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Re: Curious case of arithmetic expansion


From: Steve Amerige
Subject: Re: Curious case of arithmetic expansion
Date: Mon, 24 Apr 2017 04:18:20 -0400
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

It is essential to not break backward compatibility. Imagine the huge number of scripts that would be impacted by the semantic shift you're suggesting. If a proposal were to be made that would cause backward incompatibility, then any such proposal should be rejected unless there are truly compelling reasons for the proposal. Now, if you're proposing some new shopt (Bash-specific) or set -o (a broader set of sh shells) that would deliver some alternative behavior, then at least you wouldn't be breaking backward compatibility. However, any such addition would complicate the test matrix for shell behavior and should again be subject to very strong scrutiny and strong community desire for such a feature. I'm not sensing that this is the case for what you're describing.

Best regards,
Steve Amerige
Eggsh: A Bash-Scripting Platform
https://eggsh.com

On 4/24/2017 3:59 AM, Florian Mayer wrote:
Ok, I accept your points, but please read on and decide after that.

to do anything but assign a value to `var'.  Very few people, when asked,
would say that it were more intuitive to cause a variable named `bar' to
spring into existence with the value 7. If you want nameref behavior, you
have to explicitly declare it.
But why is nameref behavior in the evaluation part activated by default
and not for the assignments? I think that one should
either make nameref behavior the default case for both, 
assignment and evaluation, or the non-default case for both.

In the latter case, where namerefs are off for both things, the lines (1) and (2)
1) var=bar; bar=3; echo ((var))
2) var=bar; ((var=3))   or var=bar; bar=0; ((var++))
should lead to an error message!
However the current status quo is, that bash happily does (1) as you’d 
expect but gives out no error in (2) and overwrites (1).

That is variable indirection. It has superficially similar effects to what
we are discussing, which is arithmetic expansion; however, it has nothing
to do with the behavior of (( )) or $(( )).
Again, I never wrote, that, from a technical point of view, it has something to do
with (()) or $(()). Merely the visible behavior is similar in the cases I showed.

Am 24.04.2017 um 01:43 schrieb Chet Ramey <address@hidden>:

On 4/23/17 4:25 PM, Florian Mayer wrote:
That's not a reasonable expectation.
Why not? Why is it not reasonable to expect an intuitive
result from (())? The most intuitive thing, in my opinion,
would be to use nameref for side effects by default, because in order
to get a value from an id, (()) already follows namerefs.

The thing that makes that result intuitive for you is your opinion about
how things should work.  That's fine. I don't happen to share your opinion
of what is "intuitive" in this case.

However, it is unreasonable to expect

var=bar
(( var=7 ))

to do anything but assign a value to `var'.  Very few people, when asked,
would say that it were more intuitive to cause a variable named `bar' to
spring into existence with the value 7. If you want nameref behavior, you
have to explicitly declare it.


It's not indirection, and I am not sure why you show the completely
I was mentioning that, not because I use „a mental model“ that falsely unifies
both things, but because I wanted to point out that there exists something
with a similar behavior.

And if ${!<varid>} does not portray some kind of indirection, what do you
call it then?

That is variable indirection. It has superficially similar effects to what
we are discussing, which is arithmetic expansion; however, it has nothing
to do with the behavior of (( )) or $(( )).



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