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Re^4: Is there a Perl API (XS) for ->can ?

by LanX (Sage)
on Oct 08, 2021 at 17:20 UTC ( #11137361=note: print w/replies, xml ) Need Help??


in reply to Re^3: Is there a Perl API (XS) for ->can ?
in thread Is there a Perl API (XS) for ->can ?

> You have to read between the lines a little bit

What about the WARNINGS part?

    NOTE: can directly uses Perl's internal code for method lookup, and isa uses a very similar method and cache-ing strategy. This may cause strange effects if the Perl code dynamically changes @ISA in any package.

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

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Re^5: Is there a Perl API (XS) for ->can ?
by chromatic (Archbishop) on Oct 10, 2021 at 16:47 UTC

    I think that affects both the method and function forms, but I don't have code that proves it at the moment.

      OK I hacked a little demo with three class FOO > BAR > BAZ with equally named methods

      I overrode ->can in the lower two by just delegation it to the SUPER and ignoring the methods in the same class. (dunno if that even makes sense)

      The demo shows that calling UNIVERSAL::can($obj,$sym) ignores the overriding ->can methods for the good and the better, it depends what you want.

      use strict; use warnings; use Data::Dump qw/pp dd/; package FOO { sub foo { warn __PACKAGE__."::foo";}; sub bar { warn __PACKAGE__."::bar";} sub baz { warn __PACKAGE__."::baz";} }; package BAR { our @ISA = qw/FOO/; sub bar { warn __PACKAGE__."::bar";} sub can { my ( $self,$sym ) = @_ ; warn "Calling can in ".__PACKAGE__." for ".Data::Dump::pp $sel +f; FOO->can($sym); } }; package BAZ { our @ISA = qw/BAR/; sub baz { warn __PACKAGE__."::baz";} sub can { my ( $self,$sym ) = @_ ; warn "Calling can in ".__PACKAGE__." for ".Data::Dump::pp $sel +f; BAR->can($sym); } sub new { my ( $self,@args ) = @_ ; bless {@args} , __PACKAGE__; } }; package main; use Test::More; my $obj = BAZ->new(name=>"OBJ"); for my $sym (qw/foo bar baz/) { diag "****** Trying can($sym)"; diag "... as ->meth\n"; my $meth = $obj->can($sym); $obj->$meth() if $meth; diag "... as sub()\n"; my $sub = UNIVERSAL::can($obj,$sym); $obj->$sub() if $sub; is($meth,$sub,"same for $sym"); } done_testing;

      -*- mode: compilation; default-directory: "d:/tmp/pm/" -*- Compilation started at Sun Oct 10 20:52:31 C:/Strawberry/perl/bin\perl.exe -w d:/tmp/pm/tst_can.pl # ****** Trying can(foo) # ... as ->meth Calling can in BAZ for bless({ name => "OBJ" }, "BAZ") at d:/tmp/pm/ts +t_can.pl line 36. Calling can in BAR for "BAR" at d:/tmp/pm/tst_can.pl line 22. FOO::foo at d:/tmp/pm/tst_can.pl line 8. # ... as sub() FOO::foo at d:/tmp/pm/tst_can.pl line 8. ok 1 - same for foo # ****** Trying can(bar) # ... as ->meth Calling can in BAZ for bless({ name => "OBJ" }, "BAZ") at d:/tmp/pm/ts +t_can.pl line 36. Calling can in BAR for "BAR" at d:/tmp/pm/tst_can.pl line 22. FOO::bar at d:/tmp/pm/tst_can.pl line 9. # ... as sub() BAR::bar at d:/tmp/pm/tst_can.pl line 18. not ok 2 - same for bar # Failed test 'same for bar' # at d:/tmp/pm/tst_can.pl line 67. # got: 'CODE(0x6c5730)' # expected: 'CODE(0x67f920)' # ****** Trying can(baz) # ... as ->meth Calling can in BAZ for bless({ name => "OBJ" }, "BAZ") at d:/tmp/pm/ts +t_can.pl line 36. Calling can in BAR for "BAR" at d:/tmp/pm/tst_can.pl line 22. FOO::baz at d:/tmp/pm/tst_can.pl line 10. # ... as sub() BAZ::baz at d:/tmp/pm/tst_can.pl line 32. not ok 3 - same for baz # Failed test 'same for baz' # at d:/tmp/pm/tst_can.pl line 67. # got: 'CODE(0x6c57d8)' # expected: 'CODE(0x26cbc28)' 1..3 # Looks like you failed 2 tests of 3. Compilation exited abnormally with code 2 at Sun Oct 10 20:52:32

      UPDATE

      I think overriding ->can is a very tricky business.

      • $obj->baz()

        tells me

        BAZ::baz at d:/tmp/pm/tst_can.pl line 32

      • my $meth =$obj->can('baz'); $obj->$meth();

        tells me

        FOO::baz at d:/tmp/pm/tst_can.pl line 10.

      so overriding ->can should only be used to "publish" NEW dynamic methods via AUTOLOAD and not trying to hide static ones.

      Cheers Rolf
      (addicted to the Perl Programming Language :)
      Wikisyntax for the Monastery

        I think overriding ->can is a very tricky business.

        I think the biggest problem here is hard-coding the invocant. You wouldn't do that in a constructor, would you?

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