Re: Why *not* use UNIVERSAL qw( isa can ) ; ??

Message-Id: <20011113105620.B24192@sizzle.whoville.com>
(Test applied with minor tweaks and ->isa("HASH") omission.)

p4raw-id: //depot/perl@12975

diff --git a/lib/UNIVERSAL.pm b/lib/UNIVERSAL.pm
index a9cb478..92b4fcd 100644 (file)
--- a/lib/UNIVERSAL.pm
+++ b/lib/UNIVERSAL.pm
@@ -4,10 +4,13 @@ our $VERSION = '1.00';
 
 # UNIVERSAL should not contain any extra subs/methods beyond those
 # that it exists to define. The use of Exporter below is a historical
-# accident that should be fixed sometime.
+# accident that can't be fixed without breaking code.  Note that we
+# *don't* set @ISA here, don't want all classes/objects inheriting from
+# Exporter.  It's bad enough that all classes have a import() method
+# whenever UNIVERSAL.pm is loaded.
 require Exporter;
 *import = \&Exporter::import;
-@EXPORT_OK = qw(isa can);
+@EXPORT_OK = qw(isa can VERSION);
 
 1;
 __END__
@@ -18,32 +21,62 @@ UNIVERSAL - base class for ALL classes (blessed references)
 
 =head1 SYNOPSIS
 
-    $io = $fd->isa("IO::Handle");
-    $sub = $obj->can('print');
+    $is_io = $fd->isa("IO::Handle");
+    $is_io = Class->isa("IO::Handle");
 
-    $yes = UNIVERSAL::isa($ref, "HASH");
+    $sub = $obj->can("print");
+    $sub = Class->can("print");
+
+    use UNIVERSAL qw( isa can VERSION );
+    $yes = isa $ref, "HASH" ;
+    $sub = can $ref, "fandango" ;
+    $ver = VERSION $obj ;
 
 =head1 DESCRIPTION
 
 C<UNIVERSAL> is the base class which all bless references will inherit from,
-see L<perlobj>
+see L<perlobj>.
 
-C<UNIVERSAL> provides the following methods
+C<UNIVERSAL> provides the following methods and functions:
 
 =over 4
 
-=item isa ( TYPE )
+=item $obj->isa( TYPE ), CLASS->isa( TYPE ), isa( VAL, TYPE )
+
+    C<TYPE> is a package name
+    $obj is a blessed reference or a string containing a package name
+    C<CLASS> is a package name
+    C<VAL> is any of the above or an unblessed reference
+
+When used as an instance or class method (C<$obj->isa( TYPE )>), C<isa>
+returns I<true> if $obj is blessed into package C<TYPE> or inherits from
+package C<TYPE>.
+
+When used as a class method (C<CLASS->isa( TYPE )>; sometimes referred to as a
+static method), C<isa> returns I<true> if C<CLASS> inherits from (or is itself)
+the name of the package C<TYPE> or inherits from package C<TYPE>.
+
+When used as a function, like
+
+   use UNIVERSAL qw( isa ) ;
+   $yes = isa $h, "HASH";
+   $yes = isa "Foo", "Bar";
 
-C<isa> returns I<true> if C<REF> is blessed into package C<TYPE>
-or inherits from package C<TYPE>.
+or
 
-C<isa> can be called as either a static or object method call.
+   require UNIVERSAL ;
+   $yes = UNIVERSAL::isa $a, "ARRAY";
 
-=item can ( METHOD )
+, C<isa> returns I<true> in the same cases as above and also if C<VAL> is an
+unblessed reference to a perl variable of type C<TYPE>, such as "HASH",
+"ARRAY", or "Regexp".
 
-C<can> checks if the object has a method called C<METHOD>. If it does
-then a reference to the sub is returned. If it does not then I<undef>
-is returned.
+=item $obj->can( METHOD ), CLASS->can( METHOD ), can( VAL, METHOD )
+
+C<can> checks if the object or class has a method called C<METHOD>. If it does
+then a reference to the sub is returned. If it does not then I<undef> is
+returned.  This includes methods inherited or imported by C<$obj>, C<CLASS>, or
+C<VAL>.
 
 C<can> cannot know whether an object will be able to provide a method
 through AUTOLOAD, so a return value of I<undef> does not necessarily mean
@@ -54,7 +87,13 @@ will still return a code reference, which, when called, will fall through
 to the AUTOLOAD. If no suitable AUTOLOAD is provided, calling the coderef
 will cause an error.
 
-C<can> can be called as either a static or object method call.
+C<can> can be called as a class (static) method, an object method, or a
+function.
+
+When used as a function, if C<VAL> is a blessed reference or package name which
+has a method called C<METHOD>, C<can> returns a reference to the subroutine.
+If C<VAL> is not a blessed reference, or if it does not have a method
+C<METHOD>, I<undef> is returned.
 
 =item VERSION ( [ REQUIRE ] )
 
@@ -63,42 +102,9 @@ package the object is blessed into. If C<REQUIRE> is given then
 it will do a comparison and die if the package version is not
 greater than or equal to C<REQUIRE>.
 
-C<VERSION> can be called as either a static or object method call.
-
-=back
-
-The C<isa> and C<can> methods can also be called as subroutines
-
-=over 4
-
-=item UNIVERSAL::isa ( VAL, TYPE )
-
-C<isa> returns I<true> if one of the following statements is true.
-
-=over 8
-
-=item *
-
-C<VAL> is a reference blessed into either package C<TYPE> or a package
-which inherits from package C<TYPE>.
-
-=item *
-
-C<VAL> is a reference to a C<TYPE> of Perl variable (e.g. 'HASH').
-
-=item *
-
-C<VAL> is the name of a package that inherits from (or is itself)
-package C<TYPE>.
-
-=back
-
-=item UNIVERSAL::can ( VAL, METHOD )
+C<VERSION> can be called as either a class (static) method, an object method or
+or a function.
 
-If C<VAL> is a blessed reference which has a method called C<METHOD>,
-C<can> returns a reference to the subroutine.   If C<VAL> is not
-a blessed reference, or if it does not have a method C<METHOD>,
-I<undef> is returned.
 
 =back
 

diff --git a/t/op/universal.t b/t/op/universal.t
index efda2a5..f8c15d7 100755 (executable)
--- a/t/op/universal.t
+++ b/t/op/universal.t
@@ -9,7 +9,7 @@ BEGIN {
     $| = 1;
 }
 
-print "1..87\n";
+print "1..90\n";
 
 $a = {};
 bless $a, "Bob";
@@ -46,7 +46,12 @@ $Alice::VERSION = 2.718;
 package main;
 
 { my $i = 2;
-  sub test { print "not " unless shift; print "ok $i\n"; $i++; }
+  sub test {
+      print "not " unless $_[0];
+      print "ok ", $i++;
+      print "# at ", (caller)[1], ", line ", (caller)[2] unless $_[0];
+      print "\n";
+  }
 }
 
 $a = new Alice;
@@ -115,10 +120,12 @@ test ! (eval { $a->VERSION(2.719) }) &&
 test (eval { $a->VERSION(2.718) }) && ! $@;
 
 my $subs = join ' ', sort grep { defined &{"UNIVERSAL::$_"} } keys %UNIVERSAL::;
+## The test for import here is *not* because we want to ensure that UNIVERSAL
+## can always import; it is an historical accident that UNIVERSAL can import.
 if ('a' lt 'A') {
-    test $subs eq "can isa VERSION";
+    test $subs eq "can import isa VERSION";
 } else {
-    test $subs eq "VERSION can isa";
+    test $subs eq "VERSION can import isa";
 }
 
 test $a->isa("UNIVERSAL");
@@ -152,3 +159,12 @@ test ! UNIVERSAL::can($b, "can");
 test ! $a->can("export_tags"); # a method in Exporter
 
 test ! UNIVERSAL::isa("\xff\xff\xff\0", 'HASH');
+
+{
+    package Pickup;
+    use UNIVERSAL qw( isa can VERSION );
+
+    main::test isa "Pickup", UNIVERSAL;
+    main::test can( "Pickup", "can" ) == \&UNIVERSAL::can;
+    main::test VERSION "UNIVERSAL" ;
+}