Re^11: The fallacy of the *requirement* for read-only instance variables.

Strings mostly act as if they were immutable in Perl 5, so my statement should be taken with care. It's only when you look at the XS or C layer that you see mutable strings:

use Devel::Peek;

my $x = "foo";

Dump $x;
vec($x, 0, 2) = 3;
Dump $x;
[download]

Output:

SV = PV(0x1b6a088) at 0x1b8b340
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x1b7a9a0 "foo"\0
  CUR = 3
  LEN = 8
SV = PV(0x1b6a088) at 0x1b8b340
  REFCNT = 2
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x1b7a9a0 "goo"\0
  CUR = 3
  LEN = 8
[download]

You see two different strings at the same memory location, indicating mutability. Though it could of course be a newly allocated copy of the string, while the old one is gone.

Maybe it would have been better if I had talked about hashes being mutable and numbers being immutable.

My main point remains: Perl has some mutable and some immutable data types, and that all languages I know have that distinction too. So a line must be drawn, and in some contexts it might sense to create immutable objects.

Perl 6 - second systems done right

Comment on Re^11: The fallacy of the requirement for read-only instance variables. Select or Download Code

Replies are listed 'Best First'.
Re^12: The fallacy of the requirement for read-only instance variables. by JavaFan (Canon) on Apr 18, 2011 at 14:34 UTC
Ah, so, number are immutable because the literals are, and string are mutable because variables holding the strings are. Right. Maybe it would have been better if I had talked about hashes being mutable and numbers being immutable. Same problem. You're only able to modify a hash by stuffing it into a variable - but once you do that, numbers are mutable as well. Anything that can appear on the LHS of an assignment is mutable (although the reverse doesn't automatically follow). Now, one can argue that there are no hash literals (they're either lists, or hashrefs), but that invalidate my arguments.	[reply]
Re^12: The fallacy of the requirement for read-only instance variables. by BrowserUk (Patriarch) on Apr 18, 2011 at 12:50 UTC
numbers being immutable. Same address, different value. In Perl, numeric variables are mutable. `C:\test>perl -MDevel::Peek -E"my $x = 12345; Dump $x; $x ^= 1; Dump $x +" SV = IV(0x2174e0) at 0x2174e8 REFCNT = 1 FLAGS = (PADMY,IOK,pIOK) IV = 12345 SV = IV(0x2174e0) at 0x2174e8 REFCNT = 1 FLAGS = (PADMY,IOK,pIOK) IV = 12344` [download] Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l]