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From: rav@goanna.cs.rmit.EDU.AU (++           robin)
Subject: Re: next "big" language?? (disagree)
Date: 1996/06/11
Message-ID: <4pj8p7$h9r@goanna.cs.rmit.EDU.AU>#1/1
X-Deja-AN: 159599603
references: <4p1l65$35qi@info4.rus.uni-stuttgart.de>
 <4p60nk$imd@euas20.eua.ericsson.se> <4p8lmq$oq7@goanna.cs.rmit.edu.au>
organization: Comp Sci, RMIT, Melbourne, Australia
newsgroups: 
 comp.lang.pascal,comp.lang.c,comp.lang.misc,comp.lang.pl1,comp.lang.ada
nntp-posting-user: rav
Date: 1996-06-11T00:00:00+00:00
List-Id: <comp.lang.ada>


	ok@goanna.cs.rmit.edu.au (Richard A. O'Keefe) writes:

	>euahjn@eua.ericsson.se (Henrik Johansson) writes:

	>>*But* Ada lacks the general
	>>assertion expression found in other languages like Eiffel (preconditions,
	>>postconditions, assertions).

	>	subtype Assertion is Boolean range True .. True;
	>	...
	>	X_Is_Even: Assertion := X mod 2 = 0;

	>This will raise a constraint exception if the assertion is false.
	>Looks like an assertion to me.

---What is the point of this?

   A declaration buried somewhere else, and a test.

   Why not something in clear english, which doesn't need a
reference to some other part of the program, like:

   if mod(x, 2) ^= 0 then
	put ('The value of x is not odd.');

This has the aditional benefit that the user of the program
has a clear unequivocal message about the cause of the problem.
If it's important that the whereabouts of this test be known,
the programmer can include a statement such as "signal error;"
or "signal condition (xyz);" or "put (' at line ', sourceline );"
along with the put statement. [PL/I example]