Re: What I Most Recently Learned in Perl, But Should Have Already Known

I recently learned how to find out what Perl thinks I mean by my code. The magic of B::Deparse. This answered some mysteries, for example why this code will not stop on values that are seen as false:

while (<>) {
}
[download]

When writing that loop explicitly I would be testing it like this:

while (defined( my $line = <>)) {
}
[download]

It turns out that is exactly what perl is doing as well, as seen below:

perl -MO=Deparse -e 'while (<>) {}'
while (defined($_ = <ARGV>)) {
    ();
}
[download]

It's also interesting to see what the parser thinks about other code, such as:

perl -MO=Deparse -e ' if(1) {print "true"}'     
do {
    print 'true'
};
[download]

And

perl -MO=Deparse -e ' print "true" if 1'   
print 'true';
[download]

And it is useful for seeing what some of the code from the Obfuscation section is actually doing.

Comment on Re: What I Most Recently Learned in Perl, But Should Have Already Known Select or Download Code

Replies are listed 'Best First'.
Re^2: What I Most Recently Learned in Perl, But Should Have Already Known by Hue-Bond (Priest) on Aug 21, 2006 at 10:55 UTC
`perl -MO=Deparse -e ' print "true" if 1'` If we do `if 0` something even funnier happens: `$ /usr/bin/perl -MO=Deparse -e 'print "true" if 0' '???';` [download] But which is most amusing is the fact that the deparsed code generates a warning: `$ /usr/bin/perl -w -e 'print "true" if 0' $ /usr/bin/perl -w -e '"???"' Useless use of a constant in void context at -e line 1.` [download] -- David Serrano	[reply] [d/l] [select]
Re^3: What I Most Recently Learned in Perl, But Should Have Already Known by pKai (Priest) on Aug 23, 2006 at 09:36 UTC
`'???'` actually is no real thing. It's more like `Deparse` saying, this should be optimized away. E. g. every "`Useless use of a constant in void context`" also results in this.	[reply] [d/l] [select]
Re^2: What I Most Recently Learned in Perl, But Should Have Already Known by liverpole (Monsignor) on Aug 21, 2006 at 11:25 UTC
Hi imp, I myself didn't know about B::Deparse until about a half a year ago, and only then as a result of being pointed to it by the other monks. If you haven't seen it yet, check out this obfuscation, which I wrote to take advantage of some of the properties of B::Deparse. s''(q.S:$/9=(T1';s;(..)(..);$..=substr+crypt($1,$2),2,3;eg;print$..$/	[reply]
Re^2: What I Most Recently Learned in Perl, But Should Have Already Known by Freddo (Acolyte) on Mar 17, 2007 at 10:33 UTC
i have those in my ~/.aliases ... # Perl alias p='perl -nle' #> process file by line, chomp alias pp='perl -ple' #> process file by line, chomp and print alias pdbg='perl -de 1' #> perl debugger alias dpl='perl -MO=Deparse' #> show how perl see this code alias dpl2='perl -MO=Deparse,-l,-p,-q,-sCT' #> show how perl see this code, with line no, parens and more alias cpan='perl -MCPAN -e shell' #> Start CPAN ...	[reply]


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