# just as you can have a variable that contains strings and numbers,
# you can have a variable that contains subroutines.

my $subroutine = sub { print "Hello\n" };

# you can call this subroutine like so:

  $subroutine->();
  &$subroutine();

# you can also put a named subroutine into a variable

sub hello { print "Hi there!\n"; }
my $hello = \&hello;
&$hello();

# if you use a variable that is lexically bound to the scope
# the subroutine is defined in, you get what's called a closure

my @subs;
foreach my $adress ( "Sir", "Ma'am" )
{
    #each instance of the sub gets its own $adress 
    push @subs, sub { print "Good day $adress\n" }; 
}

$subs[0]->();
$subs[1]->();  

# if you call a subroutine that returns a subroutine, 
# you have what's called a factory

sub make_greeter {
  my $timeoday = shift;
  return sub { print "Good $timeoday\n" };
}

my $greeter = make_greeter("morning");
$greeter->();

# if you pass a subrouine off to some other code, 
# which promises to CALL that subroutine BACK 
# at some later point in time / when some event happens, 
# you have what's called a callback.
# Depending on the calling code, 
# the callback might or might not receive arguments.

sub handle_get_response {
  my $response = shift;
  # ...
}

my $get_response_handler = \&handle_get_response;

# When the request is done SomeWebClient will call $get_response_handl
+er->();
SomeWebClient->get( url => $get_response_handler );
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