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From: hbaker@netcom.com (Henry Baker)
Subject: Re: Depending on passing mechanism
Date: 1997/10/20
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Date: 1997-10-20T00:00:00+00:00
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In article <62ckao$827@mulga.cs.mu.OZ.AU>, fjh@mundook.cs.mu.OZ.AU (Fergus
Henderson) wrote:

> Well, I'd go for unique modes, rather than unique types.
> In addition to "in", "out", and "inout", add
> "unique_in", "destructive_in", "unique_out", and "unique_inout",
> with semantics similar to the unique modes in Mercury.

This points up a 'meta' level problem with Ada83, which wasn't really fixed
in Ada95: Ada separated the issues of parameter-passing 'modes' from
'types'.  Unfortunately, the author of a private/limited type wants to
control the way it is passed and returned, and allowing the user of the
type to make this choice is asking for trouble.  I seem to recall one of
Tucker Taft's early proposals in which a solution to this problem was
attempted.  Apparently, this proposal got soundly blasted, and it sank
without much of a trace.  Pity.