Re: closure clarity, please

You seem to believe that subs are scoped to the block that contain them. That's not true. Subs are global.

Based on vitoco's example and JadeNB's analysis, I believe that statement should now be considered false. There appears to be two facets of scope that subs demonstrate. On the one hand, no matter where a sub is defined, it can be called anywhere in your program. In that sense, a sub has global scope.

On the other hand, the variables that a sub can see depends on the scope in which the sub is defined. In that sense, a sub has local scope.

Here is a simplified version of vitoco's example:

use strict;
use warnings;
use 5.010;

my $val = 10;

sub f {
    say "in global f, \$val is: $val";
}

f(); #Demonstrates that the global definition of f is overwritten 
     #by the local definition of f (below) at compile time.  As a 
     #result, you won't see output from global f.  When the local f
     #executes instead, the global $val above is hidden by a local
     #$val in the definition of f (below).

say '=' x 20;

sub g {
    my $val = shift;
    say "in g, \$val is: $val";

    #This definition of f closes over $x in previous line.
    sub f {   #line 40
        say "in local f, \$val is: $val";  
    }

    f();
}

g('hello');
say '=' x 20;

g('goodbye');
say '=' x 20;

--output:--
Variable "$val" will not stay shared at 4perl.pl line 41.
Subroutine f redefined at 4perl.pl line 40.
Use of uninitialized value $val in concatenation (.) or string at 4per
+l.pl line 41.
in local f, $val is: 
====================
in g, $val is: hello
in local f, $val is: hello
====================
in g, $val is: goodbye
in local f, $val is: hello
====================
[download]

If a sub truly had global scope, then the local $val would have no affect on the locally defined sub. Instead, the sub would have closed over the global $val, and changes to the local $val would not be seen by the sub.

*Yes, I know that "local $val" and "global $val" are really my variables. However, referring to them as "the my variable $val that was defined in the same block as the nested sub definition" or "the my variable $val declared outside of any blocks" is too unwieldy."

Comment on Re: closure clarity, please Download Code

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Re^2: closure clarity, please by ikegami (Patriarch) on Nov 27, 2009 at 01:58 UTC
I believe [ the statement "Subs are global" ] should now be considered false. The scope of something is the area from which that something can be seen. Subroutines can be seen from anywhere, so they are global. On the other hand, the variables that a sub can see depends on the scope in which the sub is defined. You're talking of the scope of the variables now, not the scope of the sub. Yes, they are visible to the sub. We gave a number of examples of this. You can't have closures without this.	[reply]
Re^3: closure clarity, please by 7stud (Deacon) on Nov 27, 2009 at 03:04 UTC
You're talking of the scope of the variables now, not the scope of the sub. Yes, they are visible to the sub. We gave a number of examples of this. You can't have closures without this. No, I don't think I am. It matters where the sub is defined. Saying that a sub is global, end of discussion is not at all helpful in determining what variables a sub can see. Clearly, it matters where a sub is defined in determining what values it can see. A sub defined outside any blocks cannot see a my variable declared inside a block, yet if the sub is defined inside the block, it can see the my variable. The scope of the my variable is the same in both cases. I don't see how saying that the scope of a variable is "wherever it can be seen" is of any use. The goal is to determine where a variable can be seen.	[reply]
Re^4: closure clarity, please by ikegami (Patriarch) on Nov 27, 2009 at 03:14 UTC
No I'm not [talking of the scope of the variables now] The visibility of the variables is the scope of the variables. You were saying that the sub can see the variables. That's means the sub is within the scope of the variables. It matters where the sub is defined, which you keep saying doesn't matter, It does matter, I've said as much, and I've never said otherwise. If the sub's definition was moved outside of the scope of the vars, the sub wouldn't be able to see the vars. I've given an example of this. Saying that a sub is global, end of discussion is not at all helpful in determining what variables a sub can see I agree. I never said it was. By the way, you can ding my reputation all you want. "You" must be a reference to someone else. I haven't downvoted any of your nodes, in any thread. I've had no problem with your questions. You are putting effort. You are learning. Mind you, I haven't upvoted many of your nodes either. I always forget to vote.	[reply]