I've become persuaded that the current behavior should be kept. It is deducible from operator precedence and, once known, is not the cause of any great confusion.

What will be the effect of the proposed 'fix' on constructions like this?

{
    my $foo = 'ABC-2N322-850103';
    sub prefix () :lvalue {
        substr $foo, 0, index( $foo, '-')
    }
}
[download]

Update: Actually, the code above has a surprise in it, anyway. The result of the index call is kept instead of evaluated a second time:

{
    my $foo = 'ABC-2N322-850103';
    sub prefix () :lvalue {
        substr $foo, 0, index( $foo, '-')
    }
    sub foo () { $foo }
}
print foo, $/;
my $bar = prefix = 'DEFG';
print $bar, $/
print foo, $/;
__END__
output:
ABC-2N322-850103
DEF
DEFG-2N322-850103
[download]

I've become persuaded that the current behavior should be kept. It is deducible from operator precedence and, once known, is not the cause of any great confusion.

Somewhat agree. The existing behaviour is also IMO easier to document clearly than with the proposed change.

But to throw fuel on the fire:

$ perl -wl
$x = "abc";
for $z (substr($x,1,1)) { print ":$z:"; $z = "zz"; print ":$z:" }
__END__
:b:
:z:
[download]

If you alias something to the substr or pass it as a parameter, you can get some pretty odd results currently. OTOH, this is not a lot different from this case:

"abc"=~/.(.)/;
for $z ($1) { print ":$z:"; "zz" =~ /.(.)/; print ":$z:" }
[download]

To reply to your sub :lvalue case, the index shouldn't be kept from one call to prefix() to the next with either the existing or proposed behaviour. However there is a bug (#24200) with substr used in an :lvalue sub. That bug should be fixed whatever else happens. (The bug is that rvalue calls to the sub will not work correctly after the first lvalue call.)

Aside from that, your code is a perfect example of how things can go wrong the way things are. Just saying (prefix = "AB") = "ABC" gets rid of the first '-'; with the proposed change it would leave $foo back in its original state.

Update to respond to Zaxo's update; the "surprise" is exactly the subject of the thread. When you say $foo = substr() = $bar substr returns a magic value that is first stored and then fetched. The "problem" is that both use exactly the same length, even if the store expanded or contracted the string. substr() being replaced by an :lvalue sub is exactly the same case...the sub is called only once but the magic value returned is acted on twice, both times with the originally determined length.

Personally, I consider the current behaviour as broken as I would consider print sqrt( -10 ); printing 3.16227766016838. Except that I can conceive of the situation whereby the latter result might actually be useful. I could see someone relying upon that broken behaviour and skipping the step of detecting negative values and absing them, if that was the requirement of their application. They shouldn't, but I could see it happening.

However, the current resultant of substr as an lvalue, whilst predictable, is so twisted, and has so many edge cases, I find it inconceivable that anyone has actually found a reasonable use for the current behaviour, much less constructed an application that relies upon it.

I have personally encountered the situation (twice at least) where having the value assigned, returned would have slightly simplified my code.

As such, I would consider this change fixing a broken behaviour, rather than modifying an existing, useful one, and if it served to highlight broken code, or reliance upon undocumented, broken behaviour in existing applications, then so be it. Simplistically, if adding 1 + 1 returned 3 and someone coded their app to rely upon that, we wouldn't think twice about not maintaining backwards compatibility.

To my way of thinking, this fits in the same category.

However, the current resultant of substr as an lvalue, whilst predictable, is so twisted, and has so many edge cases, I find it inconceivable that anyone has actually found a reasonable use for the current behaviour, much less constructed an application that relies upon it.

Considering that 4-arg substr is a relative new invention (5.004? 5.005?), there must have been a lot of code that uses the lvalue-ness of substr. substr has never surprised me, but then, I've never used the 3-arg form in both rvalue and lvalue context at the same time, nor have I ever used 4-arg substr as an rvalue.

I'm not really convinced this issue is worth breaking backwards compatibility for. No doubt there is code right now that depends on this behaviour - and if the current behaviour confuses you, don't use it. Write it in two lines.

Abigail

Sorry, but my contention is that there almost certainly isn't code right now that depends upon this behaviour.

The current behaviour is so inherently non-useful, almost unpredictable, that I think it impossible that anyone has found a use for it. Leaving it as it is, just means perpetuating a non-useful, unusable behaviour where a useful behaviour could be provided.

I can't believe that you, of all people, are suggesting that the rest of the world eshew a possible, advanced behaviour just because it is not one that you personally have ever thought to try, or because it is easily achieved by the combination of two simpler ones?

Of course this is only my opinion and that carries exactly as much weight as anyone cares to give it, but the logic of:

Don't make something not useful into something useful because it might break something that is non-useful, but pre-existing.

I find faintly ludicrous.

---

Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
Hooray!
Wanted!

I think it should be fixed. This is a twist of human mind.

Backward compatibility is not really a problem. Without fixing it, this kind of broken interpretation will go die sooner or later. Instead of letting it die, why not just fix it and revive it. I don't expect many people or many application actually use this feature, the compatibility problem is minimum.

On the other hand, this kind of unnatural stuff really hurts application's maintainability, and programmer should be responsible for assessing it before using it.