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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Software landmines (loops)
Date: 1998/10/09
Message-ID: <m34stewqai.fsf@mheaney.ni.net>#1/1
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NNTP-Posting-Date: Fri, 09 Oct 1998 00:01:39 PDT
Newsgroups: comp.lang.eiffel,comp.object,comp.software-eng,comp.lang.ada
Date: 1998-10-09T00:00:00+00:00
List-Id: <comp.lang.ada>

"Robert C. Martin" <rmartin@oma.com> writes:

> The question that the post was addressing had to do with the definition of
> structured programming.  A mid exit loop does not fall within the bounds
> defined by structured programming because the loop body does not then have a
> single entry and a single exit.
> 
> This has nothing to do with whether mid exit loops are good or bad.  It only
> has to do with whether mid exit loops conform to structured programming or
> not.

The specific text of what Dijkstra said is (from section 7, "On
Understanding Programs"):

<<These flowcharts share the property that they have a single entry at
the top and a single exit at the bottom: as indicated by the dotted
block they can again be interpreted (by disregarding what is inside the
dotted lines) as a single action in a sequential computation.>> [p. 19]

He says later that:

<<Alternatively: restricting ourselves to the three mentioned types of
decomposition leads to flowcharts of a restricted topology compared with
the flowcharts one can make when arrows can be drawn from any block
leading into any other.>> [p. 20]

I think the last part is very significant.  As Bob Eachus pointed out, a
subprogram with an early return really does have "single exit," because
all returns lead to the same place, to the POINT OF CALL.  

Dijkstra's complaint was that you can't tell what the program does at
run-time by reading the program text.  One cause is the unbridled use of
the goto statement, when you can jump out of a sequential abstraction
(the block to which Dijkstra refers) into any other.

By returning early from a subprogram, you are not "jumping into another
abstraction."  You are merely tracing another edge that leads to the
SAME NODE (the point of call) in the graph of the program.

He sums up that section by noting that:

<<The moral of the story is that when we acknowledge our duty to control
the computations (intellectually!) via the program text evoking them,
that then we should restrict ourselves in all humility to the most
systematic sequencing mechanisms, ensuring that "progress through the
computation" is mapped on "progress through the text" in the most
straightforward manner.>> [p. 23]

I argue that an early return (appropriately used, of course) meets his
criterion of understanding the mapping of static program text to dynamic
program behavior.  

The whole point of structured programming is NOT to "use only these
three control flow constructs."  It is as he states in that last summary
paragraph: that programs should be intellectually manageable.  If a
mid-loop exit simplifies a loop, or an early return simplifies a 
subprogram, then by all means use it.