[SR-Dev] new operators & if($v) behaviour

[Miklos Tirpak]

Hi Andrei,

On 04/28/2009 10:33 AM, Andrei Pelinescu-Onciul wrote:
> On Apr 28, 2009 at 09:53, Miklos Tirpak <miklos at iptel.org> wrote:
>> On 04/27/2009 04:18 PM, Juha Heinanen wrote:
>>> Andrei Pelinescu-Onciul writes:
>>>
>>>> We could try some sane defaults (for if ($v)).
>>> yes, in case there are some.
>>>
>>>> For example that's what perl does:
>>>>             undefined   ""      string  0       other integer
>>>>
>>>> $foo eq undef   true    true    false   false   false
>>>> $foo == undef   true    true    true    true    false
>>>>
>>>> $foo eq ""      true    true    false   false   false
>>> if $foo is undefined, it is insane to me that it would be equal to empty
>>> string "" or 0.  also comparing two undefined things should not result
>>> in true.
> 
> 
> Actually if you think in terms of the operators it's not. The operators
> forces conversion of its operands to the type it expects.
> so for eq (which used for string): $foo eq undef is in fact
>  (str) $foo  eq (str) undef , and (str) undef is "".
> 
> 
> With undefined you have 2 options: either have valid conversion to
> string and ints or bail out with an error. Since we cannot stop the
> script or abort(),  even if we report an error we still have to come up 
> with a result.
> 
>> yes, this is really strange indeed. And ("abcd" == undef) is also true 
>> according to the table.
> 
> Bear in mind, that == in this case (perl) is an int operator,
> so "abcd" == undef  is in fact (int) "abcd" == (int) undef and
> (int) "abcd" is 0
> (int) undef is 0
> => 0 == 0.

Yes, I understand, but == was not an int operator in SER so far. So the 
question is whether or not we want to change the script syntax, or shall 
we keep the support for the current scripts.

> 
>>> what would make sense to me is that comparing an undefined thing to
>>> anything that is defined, would result in false.
>> I agree.
> 
> I don't. We have special operators for checking if something is defined
> or not. If you don't use it you want implicit conversion.
> In almost all script languages, if a var is undefined then it's equal to
> "".

OK, this could work also in SR. Let's decide whether or not we introduce 
another operator for string comparisons ('eq'), or stick to '=='.

> 
> 
> If you think some other behaviour is better, please fill all the table
>  and think also about normal operations, e.g. $v int+ $x, $v str+ $x.
> Bear also in mind that we support 2 types: int and str. We don't
> support bool (bool is int) so all logical expressions are evaluated to
> int (so you cannot say bool("a") == true but int("a")==0).
> Also think about how other scripting languages handle that.

What do you think about this? Undef is either 0 or "", and == is used 
both for string and int.

             undefined   ""      string  0       other integer

$foo == undef   true    true    false   true    false
$foo == 0       true    false   false   true    false

(The right value is converted to the type of the left value if 
necessary. So "0" == 0, but "" != 0.)

bool($foo)      false   false   true    false   true
int($foo)       0       0/error I/error 0       I
str($foo)       ""      ""      S       "0"     "I"

0/error, and I/error means that the expression should be evaluated  in 
the 'if' conditions, and in the mathematical expressions slightly 
differently:

if (0 == "") -> false, (0 == "0") would be true.
if (0 == "abcd") -> false
if (1 == "1") -> true

$v + "" -> $v -> I think we have no other choice.
$v + "abcd" -> $v
$v + "1" -> $v + 1

I do not whether or not it's close to any scripting language, feel free 
to tell me if it's a complete mess. But I think it's closer to the 
current SER script language at least.

Miklos

> 
> 
> 
> Andrei