Ref to a list not being equivalent to a list of refs to each element

Hue-Bond has asked for the wisdom of the Perl Monks concerning the following question:

(This is not an XY problem, nor something that will go to production; just a question that arose while I was playing with obfuscated code).

Let's declare a constant:

*a = \"cons";
$a = "something";
print "$a\n";
__END__
Modification of a read-only value attempted at - line 2.
[download]

That's expected behaviour. Now, a little twisted:

(undef, *a) = \(undef, "cons");
$a = "something";
print "$a\n";
__END__
something
[download]

Surprise! What's that? Isn't it a constant anymore? Isn't that reference to a list, a list of references to each element? Well, let's make an explicit list of references then:

(undef, *a) = (\undef, \"cons");
$a = "something";
print "$a\n";
__END__
Modification of a read-only value attempted at - line 2.
[download]

This is expected again. But, why does the second snippet behave different?

--
David Serrano

Comment on Ref to a list not being equivalent to a list of refs to each element Select or Download Code

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Re: Ref to a list not being equivalent to a list of refs to each element by broquaint (Abbot) on Jun 22, 2006 at 16:22 UTC
I believe you're seeing this behavior because `\("foo");` [download] creates a reference to a temporary value whereas `\"foo";` [download] creates a reference to a constant value. I don't know if that is explicitly documented anywhere, but there you have it. HTH `_________ broquaint`	[reply] [d/l] [select]
Re: Ref to a list not being equivalent to a list of refs to each element by Fletch (Bishop) on Jun 22, 2006 at 16:21 UTC
OK, only a shot in the dark after playing with B::Concise and B::Deparse but here goes . . . In the first and third cases the reference is to a readonly PV value. This is copied directly into the SV slot of `a` and when you try and modify it you get the error. In the middle case either a non-readonly copy of the PV is pushed onto the stack, or the make-reference-to-list opcode (`refgen` I think) makes a non-readonly copy. Whichever case it is, the PV that's stuck in the SV slot of `a` is mutable.	[reply] [d/l] [select]
Re: Ref to a list not being equivalent to a list of refs to each element by davido (Cardinal) on Jun 22, 2006 at 16:55 UTC
That's an interesting quirk you've found. This discussion does present the opportunity to discuss the creation of constants. There's always the `constant` pragma, but it makes interpolation difficult and syntax confusing. There's your typeglob method, but the fact that `*variable = \'value';` creates a sort of immutable constant is itself simply a side effect, easily forgotten unless one brushes up on the POD periodically. How about the `use Readonly;` solution? `use strict; use warnings; use Readonly; Readonly my $VAR => 'value'; #......` [download] It's clear, it states what it does explicitly, and it interpolates properly. Are there negatives to this approach? The ones I can think of are that not everyone will have Readonly.pm installed (but it's easy), and also, unless Readonly::XS.pm is also installed, the new readonly variables will incur a slight performance hit. Dave	[reply] [d/l] [select]
Re: Ref to a list not being equivalent to a list of refs to each element by Errto (Vicar) on Jun 22, 2006 at 18:23 UTC
A related quirk I ran across earlier, and which I also am not sure is documented (though I may have missed it), is that references to literal strings are read-only, but references to computed strings, including interpolated literals, are not. For example `my $x = 'foo'; my $y = \"bar$x"; $$x = 'baz';` [download] does not throw any errors or warnings. Does anyone know why exactly that is?	[reply] [d/l]
Re^2: Ref to a list not being equivalent to a list of refs to each element by broquaint (Abbot) on Jun 23, 2006 at 17:26 UTC
This occurs for the same reasons given above as `\"foo";` [download] creates a references to a constant value whereas `\"foo$x";` [download] is creating a reference to a temporary value. If you want to see the evidence then just put `Devel::Peek` to work (take note of the `FLAGS` values for the reference): `use Devel::Peek; my $x = 'foo'; Dump \'foo'; Dump \"bar$x"; __output__ SV = RV(0x183c90c) at 0x225fe4 REFCNT = 1 FLAGS = (PADBUSY,PADTMP,ROK,READONLY) RV = 0x225ffc SV = PV(0x22626c) at 0x225ffc REFCNT = 1 FLAGS = (POK,READONLY,pPOK) PV = 0x1831d04 "foo"\0 CUR = 3 LEN = 4 SV = RV(0x183c92c) at 0x2252ac REFCNT = 1 FLAGS = (TEMP,ROK) RV = 0x225150 SV = PV(0x2261a0) at 0x225150 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x1831cac "barfoo"\0 CUR = 6 LEN = 7` [download] HTH `_________ broquaint`	[reply] [d/l] [select]