Perl5

tags: Perl 5

It’s been a while since I could enjoy daily programming life with Perl 5.
I tried to find a chance to make any program with Perl, but it’s not easy especially when co-workers does not looks very happy reading the Perl code.
There are many languages showing off their fancy features and paradigm, like Kotlin and Haskell or even new projects on going with OpenJDK, however I believe perl is still a very powerful tool for any kind of utilization.
The purpose of posts regarding the perl is mostly like a memo to myself.

perldoc

  • Extension of perldoc(POD: Plain Old Documentation) is often .pod or .pm(perl module)
  • .pm extension is used if .pm file has embedded POD and the .pod is not found.
  • Source code of the module is also visible from perldoc bash perldoc -m MeCab perldoc -m Try::Tiny

Documents structure

perl perlfaq perltoc perldiag perldoc CGI

useful switchs

-D : Describes search for the item in detail -q : perldoc -q "something I'm looking for" perlfaq-search-regexp -l : perldoc -l Time::HiRes Display only the file name of the module found Type > perldoc perldoc for details

shebang lines

The third line of below code block is to tell perl to use env program to find out which perl is set as the default perl on the system.

#!/usr/bin/perl
#!/usr/local/bin/perl
#!/usr/bin/env perl

Module

  • The perl finds module by looking through the dirctories in the @INC, which contains module search path bash $ perl -V $ perl -le "print for @INC" perldoc -l Time::HiRes # see location perl -MTime::HiRes -e 'print $INC{"Time/HiRes.pm"}.\n'

Module Path

Set path in program

Following code doesn’t work becuase unshift is executed at runtime and the use pragma evaluated at compile time.

unshift @INC, '/Users/wonha/lib';
use Navigation::SeatOfPandts;

Correct way is to use BEGIN block :

BEGIN {unshift @INC, '/Users/gilligan/lib'; }
use Navigation::SeatOfPandts;

Context

  • It is comma operator impliciting the list context, not paranthese.
  • But in order to make empty list or one-element list, use paranthese, not comma perl ($dino) = something; @arr = ( );

Scoping

  • my keyword declares lexical variable, which is same as private varibale of that scope
  • state is similar to final keyword in Java perl use feature 'state'

Special Variables

> perldoc perlvar
VARIABLE DESCRIPTION
@_ a
@INC Contains paths to look for files loaded with do, require, or use.
%INC Contains entries for every file loaded with do, require, or use.
$^V The current Perl version
$^X The executable used to execute your program
$@ Perl syntax error trapped by eval.
$/ input record separator (defaultly, newline)
$ output record separator
$” list separator ( print “@array\n”;)
$! contain system call failure reason
$l set this to 1 will set the currently selected filehande(by select operator) buffer flushed after each output operation
%ENV .
$$ current process ID

String

  • ” “, , , qq{}: escape and interpolate have full power.
  • ’ ‘, qw//, q{}: escape single quote(or used delimiter) and backslach.
    perl my $letter = <<"SQL"; SELECT c.id FROM customers c JOIN orders o On c.id = customer_id SQL

Unicode

Basic control flow with Hash & Array

my %french_for = qw/1 un 2 deux 3 trois/;

###

my %perl = map { $_, 1 } @required;
for my $item (@required) {
	if ( $perl{$item} ) {
		print "perl have the item\n";
	}
}

###

@arry = %people = ( %classmate, austen => 'Jane', Lincoln => "Abraham" );

### 

my(undef, undef, undef, undef, $mtime, $ctime) = stat $some_file # this is annoying
my ($ctime1, $mtime, $ctime2) = (stat $some_file)[5,4,5]; # use a list slice

###

while ( ( $index, $value ) = each @rocks ) {
	print "$index: $value\n";
}

###

my @fields = split /:/, ":::a:b:c:::"; # gives ("", "", "", "a", "b", "c")

Caching recursion : Memoiz

  • tail call recursion : goto
  • caching previous result during recursive call : memoize module ```perl use Memoize; memoize(‘F’);

sub F { my $n = shift; return 0 if $n == 0; return 1 if $n == 1; return F($n-1) + F($n-2); } print F(7); ```