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From: diamond@jit533.swstokyo.dec.com (Norman Diamond)
Newsgroups: comp.lang.eiffel,comp.object
Subject: Re: Functions without side effects (was Old confusion)
Message-ID: <1991Jun5.083241.13145@tkou02.enet.dec.com>
Date: 5 Jun 91 08:32:41 GMT
References: <130242@tut.cis.ohio-state.edu> <1991Jun5.072556.19870@irisa.fr>
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Reply-To: diamond@jit533.enet@tkou02.enet.dec.com (Norman Diamond)
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Organization: Digital Equipment Corporation Japan , Tokyo
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In article <1991Jun5.072556.19870@irisa.fr> jezequel@irisa.fr (Jean-Marc Jezequel) writes:

>Let me summarize even further: It seems that we all agree on:
>1) Functions with side effects are dangerous, and don't follow the
>road towards better software quality
>2) Procedures which return values are often usefull
>
>So why not trying to make a syntatical difference between the two
>concepts (keyword like "pure" for pure functions)?
>I guess it is too late for Eiffel V3, but may be for V4, when we (hope
>to) have tools to make static analysis of programs in order to prove
>correctness of pieces of code (i.e. ensuring postconditions and
>invariants from preconditions and code), and so on.

There are degrees of purity.  Idempotency does not imply an absence of
side effects.  Even a function which is "practically" "pure" should be
able to signal an error handler on occasion, which is likely to cause
side effects in both I/O and in the I/O support routines' knowledge of
the external state.  I think the keywords will have to be on the order
of "idempotent" and other precise descriptions.
--
Norman Diamond       diamond@tkov50.enet.dec.com
If this were the company's opinion, I wouldn't be allowed to post it.
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