Re: Is there a language that Dijkstra liked? (was: Re: Software landmines (loops))

["Pat Rogers" <progers@NOclasswideSPAM.com>]

John Woodruff wrote in message ...
>>>>>> "Pat" == Pat Rogers <progers@NOclasswideSPAM.com> writes:
>In article <712r19$rs5$1@supernews.com> "Pat Rogers"
<progers@NOclasswideSPAM.com> writes:
>
>    > Ehud Lamm wrote in message ...
>    >> On Mon, 26 Oct 1998 dennison@telepath.com wrote:
>
>    > <good points all>
>
>    >> Still - Remember you can achieve all the run time checking
>    > functionality
>    >> in any language. It is just that in some languages you have
to
>    >> code
>    > it
>    >> explicitly. But you want your code to be of quallity - you
just
>    > have to do
>    >> it.
>
>
>    > Explicitly coded checks come at a price, though, that
>    > language-defined checks may be able to avoid.  ......
>
>Another consideration occurs when the programmer offers to code his
>(her) own checks, in defense against the kind of errors that Ada's
>checking prevents:
>
>    > ...... In Ada, the fact
>    > that the check is defined by the language means that we don't
>    > explicitly write the it ourselves, and the optimizer then has
>    > freedom to help us with performance.  Ironic, isn't it?
>
>The programmer is setting out to write additional code, and that
code
>itself is susceptible to some defects.  Shouldn't we worry that
these
>defects injected into the *checking* code might lower the quality
of the
>product?

Yes, any "extra" code is unfortunate, since it is just that much
more that can go wrong.

One does have to be careful, though.  Consider:

  function Next( This : Some_Discrete_Type ) return
Some_Discrete_Type is
  begin
    return Some_Discrete_Type'Succ(This);
  exception
    when Constraint_Error =>
      return Some_Discrete_Type'First;
  end Next;

Although the language will check that we don't "go off the end of
the world" when This is Some_Discrete_Type'Last, if some poor
maintenance programmer comes along and suppresses the check we are
in for Trouble.  In such cases the explicit, hand-coded check is
preferable, IMHO:

  function Next( This : Some_Discrete_Type ) return
Some_Discrete_Type is
  begin
    if This = Some_Discrete_Type'Last then
      return Some_Discrete_Type'First;
    else
      return  Some_Discrete_Type'Succ(This);
    end if;
  end Next;