Re^2: How to give input and collect output of a perl script using another perl script

GF: Thank you for short intro to perl modules

I am learning to use perl modules with a small example. Please go through below example, I have a couple of questions at the end

test2.pl is a script that uses perl module test.pm

Contents of test.pl

#!/usr/bin/perl
use strict;
use warnings;
use test;

my @gaga = ("paras", "kansas");

my ($x, $y) = test::test(@gaga);

print "$x\n$y\n";
[download]

contents of test.pm

package test;

sub test
{
       my $tag = $_[0];
       $tag =~ s/p/n/;
       my $bag = $_[1];
       $bag =~ s/k/p/;
       
       return $tag, $bag
}
1;
[download]

output

napas
panas
[download]

above script and module work very well. What I don't understand is..

Query-1: What is function of 1 at the end of test.pm ?

To see whether module works without a subroutine, I modified test.pm, by removing subroutine (see code below)

Contents of test2.pl

#!/usr/bin/perl
use strict;
use warnings;
use test;

my @gaga = ("paras", "kansas");

my ($x, $y) = test(@gaga);    

print "$x\n$y\n";
[download]

contents of test.pm

package test;

         my $tag = $_[0];
       $tag =~ s/p/n/;
       my $bag = $_[1];
       $bag =~ s/k/p/;
       
       return $tag, $bag

1;
[download]

Expected result is...

napas
panas
[download]

But error message reads...

test.pm did not return a true value at test2.pl line 4.
BEGIN failed--compilation aborted at test2.pl line 4.
[download]

Query-2: Is it must to have a subroutine inside a module ?

Comment on Re^2: How to give input and collect output of a perl script using another perl script Select or Download Code

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Re^3: How to give input and collect output of a perl script using another perl script by GrandFather (Saint) on Mar 23, 2011 at 20:02 UTC
1/ When you use or require a module Perl loads it and compiles it. Perl then executes any "main line" code in the module and checks that the result is true. For most modules the "main line" code may be as simple as `1;` which is equivalent to `return 1;` and returns the true result Perl expects. More interesting work done by a module during loading includes initialising variables and perhaps performing any other one time house keeping that the module may need (even including connecting to a database for example). From 1/ above you may figure out that the load process will see `return $tag, $bag 1;` as the result from loading the module. If you had used strictures in the module (use strict; use warnings;), which you should always do, you would have received a number of "Use of uninitialized value" messages too. Because there are no parameters passed in $tag and $bag are undefined which is false so the module load fails. Module Test (note the upper case first letter - it's important) should look like: `use strict; use warnings; package Test; sub SubInTest { my $tag = $_[0]; $tag =~ s/p/n/; my $bag = $_[1]; $bag =~ s/k/p/; return $tag, $bag; } 1;` [download] Note that strictly lower case module names are reserved by Perl for use as pragmata such as strict and warnings. You can use such names, but shouldn't in case Perl decides it wants to in some future version. 2/ By now it should be clear that, except for unusual modules (which only export data for example), it is futile to have a subroutine-less module because you can't execute any code in it after it's been loaded. True laziness is hard work	[reply] [d/l] [select]
Re^4: How to give input and collect output of a perl script using another perl script by chakreey (Acolyte) on Apr 07, 2011 at 10:09 UTC
thank you very much GF. Very nice explanation, now, I understand why module test.pm did not work ! For the first time, I could group all my perl scripts into single package. With-in this package, I placed one main executable script. Any one who wants to use my scripts, has to run this main executable script. Depending upon user requirements, appropriate module will be called by that main script.	[reply]
Re^3: How to give input and collect output of a perl script using another perl script by Anonymous Monk on Mar 23, 2011 at 14:11 UTC
What is function of 1 at the end of test.pm ? See Simple Module Tutorial, use, require Query-2: Is it must to have a subroutine inside a module ? Its like wheels on an automobile, its not a must, but the car won't roll otherwise and the script doesn't work ! What does that mean? How do I post a question effectively?	[reply]
Re^4: How to give input and collect output of a perl script using another perl script by chakreey (Acolyte) on Mar 23, 2011 at 15:57 UTC
and the script doesn't work ! What does that mean? How do I post a question effectively? I updated my reply. Hope, my query is more meaningful now. btw, thanks for directing me to those links !	[reply]
Re^5: How to give input and collect output of a perl script using another perl script by GrandFather (Saint) on Mar 23, 2011 at 19:34 UTC
Please note that when you make a substantive change to a node, especially if it pertains to something a reply node refers to you should add an update line like: Update: added a sample update line. True laziness is hard work	[reply]