for / while else doesn't make sense

[Steven D'Aprano]

On Wed, 15 Jun 2016 09:19 pm, Rustom Mody wrote:

> On Wednesday, June 15, 2016 at 8:42:33 AM UTC+5:30, Steven D'Aprano wrote:
>> On Wed, 15 Jun 2016 01:33 am, Rustom Mody wrote:
[...]
>> > And break is a euphemism for goto
>> 
>> Sort of. A break is a jump, and a goto is a jump, but apart from that,
>> they're not really the same thing. A goto can jump (almost) anywhere.
>> Depending on the language, they can jump into the middle of functions, or
>> into the middle of loops. That's what makes them powerful enough to break
>> compositionality. But break can only jump to a single place: to the
>> statement that follows the for...else compound statement. It's more like
>> a return than a goto.
> 
> I thought there'd be many examples for showing that break is just goto in
> disguise... Evidently not
> 
> So here is an example in more detail for why/how   break=goto:
> 
> http://blog.languager.org/2016/06/break-is-goto-in-disguise.html

Quote: "For now lets just agree that break is not very different from goto"

Let's not.

Your argument seems to be:

"I can use GOTO to jump outside of a loop. I can use BREAK to jump outside
of a loop. Careless use of jumping out of a loop leads to a bug, regardless
of whether that jump is written as GOTO or BREAK. Therefore GOTO and BREAK
are the same thing."

but you have failed to consider:

"I can use GOTO to jump inside a loop. I *cannot* use BREAK to jump inside a
loop. I can use GOTO to jump back to before the loop. I *cannot* use BREAK
to jump back before the loop. In the most general case of completely
unstructured programming, I can use GOTO to jump I can use GOTO to jump out
of one function into the middle of another function, or any other arbitrary
line of code anywhere in my program. I *cannot* do the same with BREAK."


If you only consider the similarities, and not the differences, you would
erroneously conclude that all sorts of things that are actually quite
different are "similar".

"Cars are actually not so different from the Space Shuttle. Here's video of
the shuttle taxiing down the runway after returning from space. Here's a
video of a car driving along a road. Careless driving can run over
pedestrians, regardless of whether you are driving a car or taxiing a space
shuttle. Therefore cars and space shuttles are not that different."

Congratulations, you have discovered that mechanically replacing careless
use of GOTO with careless use of BREAK doesn't necessarily make the code
less buggy. That's a valuable lesson for cargo-cult programmers who think
that its the *word* "goto" that causes bad things to happen, and changing
the word magically will fix the bug. But to the rest of us, it's about as
insightful as the discovery that mechanically replacing "x*5"
with "x+x+x+x+x" won't fix the off-by-one error that you actually
needed "x*6".



-- 
Steven