Philippine Linux Users Group (PLUG) ()

headers Silverlokk | 1 Dec 02:40

_________________________________________________
Philippine Linux Users' Group (PLUG) Mailing List
http://lists.linux.org.ph/mailman/listinfo/plug
Searchable Archives: http://archives.free.net.ph
Choice quote: “Hey, if you just press Enter, it usually works out."
http://www.fsdaily.com/Community/The_Un_Scary_Screwdriver, or
http://www.gnomejournal.org/article/88/the-un-scary-screwdriverhttp://www.gnomejournal.org/article/88/the-un-scary-screwdriver
for the direct link.

--

-- 
Daniel O. Escasa
independent IT consultant and writer
contributor, Free Software Magazine (http://www.freesoftwaremagazine.com)
personal blog at http://descasa.i.ph
Twitter page at http://www.twitter.com/silverlokk
If we choose being kind over being right, we will be right every time.

Pablo Picasso  - "Computers are useless. They can only give you
answers." - http://www.brainyquote.com/quotes/authors/p/pablo_picasso.html
_________________________________________________
Philippine Linux Users' Group (PLUG) Mailing List
http://lists.linux.org.ph/mailman/listinfo/plug
Searchable Archives: http://archives.free.net.ph

On Tue, Dec 1, 2009 at 2:44 PM, Daniel Escasa <descasa@...> wrote:
> Choice quote: “Hey, if you just press Enter, it usually works out."
> http://www.fsdaily.com/Community/The_Un_Scary_Screwdriver, or
> http://www.gnomejournal.org/article/88/the-un-scary-screwdriverhttp://www.gnomejournal.org/article/88/the-un-scary-screwdriver
> for the direct link.
I clicked the email while the girl was still five years old, and by
the time I clicked the article, the headline already said 7 years old!
Kids these days, they grow up so fast! >:D

>
> --
> Daniel O. Escasa
> independent IT consultant and writer
> contributor, Free Software Magazine (http://www.freesoftwaremagazine.com)
> personal blog at http://descasa.i.ph
> Twitter page at http://www.twitter.com/silverlokk
> If we choose being kind over being right, we will be right every time.
>
>
> Pablo Picasso  - "Computers are useless. They can only give you
> answers." - http://www.brainyquote.com/quotes/authors/p/pablo_picasso.html
> _________________________________________________
> Philippine Linux Users' Group (PLUG) Mailing List
> http://lists.linux.org.ph/mailman/listinfo/plug
> Searchable Archives: http://archives.free.net.ph
>

--

-- 
This email is:    [ ] actionable   [ ] fyi        [ ] social
Response needed:  [ ] yes          [ ] up to you  [ ] no
Time-sensitive:   [ ] immediate    [ ] soon       [ ] none
_________________________________________________
Philippine Linux Users' Group (PLUG) Mailing List
http://lists.linux.org.ph/mailman/listinfo/plug
Searchable Archives: http://archives.free.net.ph

Hi:

   I'm having some hard time understand perl closure:

   Here's an explanation from Intermediate Perl:
   "The kind of subroutine that can access lexical variables that existed at the time we declare it is called a
closure .....In Perl terms, a closure is just a subroutine that references a lexical variable that has
gone out of scope".

   and here is the sample code:
   use File::Find;

   my $total_size = 0;
   find (sub {$total_size += -s if -f},'.');
   print $total_size, "\n";

    My question is "$total_size" is not out of scope since it is declared before "find(sub.." right?

   Can we say closures are anonymous coderefs that acesses a pre declared variable?

   Thanks in advance.

Ludwig

_________________________________________________
Philippine Linux Users' Group (PLUG) Mailing List
http://lists.linux.org.ph/mailman/listinfo/plug
Searchable Archives: http://archives.free.net.ph

Hi Ludwig,

"$total_size" is not out of scope because it is implicitly global  
since it was not declared with "my" in the subroutine.

If you remove the initial declaration, the code still works.

If, within the subroutine, you do...

	find (sub {my $total_size += -s if -f},'.');

... the code breaks, because "$total_size" would then have a local  
scope with each invocation of the subroutine.

Hope that made sense!

Rob

On 12 1, 09, at 10:56 PM, Ludwig Isaac Lim wrote:

> Hi:
>
>   I'm having some hard time understand perl closure:
>
>   Here's an explanation from Intermediate Perl:
>   "The kind of subroutine that can access lexical variables that  
> existed at the time we declare it is called a closure .....In Perl  
> terms, a closure is just a subroutine that references a lexical  
> variable that has gone out of scope".
>
>   and here is the sample code:
>   use File::Find;
>
>   my $total_size = 0;
>   find (sub {$total_size += -s if -f},'.');
>   print $total_size, "\n";
>
>
>    My question is "$total_size" is not out of scope since it is  
> declared before "find(sub.." right?
>
>   Can we say closures are anonymous coderefs that acesses a pre  
> declared variable?
>
>   Thanks in advance.
>
> Ludwig
>
>
>
>
>
>
> _________________________________________________
> Philippine Linux Users' Group (PLUG) Mailing List
> http://lists.linux.org.ph/mailman/listinfo/plug
> Searchable Archives: http://archives.free.net.ph

_________________________________________________
Philippine Linux Users' Group (PLUG) Mailing List
http://lists.linux.org.ph/mailman/listinfo/plug
Searchable Archives: http://archives.free.net.ph

First of all, may I apologize for my lack of perl-fu so I may be doing
this wrong.

On Tue, Dec 1, 2009 at 10:56 PM, Ludwig Isaac Lim <ludz_lim@...> wrote:
>    My question is "$total_size" is not out of scope since it is declared before "find(sub.." right?
Yes and no. Without strict, $total_size would still be in scope
because it would be initialized to the first value of +=. Declaring it
beforehand demonstrates the dynamic, not necessarily lexical scoping,
of Perl in that Perl subs can access variables outside of it.

A more useful example of lexical scoping makes a function that
generates other functions based on how you initialize it:
----
use strict;

sub hellofn {
	my $subject = @_[0];
	return sub { print "Hello $subject!\n" };
}

my $helloworld = hellofn ("World");
my $helloburrito = hellofn ("Burrito");

... some magic...

&$helloworld(); #says "Hello World!\n"
&$helloburrito(); #says "Hello Burrito!\n"
----
In the above, hellofn is a "closure" - we define it so that we can
generate other functions that work similarly. Think of it like a class
that, when instantiated, returns an object with a single public
method. Each "instantiation" of your closure has its own persistent
storage. Paticularly, the "subject" of the first hellofn is stored
separately from the subject of the next hellofn generated.

You can use closures to make counters, something like:
sub gcounter { my $count=0; return sub { print $count+=1, "\n" }}
$c1 = gcounter();
$c2 = gcounter();
&$c1(); &$c2(); &$c1(); &$c2(); &$c1(); &$c2();

gcounter is a closure that generates counters which increment a $count
variable. But the $count variables in the generated counters will
increment independently of each other. This largely depends on the
"my" making the $count variable private to gcounter and its subs.

>   Can we say closures are anonymous coderefs that acesses a pre declared variable?
Something like that.

I guess you could think of it as a kind of function template which
gives persistent storage. Closure'd variables are private to the
functions generated.

>
>   Thanks in advance.
>
> Ludwig
>
>
>
>
>
>
> _________________________________________________
> Philippine Linux Users' Group (PLUG) Mailing List
> http://lists.linux.org.ph/mailman/listinfo/plug
> Searchable Archives: http://archives.free.net.ph
>

--

-- 
This email is:    [ ] actionable   [ ] fyi        [ ] social
Response needed:  [ ] yes          [ ] up to you  [ ] no
Time-sensitive:   [ ] immediate    [ ] soon       [ ] none
_________________________________________________
Philippine Linux Users' Group (PLUG) Mailing List
http://lists.linux.org.ph/mailman/listinfo/plug
Searchable Archives: http://archives.free.net.ph

Hi!

On Tue, Dec 1, 2009 at 10:56 PM, Ludwig Isaac Lim <ludz_lim <at> yahoo.com> wrote:
> Hi:
>
>   I'm having some hard time understand perl closure:
>
>   Here's an explanation from Intermediate Perl:
>   "The kind of subroutine that can access lexical variables that existed at the time we declare it is
called a closure .....In Perl terms, a closure is just a subroutine that references a lexical variable
that has gone out of scope".
>
>   and here is the sample code:
>   use File::Find;
>
>   my $total_size = 0;
>   find (sub {$total_size += -s if -f},'.');
>   print $total_size, "\n";
>
>
>    My question is "$total_size" is not out of scope since it is declared before "find(sub.." right?

Not out of scope in where find() is, but it is out of scope in
find()'s code where the coderef gets evaluated.

>   Can we say closures are anonymous coderefs that acesses a pre declared variable?

Usually, but not always.

Closures in Perl are best illustrated using BLOCKs, e.g.

{
    my $scalar = "Just Another Perl Hacker";

    sub get_scalar { return $scalar }
}

{
    my $scalar = "Yet Another Perl Hacker";

    use feature ':5.10'
    say $scalar;        # "Yet Another Perl Hacker"
    say get_scalar;     # "Just Another Perl Hacker"
}

Notice the above two blocks.  Both blocks contain a definition of the
the lexical $scalar, but the first block contains a named subroutine
that gets its own "closed" $scalar.  The first block's $scalar is
inaccessible by any code outside of the block, except through the
subroutine.  So, as the latter block contains its own $scalar
variable, it can still get the value of the other $scalar through the
subroutine closure.

Another example of closures relates to their use as encapsulations for
an object's implementation.  Consider a simple Point object: here's a
classic implementation:

package Point::Simple;

sub new {
    my ( $class, $x, $y ) = shift;
    my $self = {
        x => $x,
        y => $y,
    };
    return bless $self => $class;
}

sub x { return $_[0]->{x} }
sub y { return $_[0]->{y} }

1;

And here's a closure-based implementation:

package Point::Closure;

sub new {
    my ( $class, $x, $y ) = @_;
    my $self = {
        x => undef,
        y => undef,
    };
    my $c = sub {
        my $field = shift;
        if (@_) { $self->{$field} = shift }
        return    $self->{$field};
    };
    return bless $c => $class;
}

sub x { $self->("x", @_[ 1 .. $#_ ] ) }
sub y { $self->{"y", @_[ 1 .. $#_ ] ) }

1;

Granted, the Point::Closure implementation looks more complex, but on
the plus side, it adds a place where one can do some parameter
validation or preprocessing before anything else.  Say, one could
sprinkle some Carp::cluck calls in new() to add some boundary checks.

(As an aside, object-oriented programming in Perl is now a lot
simpler, thanks to Moose and friends.)

--

-- 
Zak B. Elep  ||  zakame.net
1486 7957 454D E529 E4F1  F75E 5787 B1FD FA53 851D
_________________________________________________
Philippine Linux Users' Group (PLUG) Mailing List
http://lists.linux.org.ph/mailman/listinfo/plug
Searchable Archives: http://archives.free.net.ph
On Tue, Dec 1, 2009 at 10:56 PM, Ludwig Isaac Lim <ludz_lim@...> wrote:
>
>   Can we say closures are anonymous coderefs that acesses a pre declared variable?

Nahuli na ba ako?  In Perl, a closure is just a subroutine that has
access to persistent private variables. It can either be a named subroutine
or anonymous subroutine.

A named closure is usually used to get the same effect of static variables
in C and it's usually enclosed in a BEGIN{} block to make sure that
the private variables they refer to are available at compile time. The BEGIN{}
block is not needed for subroutines that are part of a package (Perl modules),
because it will be evaluated at compile time when the package is use-d.

HTH
_________________________________________________
Philippine Linux Users' Group (PLUG) Mailing List
http://lists.linux.org.ph/mailman/listinfo/plug
Searchable Archives: http://archives.free.net.ph

sshfs is nice
_________________________________________________
Philippine Linux Users' Group (PLUG) Mailing List
http://lists.linux.org.ph/mailman/listinfo/plug
Searchable Archives: http://archives.free.net.ph

_________________________________________________
Philippine Linux Users' Group (PLUG) Mailing List
http://lists.linux.org.ph/mailman/listinfo/plug
Searchable Archives: http://archives.free.net.ph

Mon	Tue	Wed	Thu	Fri	Sat	Sun

	1	2	3	4	5	6
7	8	9	10	11	12	13
14	15	16	17	18	19	20
21	22	23	24	25	26	27
28	29	30	31

16	January 2012
13	December 2011
35	November 2011
38	October 2011
109	September 2011
68	August 2011
90	July 2011
66	June 2011
41	May 2011
69	April 2011
124	March 2011
139	February 2011
145	January 2011
104	December 2010
27	November 2010
58	October 2010
105	September 2010
79	August 2010
84	July 2010
74	June 2010
38	May 2010
41	April 2010
60	March 2010
86	February 2010
95	January 2010
32	December 2009
107	November 2009
384	October 2009
96	September 2009
45	August 2009
257	July 2009
149	June 2009
143	May 2009
376	April 2009
278	March 2009
139	February 2009
143	January 2009
137	December 2008
149	November 2008
189	October 2008
248	September 2008
180	August 2008
341	July 2008
353	June 2008
350	May 2008
282	April 2008
305	March 2008
347	February 2008
250	January 2008
360	December 2007
336	November 2007
538	October 2007
437	September 2007
448	August 2007
501	July 2007
430	June 2007
432	May 2007
430	April 2007
628	March 2007
541	February 2007
347	January 2007
556	December 2006
341	November 2006
528	October 2006
644	September 2006
528	August 2006
650	July 2006
508	June 2006
544	May 2006
437	April 2006
618	March 2006
936	February 2006
729	January 2006
392	December 2005
370	November 2005
484	October 2005
681	September 2005
498	August 2005
371	July 2005
574	June 2005
433	May 2005
866	April 2005
730	March 2005
355	February 2005
435	January 2005
470	December 2004
542	November 2004
589	October 2004
732	September 2004
539	August 2004
777	July 2004
430	June 2004
372	May 2004
518	April 2004
418	March 2004
556	February 2004
309	January 2004
238	December 2003
648	November 2003
464	October 2003
160	September 2003
1	September 2002
1	August 2002
1	June 2002

Hardware is hard - The End Of The CrunchPad

Sent to you by Silverlokk via Google Reader:

Hardware is hard - The End Of The CrunchPad

Things you can do from here:

A 5-year-old girl installs Ubuntu

Re: A 5-year-old girl installs Ubuntu

Question about Perl Closure [OT]

Re: Question about Perl Closure [OT]

Re: Question about Perl Closure [OT]

Re: Question about Perl Closure [OT]

Re: Question about Perl Closure [OT]

tool appreciation - sshfs

Re: Invitation to the IBM Innovation Center Launch at UP-Ayala Technohub