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Subject: Re: Interesting article on ARG work
From: "Dan'l Miller" <optikos@verizon.net>
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Xref: reader02.eternal-september.org comp.lang.ada:51393
Date: 2018-04-07T17:14:04-07:00
List-Id: <comp.lang.ada>

On Saturday, April 7, 2018 at 11:00:02 AM UTC-5, Dmitry A. Kazakov wrote:
> On 2018-04-07 17:27, Dan'l Miller wrote:
> > On Saturday, April 7, 2018 at 5:11:35 AM UTC-5, Dmitry A. Kazakov wrote=
:
> >> On 2018-04-07 04:25, Randy Brukardt wrote:
> >>
> >>> The biggest problem with a lexical solution (and the reason that Ichb=
iah
> >>> hated them) is that it's trivial to accidentally create a program tha=
t is
> >>> only legal in one mode or the other. One has to continually compile
> >>> everything in each possible mode to ensure that there aren't silly sy=
ntax
> >>> errors involved. A common mistake that I make all of the time is:
> >>>
> >>>       if Foo then
> >>>              Bar;
> >>>       else
> >>> @        Internal_Error;
> >>>       end if;
> >>>
> >>> With the conditional compilation off, the "Internal_Error" is comment=
ed out
> >>> and the above isn't syntactically legal.
> >>
> >> Yes, this is important but I think this could be resolved, e.g. by
> >> compiling conditionals into corresponding null-effect constructs.
> >=20
> > Yeah, right.  This is an artificially contrived =E2=80=9Cproblem=E2=80=
=9D that is in fact not extant in reality.  Simply put the @ symbols in fro=
nt of the =E2=80=A2entire=E2=80=A2 construct to elide:  the entire else bra=
nch.
> >=20
> >      if Foo then
> >             Bar;
> > @ else
> > @        Internal_Error;
> >      endif;
> >=20
>=20
> "@ else"
> "@        Internal_Error;"
>=20
> Is not a valid sequence of statements.

That is a non sequitur, Dmitry.=20

     if Foo then=20
           Bar;=20
@ else=20
@        Internal_Error;=20
     endif;

produces either this legal Ada if-else statement, where =E2=90=A0 is the AS=
CII/ISO646 space character:

     if Foo then=20
           Bar;=20
=E2=90=A0=E2=90=A0 else=20
=E2=90=A0=E2=90=A0       Internal_Error;=20
     endif;

or this legal Ada if statement:

     if Foo then=20
           Bar;=20
-- else=20
--       Internal_Error;=20
     endif;

Compare & contrast, and then you will understand what Randy was saying.