for / while else doesn't make sense

Rustom Mody rustompmody at gmail.com
Tue Jun 14 23:38:03 EDT 2016


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:
> 
> > On Tuesday, June 14, 2016 at 8:13:53 AM UTC+5:30, Steven D'Aprano wrote:
> >> No. The sun exploding was me gently mocking you for your comment
> >> disputing the "unconditional" part. Yes, you are technically right that
> >> technically the "else" block will only run if no "break" is reached, and
> >> no "return" is reached, no exception is raised, also that os._exit or
> >> os.abort aren't called, the CPU doesn't catch fire, and the world isn't
> >> destroyed.
> >> 
> >> If we try to enumerate all the things which could prevent the "else"
> >> block from running, we'll be here for decades. But, and this is the point
> >> that everyone seems to have missed, * every single one of those things*
> >> is completely independent of the for...else statement.
> >> 
> >> *Including* the presence or absence of a "break".
> > 
> > This is real wild:  A break that is inside a for is independent of the
> > for?!?!
> 
> If that's what I said, you would be right to question me. But that's not
> what I said.
> 
> It is legal syntax to have for...else without a break, or a break inside a
> for block with no else. And, if you really want to nitpick, you can even
> have a break statement without a for. (Just stick it inside a while loop
> instead.)
> 
> I know that's it's great fun to pick at nits without making a good faith
> effort to communicate, but honestly Rustom, your following comments do
> suggest that you understood what I was saying.
> 
> 
> [...]
> > This *desire* for what you call isolation is a standard tenet of
> > semantics and is right
> > 
> > It is called compositionality
> > 
> > Inn programming:
> > https://en.wikipedia.org/wiki/Denotational_semantics#Compositionality
> > 
> > More general linguistics:
> > https://en.wikipedia.org/wiki/Principle_of_compositionality
> 
> Thanks for that.
> 
> 
> > However gotos break compositionality unless one introduces heavy artillery
> > like continuations
> > 
> > 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.
> 
> You can reason about for...else without the break, then reason about what
> the break does. This isn't hard, and its what people do even in the common
> use-case:
> 
> for x in seq:
>     process(x)
>     if condition:
>         break
> else:
>     fnord()
> spam()

Yon need to take an example of the
if condition: break
nested inside some more ifs
with those other conditions giving validity to the condition
eg outer condition being say y != 0
Inner if being 
if x/y == 0 : break

Now you would see that your reasoning about the inner needs potentially the 
FULL CONTEXT  of the outer.
This need to carry large context is the essential property of non-compositional semantics.


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