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From: phil.clayton@lineone.net
Newsgroups: comp.lang.ada
Subject: Re: Ada to C translator for small microcontrollers
Date: Fri, 30 Mar 2012 04:54:08 -0700 (PDT)
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Date: 2012-03-30T04:54:08-07:00
List-Id: <comp.lang.ada>

On Wednesday, March 28, 2012 10:03:14 PM UTC+1, Georg Bauhaus wrote:
> I do not know (yet) why a no-op in the language is a must
> in a theory-friendly programming language, I have seen that
> it is. Ada has it, in any case.

At a practical level, without null, using rules to reason about programs can become tedious.  For example, null statements allow us to write the rule

  if C
  then
    P
  else
    null;
  end if;
=
  if C
  then
    P
  end if;

where C is a boolean expression and P is any program statement.  

That is useful because no other rules then need to deal with an implicit else null branch.  Without null statements and the above rule, we would then need four versions of the following rule, for all combinations of omitting the else branch:

  if C
  then
    if C
    then
      P
    else
      Q
  else
    R
  end if;
=
  if C
  then
    P
  else
    R
  end if;

At a more abstract level, a null statement may be needed so that sequential composition has an identity to appeal to some other theoretical result.