[Wellington-pm] A question about scopes

[Cliff Pratt]

Jacinta Richardson wrote:
> G'day Cliff,
> 
> I don't know how useful you'd find it, but we cover scoping and scopes in our
> Introduction and Intermediate Perl course notes, available at
> http://perltraining.com.au/notes.html
> 
> Scope itself is covered in the Intro Conditionals chapter, with further coverage
> when we get to Modules and Packages in the Intermediate course.
> 
> Cliff Pratt wrote:
> 
>>Say I have a Package and a PERL script.
> 
> 
> It's "Perl" typically.  ;)
> 
> 
>>If the package uses a variable 
>>$foo, and so does the script, I should be able to access the script's 
>>version of $foo by $main::foo in the package. Am I correct?
> 
> 
> Consider the following case:
> ---------------------------------------------------
> #!/usr/bin/perl -w
> use strict;
> 
> my $foo = 4;         # $foo in the main package
> my $bar = 10;        # $bar in the main package
> 
> print "$foo\n";      # prints "4".
> 
> if(1) {
>      my $foo = 1;    # $foo in the main package, but over-shadows previous
> 
>      $foo++;
> 
>      print "$foo\n"; # prints "2"
> }
> 
> print "$foo\n";      # still prints "4"
> 
> 
> package bar;
> 
> my $foo = 7;         # $foo in the bar package (generates warning if in
>                        same file as above code)
> 
> print "$foo\n";      # prints "7".
> 
> print $main::foo;    # undefined!
> print $main::bar;    # undefined!
> 
> print "$bar\n";      # prints "10" but only if this is all in the same file
> ---------------------------------------------------
> 
> 
> $main::foo is undefined because $foo was declared with "my".  Declaring a
> variable with "my" means that it lasts from that location to the end of the
> enclosing block ( }  or end of tile ).  "my" variables are not "package"
> variables, but are rather lexical variables.
> 
I understand this and I don't understand this! Yes, I see what you are 
saying, but doesn't $main:: say to look in $main's Symbol Table and 
hence know the location of foo even though it is out of scope? Or is 
scope not just matter of knowing where to look?

Cheers,

Cliff

-- 

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