in reply to Trying to Understand Callback(s)

# 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 );


You can lead your users to water, but alas, you cannot drown them.

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Re^2: Trying to Understand Callback(s)
by Laurent_R (Canon) on Nov 23, 2017 at 23:47 UTC
    Hi holli,

    congratulations for these very nice explanations.

    You really achieve to show quite a lot, and fairly clearly in my opinion, in a relatively short post. I really wish I could upvote your post a few more times. ;-)

Re^2: Trying to Understand Callback(s)
by DanielSpaniel (Scribe) on Nov 24, 2017 at 01:17 UTC
    Your post has definitely given me a much clearer idea. I really appreciate your time and effort in responding to me. I'll need to re-read the posts on here a few times more, but I'm very grateful for your reply, and the others of course too. Thank you very much!