namespaces and variable initilization with require 'file'

weltyj has asked for the wisdom of the Perl Monks concerning the following question:

I'm not understanding something about namespaces when using "require". For example

#this filename is test.pl
use strict ;
use warnings ;
my $a ;
require "set_a.pl"
print "a is $a\n" ;
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And

#this filename is set_a.pl
$a=1 ;
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When I run test.pl, $a is not initialized. I thought "set_a.pl" would run in the "main" namespace, so the line "$a=1" in set_a.pl would be initializing the variable $a in the main namespace. What did I forget about namespaces, or never learn? :-)

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Re: namespaces and variable initilization with require 'file' by JavaFan (Canon) on Oct 01, 2008 at 15:11 UTC
You are quite right; in set_a.pl the variable is initialized in the main name space. However, in test.pl, you have 'my $a', so when you print $a, you are referring to the lexical $a, not the package variable $main::a (which you initialize in set_a.pl). Lexicals and package variable are quite distinct. And I hope you are using '$a' just for this example - $main::a is special for 'sort'.	[reply]
Re^2: namespaces and variable initilization with require 'file' by weltyj (Novice) on Oct 01, 2008 at 15:29 UTC
Thanks, that was it. Remembering the nuance of lexicals are too often my nemesis. And yes, the $a was just for the example.	[reply]
Re: namespaces and variable initilization with require 'file' by broomduster (Priest) on Oct 01, 2008 at 15:46 UTC
As an alternative to `$main::a` (noted by JavaFan), you can declare using our.	[reply] [d/l]
Re: namespaces and variable initilization with require 'file' by ikegami (Patriarch) on Oct 01, 2008 at 19:05 UTC
`require` is for modules (files with a `package` directive). `do` is for ~~modules~~scripts (files without a `package` directive). It might even fix your problem without further change, but I'm not sure.	[reply] [d/l] [select]
Re^2: namespaces and variable initilization with require 'file' by JadeNB (Chaplain) on Oct 01, 2008 at 22:04 UTC
Did you really mean to use the word 'modules' for both kinds of files? I guess in any case that you didn't mean that `require` can only be used on files with a `package` directive, because that's certainly not true: `$ echo 'print "Included a.pm\n";' > a.pm $ perl -e 'require a' # Included a.pm` [download] works just fine for me. I understood a big difference (though by no means the only one) to be between whether you want to allow the file to be read multiple times, in the sense that `$ perl -e 'require a; require a' # Included a.pm $ perl -e 'do "a.pm"; do "a.pm"' # Included a.pm\ # Included a.pm` [download] do different things.	[reply] [d/l] [select]
Re^3: namespaces and variable initilization with require 'file' by ikegami (Patriarch) on Oct 02, 2008 at 02:02 UTC
The repeated use of "modules" was a copy and paste error. And right, I wasn't talking about the abilities of the respective commands, but their purpose. `do` executes a file. So does `require`. The difference is that `require` will only execute a file once per run, so it only makes sense to use `require` for files that load a module.	[reply] [d/l] [select]