Re^2: How to safely use $_ in a library function?

If I understand the OP’s question correctly, the programme’s structure is something like this:

#! perl
use strict;
use warnings;

for (qw(literal1 literal2 literal3))
{
    print "$_\n";
    foo('wilma');
}

sub foo
{
    $_ = 'fred';
    goto &bar;
}

sub bar
{
    my ($name) = @_;
    print "foo --> name = $name, \$_ = $_\n";
}
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Unfortunately, changing the line $_ = 'fred'; to local $_ = 'fred'; doesn’t work: it does fix the syntax error, but within sub bar $_ is still literal1, literal2, and literal3 on successive iterations of the for loop. From the documentation for goto (underlining added):

The goto-&NAME form ... exits the current subroutine (losing any changes set by local()) and immediately calls in its place the named subroutine using the current value of @_.

The problem is that on each iteration of the for loop, global $_ is made an alias of the next item in the list. When that argument is a literal, this makes global $_ readonly, and that effect is undone only when that loop iteration completes. There doesn’t appear to be any way to undo this effect. :-(

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re^3: How to safely use $_ in a library function?
by tobyink (Canon) on Sep 18, 2013 at 14:55 UTC

"There doesn’t appear to be any way to undo this effect. :-("

sub foo
{
    Internals::SvREADONLY($_, 0);
    $_ = 'fred';
    goto &bar;
}
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The intentionally-undocumented Internals package is part of Perl core (so much so that you don't even need to use it - it's completely internal, like parts of utf8 are) and has nothing much to do with the Internals module on CPAN.

use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name

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Re^4: How to safely use $_ in a library function?

by Athanasius (Archbishop) on Sep 18, 2013 at 15:09 UTC

Sounds good; but, if I add

Internals::SvREADONLY($_, 0);
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to the code I gave above, immediately before the line $_ = 'fred';, I still get:

 1:03 >perl 723_SoPW.pl
literal1
Modification of a read-only value attempted at 723_SoPW.pl line 26.

 1:03 >perl -v

This is perl 5, version 18, subversion 1 (v5.18.1) built for MSWin32-x
+86-multi-thread-64int
...
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Also, where and how can I learn about a feature that is “intentionally-undocumented”? (This is a serious question!)

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re^5: How to safely use $_ in a library function?

by hdb (Monsignor) on Sep 18, 2013 at 15:21 UTC

It reminds me more and more of code like this:

10: A=2
20: B=4
30: GOTO 200
40: PRINT C
50: END

200: C=A+B
210: GOTO 40
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Re^5: How to safely use $_ in a library function?

by tobyink (Canon) on Sep 18, 2013 at 15:28 UTC

Really? I get this...

Read more... (674 Bytes)

You can see what Internals functions you have like this:

perl -E'say for sort keys %Internals::'
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Many of these built-in functions, including stuff in the Internals, utf8, re and UNIVERSAL packages amongst others, are defined in the file universal.c in the Perl source code. Search in the file for the string "xsub_details" and you will find a list. That's not a comprehensive list though; Internals::V and mro::get_linear_isa are certainly missing.

use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name

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Re^6: How to safely use $_ in a library function?

by Athanasius (Archbishop) on Sep 18, 2013 at 15:47 UTC

Re^7: How to safely use $_ in a library function?

by tobyink (Canon) on Sep 19, 2013 at 07:10 UTC

Some notes below your chosen depth have not been shown here

Re^3: How to safely use $_ in a library function?
by perl-diddler (Chaplain) on Sep 17, 2013 at 14:27 UTC

*Bingo*.

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Re^3: How to safely use $_ in a library function?
by vsespb (Chaplain) on Sep 17, 2013 at 13:17 UTC

Indeed, you're right, seems this does not work with goto.

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Re^4: How to safely use $_ in a library function?

by perl-diddler (Chaplain) on Sep 17, 2013 at 19:39 UTC

As much as anything, not running into it is a reflection of perl-programming style -- i.e. if one rarely, if ever, uses constants hard-coded in the text of the 'for' loop (I almost always use for with an array or such -- even if it is an array of constants -- which doesn't hit this problem).

To answer the question of what happens if you call "for" in the target of the "goto", it overrides the global usage within the scope of the "for", then restores it. So the language *does* have "inherent" methods to do this, just, apparently, not explicit.

One reason (among others,) for splitting the code was to have most of the code parsed at compile time and leave as little parsing to runtime as possible. Compare that to the alternative where the entire "main sub" was in-lined in the "eval". Then, not only could I probably get around it by using "for" as a means to assign to "$_", (i.e. for ([$sav, et other vals]) { overriden value of "$_" active here; set to be ptr-to the indirect array }), but I wouldn't need to use it in the first place. *ARG* (hitting head against wall)...

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