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From: vsnyder@math.jpl.nasa.gov (Van Snyder)
Subject: Modest proposal, 1 of 3
Date: 1996/11/21
Message-ID: <572q86$g7m@netline-fddi.jpl.nasa.gov>#1/1
X-Deja-AN: 198010338
sender: vsnyder@math.jpl.nasa.gov (Van Snyder)
content-type: text/plain; charset=US-ASCII
organization: Jet Propulsion Laboratory
mime-version: 1.0
newsgroups: comp.lang.ada
Date: 1996-11-21T00:00:00+00:00
List-Id: <comp.lang.ada>


BACKGROUND

Suppose procedure A contains B, but not C and D.  Suppose furthermore
that an access-to-procedure type T that is compatible with B is
visible to all three.  Notwithstanding the visibility, A is not
able to pass access-to-B to C as an argument.

The reason is that C might store its formal argument of type access-to-
procedure into a (relatively) global variable, and then D (or somebody
else) could use it to call B after its up-level environment (A's
local variables) disappears.

The crucial problem is the ability of C to store access-to-B into
a more global variable.  Absent that ability, it's a trivial exercise
in induction to show that B can only be invoked when its up-level
exists.

PROPOSAL for the next standardization of Ada

Allow the "limited" keyword as an argument-passing mode, similar to
"in".  The interpretation is that arguments of mode "limited" can only be used
to access something, or be bound as an actual argument to a formal argument of
"limited" mode.  In particular, as for other "limited" objects, assignment is
not defined.

Thus, one cannot store a copy of the argument into a more global
variable.

Then, allow access-to-procedure constants to be bound as actual
arguments to formal arguments of the appropriate type, but only
so long as the formal argument is of "limited" mode.
-- 
What fraction of Americans believe   |  Van Snyder
Wrestling is real and NASA is fake?  |  vsnyder@math.jpl.nasa.gov