Generic const - a non-functional view

[Steven Schveighoffer]

"Bill Baxter" wrote
> Me Here wrote:
>> Walter Bright wrote:
>>
>> Perl has invariant strings, but they are implicitly invariant
>>> and so nobody notices it, they just work.
>>>
>> Sorry Walter, but thta is simply not the case: Vis:
>>
>> [0] Perl> $x = 'x' x 500e6;
>> [0] Perl> print length $x;;
>> 500000000
>> [0] Perl> substr $x, 250e6, 1, 'y';;
>> [0] Perl> print length $x;;
>> 500000000
>> [0] Perl> print substr $x, 250e6-5, 10;;
>> xxxxxyxxxx
>>
>> b.
>
> What are you disagreeing with?
>
> The fact that they're invariant?
> Or the fact that nobody notices?
>
> I have no idea if they're invariant in perl or not.  But I don't think 
> your test above is conclusive proof that they're mutable.

No it's not.  The only conclusive proof comes from observing what happens 
when you copy strings from one to the other:

#!/usr/bin/perl

print "before x\n";
sleep 20;
$x = 'x' x 100000000;
print "initialized x\n";
sleep 5;
$y = $x;
print "copied to y\n";
sleep 5;
substr $x, 3, 1, 'y';
print "did substring\n";
print substr $y, 0, 5;
print "\n";
sleep 5;

OK, so what does this do?  I set x to a string of 100 million x's, then 
assign x to y, then replace the 4th character in x with a 'y', then print 
the first 5 characters of y to see if they changed too (see if x and y 
reference the same data)

So what does this output?
before x
initialized x
copied to y
did substring
xxxxx

But this is not yet conclusive proof, we need to watch what happens with 
memory usage when each step occurs (hence the sleeps).  So using 'top', I 
observed this:

before x => mem usage 3k
initialized x => 191MB (!)
copied to y => 291MB
did substring => 291MB
xxxxx

So, what it looks like is on assignment, the string is copied, and the 
editing edits the string in-place.   But I can't really explain why it takes 
191MB to store x, where it only takes 100MB to store y.

So I'd say perl does not have invariant strings.  'course, I'm not a perl 
hacker, so I don't know if I did this correctly :)

-Steve