Re^3: Autovivification of scalars in sub calls

[Anonymous Monk]

Interesting. I knew that some builtins, like pos and keys, can be lvalues, but not that arguments to subs were. Come to think of it, I can't even say that I know what the expression "arguments to subs are lvalues" means. How is the argument in sleep( 3 ) an lvalue? Arguments to subs can be assigned to; they're lvalues in that sense. Therefore, you get these ugly situations:

 
my $var=3;
mysleep($var); # these two lines are fine

mysleep(3); # but this one causes an error!!!

sub mysleep {
           sleep($_[0]);
           $_[0]=10;
}

Or this ugly situation...:

$y=substr("hello",10,1); 
foo($y);# this runs, but substr() generates warnings because the subst
+r() is outside the string

foo( substr("hello",10,1) ); # this is a fatal error!

Why is it a fatal error? Well, because an invalid substr() is used in a lvalue context. Why is it an lvalue context? Because the subroutine *could* modify the value, and perl isn't smart enough to check to see if it does or not! Worse yet, substr() fails silently on such a fatal error, after generating the *same* warning that's nonfatal in a non-lvalue context. ***SIGH***

I've made it my policy to avoid lvalues() where possible, especially after that substr() issue killed our production code, and wasted a few hours of my time! --
Ytrew Q. Uiop

[truedfx]

Perhaps also interesting is that lvalues remain lvalues even after passing them to the unary + operator.

sub foo {}

#foo(substr "hello", 10, 1);       # fatal error as you pointed out
#foo(+substr "hello", 10, 1);      # I thought this would be a workaro
+und
foo(scalar substr "hello", 10, 1); # but you need something like this

print "This only gets printed if the last line didn't cause a fatal er
+ror\n";

And testing to make sure:

perl -e '+$a = "Hello, world!\n"; print $a'
Hello, world!

(I know it's documented that + does absolutely nothing, and I guess that does mean it doesn't convert lvalues to rvalues, but it's still somewhat surprising to me.)