local scope, so
the "global variables" are local to that scope.


Yet another design is to declare a "State_Type" generic formal for the
iterator, like ASIS does for tree processing:

generic
   type State_Type is limited private;
   with procedure Operation 
      (Target : in out Element; 
       State  : in out State_Type;
       Quit   :    out Boolean);
procedure Iterator (Target : in out List);

This complicates things for users that don't need state, but they can
just pass Integer.
 
-- 
-- Stephe