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Date: 10 Apr 93 09:39:07 GMT
From: munnari.oz.au!yoyo.aarnet.edu.au!news.adelaide.edu.au!usenet@tcgould.tn.c
ornell.edu  (Andrew Dunstan,,2285592,)
Subject: Re: and then
Message-ID: <1q64jr$o39@huon.itd.adelaide.edu.au>
List-Id: <comp.lang.ada>

>>From article <1993Apr9.180808.19494@scf.loral.com>, by bashford@srs.loral.com 
(Dave Bashford):
> In article <1q29bd$7d8@huon.itd.adelaide.edu.au> andrewd@cs.adelaide.edu.au w
rites:
>>>From article <1993Apr7.162133.3564@nosc.mil>, by sampson@nosc.mil (Charles H.
 Sampson):
>>
>>
>>There could well be cases where evaluation of both operands is wanted
>>(if evaluation of one or both has a desired side-effect). If so,
>>"optimising" evaluation of one away is highly undesirable.
> 
> I can't believe my eyes - "desired side-effect" in Ada ? Several people
> have mentioned side-effects as a reason not to optimize boolean
> expressions.  I thought one of the guiding principles of good s/w
> engineering was to minimize or eliminate side-effects. The only reason
> I can think of not to optimize to short circuit forms is to catch
> undesired side-effects.
> 
> Are there "desired side-effects" ?
> 

Certainly. One classic example should suffice. Since Ada does not have
"own" variables, a random number generator function in Ada will
normally have the desired side-effect of changing its seed. 

Of this would best be done by hiding the seed in a package, where
nothing but the function could change the seed, but that is another
issue. Ada is NOT a functional language - it is procedural, and
side-effects are common. 

None of this is to deny that over-use of side effects is bad practice.
It is, and as a teacher I have tried to encourage students to avoid
side-effects wherever possible. (Note the qualification).

Interestingly, functions in protected objects in Ada9X are to be
prohibited from having side-effects which alter the protected data
(thus permitting the optimisation of allowing several such functions
to be active at one time).

#  Andrew Dunstan                   #   There's nothing good or bad   #
#  net:                             #                                 #
#    adunstan@steptoe.adl.csa.oz.au #   but thinking makes it so.     #
#  or: andrewd@cs.adelaide.edu.au   #                                 #