asterisk users - Jul 2009 - ExecIf and empty variables (early evaluation)

Imagine that you have this code:

exten => _X!,n,Set(foo=${QUEUE_WAITING_COUNT(${QueueName})}))

If ${QueueName} happens to be unset, this will cause a warning:

[Jul 22 14:26:17] ERROR[8114]: app_queue.c:5187
queue_function_queuewaitingcount: QUEUE_WAITING_COUNT requires an
argument: queuename

The obvious solution:

exten => _X!,n,ExecIf($["${QueueName}" !=
""]?Set(foo=${QUEUE_WAITING_COUNT(${QueueName})}))

However, this doesn't actually work! Functions and variables on the
right hand side are evaluated BEFORE it is decided whether it needs to
be executed at all!

This is fairly harmless in this case, in that it just causes a warning.
However, what about this case?

exten => _X!,n,ExecIf($[${bar} < 10]?Set(foo=${INC(bar)}))

Which you can argue that this code is in poor taste, it is definitely
surprising that INC is evaluated in this case, changing ${bar} even if
${bar} >= 10.

It probably isn't possible to do something about this, but now you have
all been warned... This could be a good reason for avoiding side effects
in functions, and thereby banning ${INC()}


/Benny

You should submit this as a bug.  It may or may not get fixed, but it
definitely won't until someone reports it or takes it upon themselves to fix
it.  

-----Original Message-----
From: asterisk-users-bounces at lists.digium.com
[mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of Benny Amorsen
Sent: Wednesday, July 22, 2009 7:44 AM
To: asterisk-users at lists.digium.com
Subject: [asterisk-users] ExecIf and empty variables (early evaluation)

Imagine that you have this code:

exten => _X!,n,Set(foo=${QUEUE_WAITING_COUNT(${QueueName})}))

If ${QueueName} happens to be unset, this will cause a warning:

[Jul 22 14:26:17] ERROR[8114]: app_queue.c:5187
queue_function_queuewaitingcount: QUEUE_WAITING_COUNT requires an
argument: queuename

The obvious solution:

exten => _X!,n,ExecIf($["${QueueName}"
!""]?Set(foo=${QUEUE_WAITING_COUNT(${QueueName})}))

However, this doesn't actually work! Functions and variables on the
right hand side are evaluated BEFORE it is decided whether it needs to
be executed at all!

This is fairly harmless in this case, in that it just causes a warning.
However, what about this case?

exten => _X!,n,ExecIf($[${bar} < 10]?Set(foo=${INC(bar)}))

Which you can argue that this code is in poor taste, it is definitely
surprising that INC is evaluated in this case, changing ${bar} even if
${bar} >= 10.

It probably isn't possible to do something about this, but now you have
all been warned... This could be a good reason for avoiding side effects
in functions, and thereby banning ${INC()}


/Benny



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Benny Amorsen schrieb:
> Imagine that you have this code:
> 
> exten => _X!,n,Set(foo=${QUEUE_WAITING_COUNT(${QueueName})}))
> 
> If ${QueueName} happens to be unset, this will cause a warning:
> 
> [Jul 22 14:26:17] ERROR[8114]: app_queue.c:5187
> queue_function_queuewaitingcount: QUEUE_WAITING_COUNT requires an
> argument: queuename
> 
> The obvious solution:
> 
> exten => _X!,n,ExecIf($["${QueueName}" !=
""]?Set(foo=${QUEUE_WAITING_COUNT(${QueueName})}))
> 
> However, this doesn't actually work! Functions and variables on the
> right hand side are evaluated BEFORE it is decided whether it needs to
> be executed at all!
> 
> This is fairly harmless in this case, in that it just causes a warning.
You could split it up into multiple statements:
if ("${QueueName}" != "") {
	Set(foo=${QUEUE_WAITING_COUNT(${QueueName})});
} else {
	Set(foo=-1);  // or whatever
}
(don't remember how to write that in extensions.conf format)

Pros:
- conditional evaluation
- more readable (ExecIf() looks ugly)

Cons:
- more statements
- less readable then a ternary conditional expression in "real"
  programming languages:
  foo = ($queuename != "" ? queue_waiting_count($queuename) : -1)
> However, what about this case?
> 
> exten => _X!,n,ExecIf($[${bar} < 10]?Set(foo=${INC(bar)}))
> 
> Which you can argue that this code is in poor taste, it is definitely
> surprising that INC is evaluated in this case, changing ${bar} even if
> ${bar} >= 10.
> 
> It probably isn't possible to do something about this, but now you have
> all been warned... This could be a good reason for avoiding side effects
> in functions, and thereby banning ${INC()}
Ban ExecIf(). Use AEL. Use if(){} blocks. :-)
In order to use control structures like "if .. else"/"switch ..
case"
it's almost necessary to write your dialplan in AEL because the same
thing is so incredibly hard to read and write in extensions.conf
format (GotoIf()).
Although very basic dialplans look good in extensions.conf my
suggestion is to use AEL and let the AEL compiler figure out how to
translate that into an Asterisk dialplan.


    Philipp Kempgen
-- 
AMOOMA GmbH - Bachstr. 126 - 56566 Neuwied  ->  http://www.amooma.de
Gesch?ftsf?hrer: Stefan Wintermeyer, Handelsregister: Neuwied B14998
Asterisk: http://the-asterisk-book.com - http://das-asterisk-buch.de
Videos of the AMOOCON VoIP conference 2009 ->  http://www.amoocon.de
--

Benny Amorsen wrote:
> Imagine that you have this code:
> 
> exten => _X!,n,Set(foo=${QUEUE_WAITING_COUNT(${QueueName})}))
> 
> If ${QueueName} happens to be unset, this will cause a warning:
> 
> [Jul 22 14:26:17] ERROR[8114]: app_queue.c:5187
> queue_function_queuewaitingcount: QUEUE_WAITING_COUNT requires an
> argument: queuename
> 
> The obvious solution:
> 
> exten => _X!,n,ExecIf($["${QueueName}" !=
""]?Set(foo=${QUEUE_WAITING_COUNT(${QueueName})}))
> 
> However, this doesn't actually work! Functions and variables on the
> right hand side are evaluated BEFORE it is decided whether it needs to
> be executed at all!
Try this, as I think the IF() function works differently (I could be wrong
though):

exten => _X!,n,Exec(${IF($["${QueueName}" != 
""]?Set(foo=${QUEUE_WAITING_COUNT(${QueueName})}:NoOp())})

Leif Madsen.
http://www.leifmadsen.com
http://www.oreilly.com/catalog/asterisk