Schizophrenic var

I admit that dualvar was a term I never felt looking up until it came up recently here at the Monastery at Re^2: Rosetta Code: Long List is Long -- dualvar by marioroy. And I had a look at it. I found that Scalar::Util's dualvar can indeed create such a variable but I did not find any way of driving an existing variable into schizophrenic behaviour.

And so here is a gutty way to alter the string and/or numeric part of an existing scalar. Mind you I did a brief search on whether there was already some existing package doing that but did not find any.

This can find applications in some cases of the Schwartzian transform where the "decorations" and "undecorations" can apply on existing data rather than creating new (re: decorate/undecorate, Schwartzian_transform). The concept is not new (e.g. see comments under https://www.endpointdev.com/blog/2009/08/perls-scalarutildualvar/), but I did not find anything without creating new (dual)var but altering existing var. So here it is in a single-file SCSE form (XS can be found in ./_Inline/build dir):

## by: bliako
## on: 2022-12-05
## EDIT: kudos to https://metacpan.org/release/PEVANS/Scalar-List-Util
+s-1.63/source/ListUtil.xs (line 1674, dualvar)

use strict;
use warnings;
use Devel::Peek;
use Inline Config => clean_after_build => 0;
use Inline C => <<'EOC';
        // alter the 'numeric' slot of existing var 'asv' with that of
+ 'theiv'
    void setIV(SV *asv, SV *theiv){
        (void)SvUPGRADE(asv, SVt_PVNV);
        SvIV_set(asv, SvIV(theiv));
        SvIOK_on(asv);
    }
        // alter the 'string' slot of existing var 'asv' with that of 
+'thesv'
    void setSV(SV *asv, SV *thesv){
        sv_setsv(asv, thesv);    
    } 
EOC

my $num = 42;
my $x = "My name is Ace!";
Dump($x);

setIV($x, $num);
Dump($x);
setSV($x, "Hello Ozy!");
Dump($x);

# and here is a one-legged Schwartzian transform
# to sort strings on their lengths using above XS:
my @unsorted = ("Just", "another", "Perl\x{7}\x{7}\x{7}", "hacker\x{7}
+");
my @sorted = sort { $a <=> $b }
    map { setIV($_, length($_)); $_ } @unsorted;
print "Sorted: @sorted\n";
;
[download]

Hacking away with Perl into the new year ... (actually I think I am in 2048 already, riding on Perl)

bw, bliako

Comment on Schizophrenic var Select or Download Code

Replies are listed 'Best First'.
Re: Schizophrenic var by Discipulus (Canon) on Dec 06, 2022 at 07:39 UTC
Hello bliako, for sure in your resarch you read create-your-own-dualvars. It tells `$!` is already dualvar: perl -le "for (0..42){local $!; $! = $_; print 0 + $!, qq( - $!)}" 0 - 1 - Operation not permitted 2 - No such file or directory 3 - No such process 4 - Interrupted function call 5 - Input/output error 6 - No such device or address 7 - Arg list too long 8 - Exec format error 9 - Bad file descriptor 10 - No child processes 11 - Resource temporarily unavailable 12 - Not enough space 13 - Permission denied 14 - Bad address 15 - Unknown error 16 - Resource device 17 - File exists 18 - Improper link 19 - No such device 20 - Not a directory 21 - Is a directory 22 - Invalid argument 23 - Too many open files in system 24 - Too many open files 25 - Inappropriate I/O control operation 26 - Unknown error 27 - File too large 28 - No space left on device 29 - Invalid seek 30 - Read-only file system 31 - Too many links 32 - Broken pipe 33 - Domain error 34 - Result too large 35 - Unknown error 36 - Resource deadlock avoided 37 - Unknown error 38 - Filename too long 39 - No locks available 40 - Function not implemented 41 - Directory not empty 42 - Illegal byte sequence [download] > but I did not find any way of driving an existing variable into schizophrenic behaviour. All scalars are already schizophrenic (but using `strict` they claim for a psychologic support, not sure if for the compiler or for the programmer ;) `perl -wle "use strict; for my $x(@ARGV){ print qq(I was: $x); print qq +(adding 1 I'm: ),$x + 1; print $x > 0 ? qq(I'm more than zero! ) : qq +(I'm less than or zero! ); print $\ }" 0z 42A I was: 0z Argument "0z" isn't numeric in addition (+) at -e line 1. adding 1 I'm: 1 I'm less than zero! I was: 42A Argument "42A" isn't numeric in addition (+) at -e line 1. adding 1 I'm: 43 I'm more than zero!` [download] `perl -wle "$x = q(a); print ++$x"` prints `b` L* PS This is even more weird.. I want to see if the numeric part of a filehandle was the same of what `fileno` returned.. it would be fun.. but no fileno `perl -lwe "open my $fh, '>', 'test'; print 0 + $fh; print + $fh" 16950176 GLOB(0x102a3a0) # ..and no: 16950176 is not the sum of numerical values of characters perl -wle "$str = q!GLOB(0x102a3a0)!; for (split '',$str){ print qq($_ + = ),ord($_);$res += ord($_);} print $res" G = 71 L = 76 O = 79 B = 66 ( = 40 0 = 48 x = 120 1 = 49 0 = 48 2 = 50 a = 97 3 = 51 a = 97 0 = 48 ) = 41 981` [download] PPS `perl -wle "$x = q(a); print ++$x"` prints `b` but not all chars are numeric, sorry I'm not `JAPH,` but `J1PH1` :) `perl -we "print ++$_ for 'I@OG+' =~ /./g" Argument "@" isn't numeric in preincrement (++) at -e line 1. Argument "+" treated as 0 in increment (++) at -e line 1. J1PH1` [download] There are no rules, there are no thumbs.. Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.	[reply] [d/l] [select]
Re^2: Schizophrenic var by eyepopslikeamosquito (Archbishop) on Dec 06, 2022 at 10:39 UTC
for sure in your research you read create-your-own-dualvars Ha ha, this has now become hilarious! Yesterday in Re^3: Rosetta Code: Long List is Long (Updated Solutions - dualvar), I claimed "Despite using Perl for 20 years, I'd never heard of dualvar before" ... yet on seeing your reference to good old brian_d_foy's dualvar article just now, I rushed to my bookshelf to find brian's superb book still sitting there, along with my customary book notes scribbled on a piece of paper inside the book ... which noted the existence of dualvar on page 30 as "interesting"! Oops, looks like my memory is starting to fail. Also interesting is that the paper copy had some nice levity to it which is sadly missing from the Web version ... at least the paper copy concludes the dualvar item with "now that you know this, don't start using it everywhere. Save it for very special situations, like impressing people at conferences and cocktail parties." ... or impressing people at Perl Monks in the case of marioroy. :)	[reply]
Re^3: Schizophrenic var by marioroy (Prior) on Jul 01, 2023 at 05:58 UTC
I used "dualvar" also, in MCE::Shared::Cache. When max_age is specified during construction of the cache, the dualvar is used internally to store the expiration time with the key name in the keys array. `# update expiration $keys->[ $off ] = ( $exptime >= 0 ) ? dualvar( time + $exptime, $_[1] ) : dualvar( -1, $_[1] );` [download] Basically, the "dualvar" capability in Perl allows the "max_age" feature without increasing the memory consumption of the cache or impacting performance.	[reply] [d/l]
Re^4: Schizophrenic var (semipredicate problem) by eyepopslikeamosquito (Archbishop) on Jul 01, 2023 at 09:43 UTC
Re^5: Schizophrenic var by marioroy (Prior) on Jul 01, 2023 at 14:58 UTC
Some notes below your chosen depth have not been shown here
Re^2: Schizophrenic var by Haarg (Priest) on Dec 06, 2022 at 13:36 UTC
All scalars are already schizophrenic This has little to do with the values, and more with the operations you are doing. If a value starts with a number, it will be treated as that number when used in numeric operations. And it will issue a warning if it is followed by non-numeric data, as you demonstrated. "0z" is not comparing as "less than zero", it is comparing equal to zero. And the ++ operator has two different functions depending on what type of input it is given. This is even more weird.. I want to see if the numeric part of a filehandle was the same of what fileno returned.. it would be fun.. but no fileno The value shown is a GLOB reference, which internally contains the file handle in its IO slot. All references, aside from objects using overloading, numify to their refaddr. $ perl -le'my $ref = []; print $ref; printf "%s(0x%x)", ref $ref, 0+$ref;' ARRAY(0x15500a648) ARRAY(0x15500a648)	[reply]
Re^2: Schizophrenic var by Tux (Canon) on Dec 07, 2022 at 08:37 UTC
<shameless plug> create your own TripleVal. Show all entries in the value. </shameless plug> Enjoy, Have FUN! H.Merijn	[reply]