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Re: Trying to Understand Callback(s)

by shmem (Chancellor)
on Nov 24, 2017 at 00:29 UTC ( [id://1204184] : note . print w/replies, xml ) Need Help??


in reply to Trying to Understand Callback(s)

...but I don't really quite get the idea of callbacks, and particularly I don't understand the syntax.

Well, it's not just quite like with a phone - you ring up somebody, they hang up and call you - but rather a way to parametrize a subroutines execution chain via a subroutine reference (like a function pointer). The called sub may have been compiled into any context, and the compilation rules of that context apply. And what ikegami said.

I don't really get the first line of the get_callback routine either, when nothing appears to be being passed across

It gets something passed across but discards it.

A sub routine called as a method ($thingy->routine(@args)) by something ($thingy - irrelevant whether $thingy is a literal, a blessed reference, a scalar holding a string, or an expression which resolves to a package or a blessed reference) gets passed the invocant as its first parameter in @_.

So, having this preamble

sub routine { my($thingy, $tx) = @_; # $thingy is not used print "routine: $tx\n"; } my $ref = \&routine; # take a reference to the sub my $pkg = "main"; my $obj = bless do { \my $x }; # we could also say... # my $pkg = __PACKAGE__; # ...to get any package we are in in compile time

the following call expressions are all equivalent:

# method calls main->routine("foo"); main->$ref ("foo"); $pkg->routine("foo"); $pkg->$ref ("foo"); $obj->routine("foo"); $obj->$ref ("foo"); # or even (join'',map{chr}(109,97,105,110))->routine("foo"); __PACKAGE__->$ref("foo"); (bless \my $x)->$ref("foo"); # function calls routine ('main', "foo"); $ref -> ('main', "foo"); routine ($pkg, "foo"); $ref -> ($pkg, "foo"); &$ref ($pkg, "foo"); routine ($obj, "foo"); # etc.

More on differences between subroutine calls either as function or method: perlsub, perlmod, perlref.

The expression

my (undef, $thing) = @_;

is just a fancy way of saying

shift @_; # drop first element of argument list my ($thing) = @_;

The subroutine get_callback you quoted just seems not to care at all about it's invocant, but nonetheless expects to be called as a method. Hmm, code smell...

perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'

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Re^2: Trying to Understand Callback(s)
by DanielSpaniel (Scribe) on Nov 24, 2017 at 01:14 UTC

    Thank you all so much for your replies. I sincerely appreciate your time and efforts in making things clearer for me. I'll need to re-read a couple of times at least, but I have at least a much better understanding now. Thank you again.

      Well, it is a bit difficult to understand if you don't practice. I think you really need to start using these things in your code and make your own experiments to really grasp them. Once you get a working knowledge on them, you'll probably wonder how you could live without them before.

      If you're interesting in digging into these techniques further, please read this excellent book: https://hop.perl.plover.com/book/ by dominus. You can read it for free on-line, but I should warn you that you might end up willing to buy the paper copy. At least, that's what happen to me when I first read it about ten years ago.