Re: Idea for Ada 200x: Arguments that are procedures

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

In article <swhalenEvJBCC.6Er@netcom.com> swhalen@netcom.com (Steve Whalen) writes:

  > Also interesting is the thread you attached, especially the note that
  > this is the only thing that keeps Ada95 from being a proper superset
  > of Pascal! I guess that's one reason that Ada95's lack of downward
  > closures feels so "wrong" to me.

  Ada 83 and Ada 95 both support downward closures through use of
generics.  A few years ago I wrote some test programs not only to
verify that the language rules permitted it, but that the
implementations that used displays and/or shared generics got it
right.  At that time I talked to the implementors, in particular R&R
about how they implemented the feature, and found that, with a generic
instantiation, there is enough of a flag that something unusual is
happening that they didn't have to save and store contexts on every
procedure call, just when a generic is instantiated in an inner scope,
and the instantiation has a subprogram parameter, and that parameter
is from a nested scope.  Since the feature is "in there" for when it
is needed, I saw no reason to continue the fight for a simpler syntax.
(Jean Pierre Rosen was the first AFAIK to point out in an Ada 9X
context that generics could be used to implement downward closures, at
the meeting in Switzerland.  But there were Ada 83 validation tests
that required that it work there, so all compilers I tested
implemented it correctly.)
--

					Robert I. Eachus

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