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From: hal@newton.physics.purdue.edu.UUCP
Newsgroups: comp.lang.modula2
Subject: Re: Modula2 for Apollos and  Ada,Modula2 question(s)
Message-ID: <824@newton.physics.purdue.edu>
Date: Tue, 29-Sep-87 08:22:17 EDT
Article-I.D.: newton.824
Posted: Tue Sep 29 08:22:17 1987
Date-Received: Fri, 2-Oct-87 03:45:44 EDT
References: <281@mv03.ecf.toronto.edu> <440@trwspf.TRW.COM>
Reply-To: hal@newton.physics.purdue.edu.UUCP (Hal Chambers)
Organization: Purdue Univ. Physics Dept., W. Lafayette, IN
Lines: 26
Keywords: Modula2  Ada
Summary: Opaque/Private Types

In article <440@trwspf.TRW.COM> thomsen@trwspf.UUCP (Mark Thomsen) writes:
>In article <281@mv03.ecf.toronto.edu> apollo@ecf.toronto.edu (Vince Pugliese) writes:
>....
>....  The only features of Modula-2 that
>are not in Ada in equivalent form are procedure variables, sets, and opaque
>types (someone challenge this if you disagree -- I would be very interested
>in updating my study of this)....
>
>                                         Mark R. Thomsen

The Ada terminology for opaque types is "private" types.  Also private types
are not restricted as to structure but can be any programmer defined type.
This requires the complete definition of the type appear in the definition
part of a package but this is for the use of the compiler only; the user
cannot reference any components of the private type.  (This opens a whole
"can of worms" from the software engineering standpoint since changing the
implementation may now require changing the definition part).
The package programmer even has control of whether the default definition
of equality "=" is defined on the type ("restricted private" types).

Regarding the virtues of the two languages:
	I have access to a Modula-2 compiler but no access to an Ada
	compiler.  Any possible theoretical advantage of a language
	I can't USE is rather irrelevant to me.

Hal Chambers