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[ #233 ] Context managers in Perl Permalink

Perl Added about two months ago

Let's try to implement something like Python's with statement in Perl.

First, let's look at Python. Python lets you do something like this:

class controlled_execution:
    def __enter__(self):
        set things up
        return thing
    def __exit__(self, type, value, traceback):
        tear things down

with controlled_execution() as thing:
     some code

(code example stolen from effbot)

The idea is that when the with block exits, the __exit__ cleanup is automatically called. This is nice for dealing with files, for example:

 with f = open("somefile"):
    f.read()

... where the implicit __exit__ code closes the file for us.

Can we do this in Perl? Sure. We just don't have the luxury of using nicely named __enter__ and __exit__ functions. Here is an example:

package Dir;

use strict;
use warnings;

use Cwd;

sub cd {
    my $dir = shift;
    my $code = shift;
    my $origdir = getcwd();
    chdir($dir);
    $code->();
    chdir($origdir);
}

1;

Now the script that utilises it:

#!/usr/bin/env perl

use strict;
use warnings;

use Dir;

Dir::cd "..", sub {
    my @files = glob("*");
    print join("\n", @files);
};

This context manager is for "cd", which chages directory, runs all the code in the block, and as it exits it simply changes back to the original directory it was in. There you go! Simple context managers in Perl.

[ #218 ] Install all Perl modules Permalink

Perl, Shell, apt Added less than a year ago and last edited less than a year ago

In Debian or Ubuntu you can install all apt provided Perl modules like so:

apt-cache search "\-perl" | grep -e '^lib' | awk '{ print $1 }' | xargs sudo apt-get install

If, before running that command, you just want to check what it would install, run this:

apt-cache search "\-perl" | grep -e '^lib' | awk '{ print $1 }'

[ #210 ] Perl - Using an expensive module Permalink

Perl Added less than a year ago

(Obviously expensive in this context refers to time or resources.)

From perldoc -f use, we know that use Module; is the same as:

BEGIN { require Module; Module->import( LIST ); }

Code in BEGIN blocks is executed at compile time, not run time. ('Compilation' in perl refers to the compilation to internal bytecode format.)

Consider an expensive module that your script uses. Imagine that the functionality that this module provides is only used in one of your subroutines, and that this subroutine is not necessary run in every invocation of your script.

If you used the expensive module, it would be included every time your script is run. If, instead, you simply required this module within the subroutine that actually needs it, you could avoid the cost of including the module when you don't need it!

Consider this expensive-to-use module, Expensive.pm:

package Expensive;

sleep 5;

1;

Now consider use.pl:

#!/usr/bin/env perl

use strict;
use warnings;

use Expensive;

sub rarely_called {
    # uses Expensive.pm's functionality here
    return;
}

rarely_called if defined $ARGV[0];

versus require.pl:

#!/usr/bin/env perl

use strict;
use warnings;

sub rarely_called {
    require Expensive;
    Expensive->import();
    # uses Expensive.pm's functionality here
    return;
}

rarely_called if defined $ARGV[0];

Let's time them:

$ time ./use.pl 

real    0m5.011s
user    0m0.008s
sys     0m0.008s

$ time ./use.pl xxx # see how both invocations of use.pl take the same amount of time

real    0m5.011s
user    0m0.008s
sys     0m0.004s

$ time ./require.pl # but when require doesn't call the sub it is fast!

real    0m0.010s
user    0m0.004s
sys     0m0.004s

$ time ./require.pl xxx

real    0m5.011s
user    0m0.012s
sys     0m0.000s

Oh, and in case you are wondering, no you can't just put the use statement in the sub... all uses are processed at compile time, so it would still be expensive even if that sub was never called.

[ #208 ] Perl Modules with Custom Prefix Permalink

Perl Added less than a year ago

To install a Perl module somewhere custom, simply:

$ perl Makefile.PL PREFIX=/your/prefix/here
$ make
$ make test
$ make install

[ #207 ] Perl: tr vs. s Permalink

Perl Added less than a year ago

Benchmarking tr vs. s:

$ perl -M'Benchmark qw(:all)' -E '$a= "fhkjds fhdj fhj fdjskfl";
> cmpthese(5000000,
> {"tr" => sub { $b=$a; $b =~ tr/ /_/ },
> "s" => sub { $b=$a; $b =~ s/ /_/g }})'

        Rate    s   tr
s  1173709/s   -- -78%
tr 5263158/s 348%   --

This means that tr is faster for this type of operation (a simple string-wide character substitution)

[ #203 ] Perldoc Output Permalink

Perl, Linux Added less than a year ago

If, when you look at the documentation for a core-builtin, like so:

$ perldoc -f system

you get some funny characters in the output (long dashes, 'smart' quotes, etc) - and you wish to remove them (to make copy-pasting of code easier,) then simply:

$ perldoc -tf system

The -t flag avoids any fancy formatting and just outputs plain text.

[ #189 ] utf8 in your Perl Permalink

Perl Added less than a year ago

Just by using the utf8 pragma in your Perl, you can do things like this:

use Modern::Perl;
use utf8;

my $天 = 'sky!';

say $天;

for my $否 (split(/ /, "no no NO no! NO!")) {
    say $否;
}

[ #186 ] Colours in your PAGER Permalink

Perl, Shell Added less than a year ago

Seeing escape codes when you run perldoc?

Set your $PAGER environment variable up correctly:

export PAGER="less -R"

[ #183 ] Funny Tweets Permalink

Comedy, Perl Added less than a year ago

... between two Perlers today:

chromatic:

Some days the jerks and bugs and workarounds and fame-chasing pundits make me want to leave technology as a profession.

Larry Wall:

If you do leave, you will find jerks and bugs and workarounds and fame-chasing pundits in most other professions too.

chromatic

At least real life hides its backwards compatibility misfeatures in DNA where I can't get at it.

Haha :-)

[ #175 ] Automate your perltidy from Vim Permalink

Vim, Perl Added less than a year ago

I have set up this mapping:

nnoremap <silent> <F9> :%! perltidy -q -pbp<CR>

... now I just hit F9 and I'm set.

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Vim & Vision

[ #233 ] Context managers in Perl Permalink

[ #218 ] Install all Perl modules Permalink

[ #210 ] Perl - Using an expensive module Permalink

[ #208 ] Perl Modules with Custom Prefix Permalink

[ #207 ] Perl: tr vs. s Permalink

[ #203 ] Perldoc Output Permalink

[ #189 ] utf8 in your Perl Permalink

[ #186 ] Colours in your PAGER Permalink

[ #183 ] Funny Tweets Permalink

[ #175 ] Automate your perltidy from Vim Permalink

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