I'm getting some weird error

harangzsolt33 has asked for the wisdom of the Perl Monks concerning the following question:

I'm using TinyPerl 5.8. Why am I getting an error in this script??? I am fiddling around with functions to see what happens when I modify my code. And according to the manual, this should work. And yet it doesn't work! Why?

Btw, I am trying to learn how to use "virtual functions" in Perl. Is there a better way than doing it like this?: my $Func = sub { ... };

use strict;
use warnings;

my $myFunction = sub { print "Hey, I am from Italy! @_\n"; };

printList(1, 2, 3, 4);

$myFunction = sub { print "Hey, I am from Japan! @_\n"; };

printList(5, 6, 7);


sub printList
{
  print "\nprintList was called.\n";
  &$myFunction->();
}
exit;

#
# This program should print:
#
# printList was called.
# Hey, I am from Italy! 1 2 3 4
# printList was called.
# Hey, I am from Japan! 5 6 7
#
[download]

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Re: I'm getting some weird error by jwkrahn (Abbot) on Dec 05, 2018 at 07:10 UTC
`&$myFunction->();` [download] That should be written as: `$myFunction->(@_);` [download]	[reply] [d/l] [select]
Re^2: I'm getting some weird error by harangzsolt33 (Deacon) on Dec 05, 2018 at 07:49 UTC
Thank you! Yes, that works now. But I don't understand why mine didn't work. It was saying `printList was called. Hey, I am from Italy! 1 2 3 4 Can't use string ("1") as a subroutine ref while "strict refs" in use +at C:\DESKTOP\test.pl line 16. Press any key to continue . . .` [download] I wasn't trying to use "1" as a sub ref. What is that all about???	[reply] [d/l]
Re^3: I'm getting some weird error by Corion (Patriarch) on Dec 05, 2018 at 08:13 UTC
You called your subroutine, and then tried to call its result again, as a subroutine: `&$subref; # this calls the subroutine &$subref->(); # this calls the subroutine and tries to call the result + again, as a subroutine` [download] Using `&` is a bad idea in most cases.	[reply] [d/l] [select]
Re^3: I'm getting some weird error by LanX (Saint) on Dec 05, 2018 at 12:28 UTC
Perl offers two ways to dereference (most) datatypes. with the right sigil like `&$code_ref(ARGS)` with a deref arrow + bracket `$code_ref->(ARGS)` Same for hashes and arrays. You mixed both (and Corion explained what happend) See perlreftut and References quick reference for more. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply] [d/l] [select]
Re: I'm getting some weird error by Corion (Patriarch) on Dec 05, 2018 at 07:10 UTC
What error do you get? What you are doing is not "virtual functions", but using anonymous functions. The subroutine stored in `$myFunction` does not have a name. Your syntax for calling the function is wrong/weird: `&$myFunction->()` [download] Most likely you wanted: `$myFunction->()` [download]	[reply] [d/l] [select]
Re: I'm getting some weird error by haukex (Archbishop) on Dec 05, 2018 at 12:05 UTC
Because you're not passing any arguments in `&$myFunction->()` but are expecting to see the arguments to `printList` get output, I'm guessing that perhaps you were trying to call the `sub` like one would in `&foo;`, which leaves the original `@_` unchanged (see perlsub). However, this is nowadays not considered a good practice, as leaving `@_` unchanged can also easily lead to mistakes. Plus, using the `&foo` syntax avoids prototype checking, which is also something that one should only do if one knows what one is doing. If you do want to make use of the `&` functionality for whatever reason, the correct syntax is `&$myFunction;` - but as the others have said, it's usually better to be explicit and use `$myFunction->(@_);` instead. See also perlreftut and perlref on using references. Is there a better way than doing it like this?: my $Func = sub { ... }; I'm not sure what you mean with "virtual functions" in this context, but `my $Func = sub { ... };` is basically "the" way to create an anonymous subroutine and store a reference to it.	[reply] [d/l] [select]