closures and streams question

[grocery_stocker]

The following code generates even numbers..

[cdalten@localhost oakland]$ more even.pl
#!/usr/bin/perl -w

sub even_number_printer_gen {
# This function returns a reference to an anon. subroutine.
# This anon. subroutine prints even numbers starting from $input.
my($input) = @_;
if ($input % 2) { $input++}; # Next even number, if the given
# number is odd
$rs = sub {
print "$input "; # Using $input,which is a my variable
# declared in an outside scope
$input += 2;
};
return $rs; # Return a reference to the subroutine above
}

my $iterator = even_number_printer_gen(30);
# $iterator now points to a closure.
# Every time you call it, it prints the next successive even number.
for ($i = 0; $i < 10; $i++) {
&$iterator();
}
print "\n";

[cdalten@localhost oakland]$ ./even.pl
30 32 34 36 38 40 42 44 46 48
[cdalten@localhost oakland]$

The thing I'm not seeing why using clousres would be important in this
case. Can someone enlighten me?

 

[Jim Gibson]

The following code generates even numbers..

[cdalten@localhost oakland]$ more even.pl
#!/usr/bin/perl -w

sub even_number_printer_gen {
# This function returns a reference to an anon. subroutine.
# This anon. subroutine prints even numbers starting from $input.
my($input) = @_;
if ($input % 2) { $input++}; # Next even number, if the given
# number is odd
$rs = sub {
print "$input "; # Using $input,which is a my variable
# declared in an outside scope
$input += 2;
};
return $rs; # Return a reference to the subroutine above
}

my $iterator = even_number_printer_gen(30);
# $iterator now points to a closure.
# Every time you call it, it prints the next successive even number.
for ($i = 0; $i < 10; $i++) {
&$iterator();
}
print "\n";

[cdalten@localhost oakland]$ ./even.pl
30 32 34 36 38 40 42 44 46 48
[cdalten@localhost oakland]$

The thing I'm not seeing why using clousres would be important in this
case. Can someone enlighten me?

One reason would be so you could define two such generators. Try:

my $iter1 = even_number_printer_gen(30);
my $iter2 = even_number_printer_gen(100);
for (my $i = 0; $i < 10; $i++) {
&$iter1();
$iter2->();
}
print "\n";

__OUTPUT__

30 100 32 102 34 104 36 106 38 108 40 110 42 112 44 114 46 116 48 118

Because the subroutine is defined as a closure, there are two values of
the lexical variable $input defined instead of a single, shared one.

Note the alternate method of calling the second generator function.

 

[smallpond]

The following code generates even numbers..

[cdalten@localhost oakland]$ more even.pl
#!/usr/bin/perl -w

sub even_number_printer_gen {
    # This function returns a reference to an anon. subroutine.
    # This anon. subroutine prints even numbers starting from $input.
    my($input) = @_;
    if ($input % 2) { $input++};  # Next even number, if the given
                                  # number is odd
    $rs = sub {
        print "$input ";  # Using $input,which is a my variable
                                  # declared in an outside scope
        $input  += 2;
    };
    return $rs;   # Return a reference to the subroutine above

}

my $iterator = even_number_printer_gen(30);
# $iterator now points to a closure.
# Every time you call it, it prints the next successive even number.
for ($i = 0; $i < 10; $i++) {
    &$iterator();}

print "\n";

[cdalten@localhost oakland]$ ./even.pl
30 32 34 36 38 40 42 44 46 48
[cdalten@localhost oakland]$

The thing I'm not seeing why using clousres would be important in this
case. Can someone enlighten me?

perl 5.10 has static "state" variables, you only need the closure in
5.8.

Why is it important? Can you be enlightened? Those aren't perl
questions. As an exercise, maybe try to access and print the value
of
$input inside your for loop.

 

[Tad J McClellan]

The thing I'm not seeing why using clousres would be important in this
case. Can someone enlighten me?

The biggies are allowing multiple iterators and data encapsulation
as in the other followups.

See also:

http://en.wikipedia.org/wiki/Closure_(computer_science)