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From: lga@sma.ch (Laurent Gasser)
Subject: Re: Pascal to Ada translator/ aflex,ayacc GNAT ports
Date: 1996/12/02
Message-ID: <57uki4INNkae@maz4.sma.ch>#1/1
X-Deja-AN: 201868350
distribution: world
references: <dewar.849318953@merv>
organization: Swiss Meteorological Institute
reply-to: lga@sma.ch
newsgroups: comp.lang.ada
Date: 1996-12-02T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <dewar.849318953@merv>, dewar@merv.cs.nyu.edu (Robert Dewar) writes:
> Wiljan says
> 
> "I do not agree that this is the obvious translation.
> It depends on how the code is original written. In many cases you can
> of cause give an abvious translation to Ada but it depends from case to
> case."
> 
> 
> I am confused, you seem to be talking in generalities (which are in fact
> reasonable), but you don't seem to talk to this specific case. Can you
> show an example where you think it is inappropriate to translate the
> Pascal "with" into an Ada renaming?
> 
> Seems to me that the Pascal WITH is *exactly* analogous to the Ada renaming,
> with a little less to write, but with less flexibility.
> 


In the MacOs, you often have structures like a Rect which are

Rect = record
  top, bottom, right, left: integer;
end;

You are typically using WITH when you are calculating with them.  The intent
is to make the source easier to read, and only that.  (Well, compiler might
write more efficent code as well.)

my_rect : Rect;

with my_rect do begin  { may have a scope of many lines }
  top    := top - top_margin;
  right  := left + (3*(bottom - top)) div 4;
  bottom := bottom - bot_margin;
end;

In this case, the proposed solution would not help much.  It would be 
preferable to work as in C: no local block, and all the fields are written
in full.  An extra local variable cannot do any better.

my_rect.top    := my_rect.top - top_margin;
my_rect.right  := my_rect.left + (3*(my_rect.bottom - my_rect.top)) div 4;
my_rect.bottom := my_rect.bottom - bot_margin;

This is the case every time the structured type is directly the type
of the variable at work.  No array of, pointer to, or field in a record 
of the structured type.  The Ada renaming is quite effective when the
structured type is deeply nested in the variable call.

-- 
Laurent Gasser (lga@sma.ch)
Computers do not solve problems, they execute solutions.