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From: rogoff@sccm.Stanford.EDU (Brian Rogoff)
Subject: Re: Q: access to subprogram
Date: 1996/07/08
Message-ID: <ROGOFF.96Jul8155205@sccm.Stanford.EDU>#1/1
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reply-to: rogoff@sccm.stanford.edu
newsgroups: comp.lang.ada
Date: 1996-07-08T00:00:00+00:00
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ncohen@watson.ibm.com (Norman H. Cohen) writes:

   (By the way, I share Bob Duff's amazement at the noninclusion of downward
   closures--we called them downward at the time--in Ada 95, and I say this
   as one of those Bob described as urging the removal rather than the
   addition of proposed Ada-95 features.  I viewed the inclusion of downward
   closures as the REMOVAL of an arbitrary restriction.  The decision was
   not made in ignorance.  Bill Taylor had made what I considered an
   irrefutable case for downward closures, showing how much easier it would
   be to write iterators if downward closures were allowed.  It came down to
   a conflict between the interests of Ada programmers and the interests of
   a minority of Ada implementors, and in this case the interests of the few
   implementors using displays prevailed.)

Could you summarize or provide a reference to Bill Taylor's work? Does it 
answer the objection that downward closures impose an inefficiency on programs 
that don't use them?

I like closures myself, but I am thinking of the full (upward I guess you'd 
call them) closures of Lisp, which require garbage collection. Perhaps a 
"syntactically upwards compatible" Ada dialect supporting closures should be 
considered, especially since the Java virtual machine already imposes GC on 
any language that runs on it, and it will be a popular target.

-- Brian