Re: AQ&S Guidance on pragma Elaborate_Body

[eachus@spectre.mitre.org (Robert I. Eachus)]

In article <336089AF.6046@elca-matrix.ch> Mats Weber <Mats.Weber@elca-matrix.ch> writes:

  > IMO, state of the art belongs in the standard, not in nice compiler
  > specific features, which would have allowed the standard to take a
  > different approach: automatic elaboration order consturction with
  > pragmas to _override_ it, instead of pragmas to do the right thing in
  > the most basic cases.

    No.  State of the art should not be outlawed, and both Ada 83 and
Ada 95 went along with that.  But state of the art also argues for
"letting a milion flowers bloom," until one prefered method emerges as
"the state of the practice."

    There are Ada programs where not only is finding a workable
elaboration order hard--in the mathematical sense--but a property of
the program.  Right now, the program designer has access to
information not visible in the program, and based on that information
there is a working elaboration order.  But...

    Let me give you a true life example.  The system archictecture has
a design philosophy where each level in the architect only depends on
state in a lower level.  But for implementation reasons, lower level
routines call upper level routines all over the place.  As long as
elaboration is done in the right order, from lowest level to highest,
there are no ABE problems.  But assigning programs to levels is not
something the compiler can easily do.

    Sound like an extreme case?  Sorry, most graphical interfaces are
structured this way.  Accidently, breaking those ordering rules
becomes an obvious bug at run-time.  In Ada, using the
extra-linguistic knowledge to do elaboration means that, if it
compiles, you won't have ABE problems.

    Since the Ada model is so much easier to life with than the model
in some other languages, I'm willing to put up with the minor
irritation to allow these programs to be written.  (The problem when
you find a circular elaboration dependency and have to fix it is not
minor.  But in Ada you haven't spent months tracking it down.)


					Robert I. Eachus

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					Robert I. Eachus

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