All the Perl that's Practical to Extract and Report

A Perl one-liner grows up

masak on 2009-01-03T13:12:47

I wanted an overview of S29. More exactly, I wanted a list of functions. The table of contents in S29 itself didn't cut it, because it stopped short at the level of classes, and left out the functions. That's when I decided to extract the relevant information using a script of my own. I mean, how hard can it be?

Turns out it was easy.

Duh, right? We have Perl this is what Perl does: extract and report. (Practically.) Because it turned out to be a three-minute one-liner in Perl 5, I decided to turn it into a Perl 5 script file, and then into a Perl 6 script file.

The results were a bit interesting, and I thought I'd share.

First, a description of the problem:

  • S29 is a POD file, which means we're in luck: all the directives we need to extract are on their own lines.
  • Furthermore, all the classes are headed by =head2 $class, and the functions by =item $function.
  • There's some =over 4 and =back stuff that we don't care about.
  • Oh, an the action doesn't start until after the line =head1 Function Packages.
  • Now, produce a list of functions, indented under their respecitve class.

Here's my one-line solution:

$ perl -e '$_ = <> until /Function Packages/; while (<>) { if (/^=(\S+) (.*)/) { next if $1 eq "over"; print " " x 4 if $1 eq "item"; print $2, "\n" } }' S29-functions.pod

If you're interested in the output it produces, try running it after cd-ing into the directory docs/Perl6/Spec in the Pugs repo.

Here's the corresponding turned-into-a-file script (still Perl 5):

use strict;

open my $S29, '<', 'S29-Functions.pod' or die $!;

until (<$S29> =~ /Function Packages/) {}

while (my $line = <$S29>) {
    if ($line =~ / ^ = (\S+) ' ' (.*) /x) {
        next if $1 eq 'over';
        if ($1 eq 'item') {
            print ' ' x 4;
        }
        print $2, "\n"
    }
}

Things are a bit more indented, a bit more Best Practices, but no revolutionary changes. It's the same script.

Now, take a look at the Perl 6 script:

use v6;

my $S29 = open('S29-Functions.pod', :r) or die $!;

repeat {} until =$S29 ~~ /'Function Packages'/;

for =$S29 -> $line {
    if $line ~~ / ^ '=' (\S+) ' ' (.*) / {
        next if $0 eq 'over';
        if $0 eq 'item' {
            print ' ' x 4;
        }
        say $1;
    }
}

Noteworthy differences:

  • use strict is implicit in use v6.
  • The open syntax is different. I stumbled on this, and had to go look in a spectest file to remind myself.
  • I use repeat {} until $cond unstead of {} until $cond, because I think it looks better. The old way still works, too.
  • The while loop is a for loop. Beware, young Perl 6 hatchlings! In Perl 6 land, we for loop over our files and other stream-like things. That way, we don't have to do an assignment (explicit as in my Perl 5 script, or implicit with $_). Things are more lazy in Perl 6, so the for loop doesn't cause the file to be slurped or anything like that. (For what it's worth, I forgot to use a for loop and used a while loop instead, with segfaults as a result. That was an indication that I was doing something wrong.)
  • No parens are needed after for and if.
  • Perl 6 regexes are automatically /x.
  • The family of $1, $2, ... match variables are pushed down one step. For great justice. And consistency.
  • I'm not sure it's kosher to use say like I did (i.e. to finish off a line that has been printed in several stages), but it seemed nice, so I decided to give it a try. So far, I like it.

There you have it. A Perl one-liner grows up.

a safer one-liner

azawawi on 2009-01-04T19:52:13

Great work, masak. I started using that in Padre::Plugin::Perl6 to index Perl6 functions.

A more safe one-liner that works even on win32 platforms.

perl -e "$_ = <> until /Function Packages/; while (<>) { if (/^=(\S+) (.*)/) { next if $1 eq q{over}; print q{ } x 4 if $1 eq q{item}; print qq{$2\n} } }" S29-functions.pod

Re:a safer one-liner

masak on 2009-01-05T12:00:58

Great work, masak. I started using that in Padre::Plugin::Perl6 to index Perl6 functions.

Happy to hear that.

A more safe one-liner that works even on win32 platforms.

perl -e "$_ = <> until /Function Packages/; while (<>) { if (/^=(\S+) (.*)/) { next if $1 eq q{over}; print q{ } x 4 if $1 eq q{item}; print qq{$2\n} } }" S29-functions.pod

That version may work on win32 platforms, but it doesn't in bash, which does variable interpolation of double-quoted strings. Single quotes work.

Thanks for the tip about q{} and qq{}, though! I really should remember to use those more in one-liners, where single quotes are essentially hijacked by the -e string.

Re:a safer one-liner

azawawi on 2009-01-05T12:53:23

Cool. Thanks for the bash tip.

Please take a look at the updated code for parsing S29 for documentation and selecting the correct one. I think S29 needs to be updated in some =item-s to be more consistent.

http://padre.perlide.org/browser/trunk/Padre-Plugin-Perl6/lib/Pad re/Plugin/Perl6.pm.