Bignum breaks Time::Local?

cunningrat has asked for the wisdom of the Perl Monks concerning the following question:

This drove me nuts for several hours today until I figured it out. Sample code below:

#!/usr/bin/perl
use strict;
use Time::Local;
#use bignum(p => -3);

my $key='121005';
print $key." ==> ".convtime($key)."\n";

sub convtime {
 my @months=qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
 my @dow=qw{Sun Mon Tue Wed Thu Fri Sat};
 my $date=@_[0];
 my $year=substr $date,0,2;
 $year += 100;
 my $month=substr $date,2,2;
 $month -= 1;
 my $day=substr $date,4;
 my $gmt = timegm(0,0,0,$day,$month,$year);
 my @gmtime = gmtime($gmt);
 my $convstring=$dow[$gmtime[6]]." ".$months[$gmtime[4]]." ".$gmtime[3
+];
 return $convstring
}
[download]

If you run this, it returns "121005 ==> Fri Oct 5", as it should. If you uncomment the "use bignum" line, however, it returns "121005 ==> Mon Jan 23".

What I *think* (after some testing) is going on is that the Math::BigInt and Math::Bigfloat overload the arithmetic operators, which breaks timegm().

Is this expected behavior and/or documented anywhere?

Comment on Bignum breaks Time::Local? Download Code

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Re: Bignum breaks Time::Local? (constants) by tye (Sage) on Nov 05, 2012 at 23:34 UTC
What I think* (after some testing) is going on is that the Math::BigInt and Math::Bigfloat overload the arithmetic operators, which breaks timegm().* No, overloading of operators is based only on the arguments given to them. But 'use bignum;' overloads numeric constants. So, for example: `timegm(0,0,0,$day,$month,$year)` [download] becomes `timegm( Math::BigInt->new(0), Math::BigInt->new(0), Math::BigInt->new(0), $day, $month, $year )` [download] And Math::BigInt might not be careful enough to have Math::BigInt->new(0) act enough like just plain 0 for timegm()'s usage (not surprisingly). Is this expected behavior and/or documented anywhere? It sounds like you probably read the first sentence of the bignum documentation. Perhaps you failed to go on to read the second sentence. This makes bignum.pm a cute demonstration that can be convenient for a one-liner. Otherwise, you should just use Math::BigInt and/or Math::BigFloat directly, so you can avoid making every single time you use a numeric constant magically turn into a complex object that infects the results of any computations that touch it. Most of the time you only even need to call Math::BigInt->new() once, so a wrapper like bignum.pm seems of dubious benefit. bignum.pm should probably come with a big warning like "This transforms simple-looking Perl code into something very different and can easily break code". - tye	[reply] [d/l] [select]
Re: Bignum breaks Time::Local? by tobyink (Canon) on Nov 05, 2012 at 22:54 UTC
I see the bug you describe in Perl 5.8 and 5.10, but it seems to be fixed in 5.12, 5.14 and 5.16. Having checked the changelogs for bignum and Time::Local and also perl5120delta I can't figure out what exactly fixed this. `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply]
Re^2: Bignum breaks Time::Local? by cunningrat (Acolyte) on Nov 06, 2012 at 14:41 UTC
That explains it, Toby. Thanks. (I'm stuck using perl 5.8, unfortunately.)	[reply]