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diff --git a/Master/tlpkg/tlperl0/lib/pods/perlbot.pod b/Master/tlpkg/tlperl0/lib/pods/perlbot.pod new file mode 100755 index 00000000000..91723b70d78 --- /dev/null +++ b/Master/tlpkg/tlperl0/lib/pods/perlbot.pod @@ -0,0 +1,535 @@ +=head1 NAME + +perlbot - Bag'o Object Tricks (the BOT) + +=head1 DESCRIPTION + +The following collection of tricks and hints is intended to whet curious +appetites about such things as the use of instance variables and the +mechanics of object and class relationships. The reader is encouraged to +consult relevant textbooks for discussion of Object Oriented definitions and +methodology. This is not intended as a tutorial for object-oriented +programming or as a comprehensive guide to Perl's object oriented features, +nor should it be construed as a style guide. If you're looking for tutorials, +be sure to read L<perlboot>, L<perltoot>, and L<perltooc>. + +The Perl motto still holds: There's more than one way to do it. + +=head1 OO SCALING TIPS + +=over 5 + +=item 1 + +Do not attempt to verify the type of $self. That'll break if the class is +inherited, when the type of $self is valid but its package isn't what you +expect. See rule 5. + +=item 2 + +If an object-oriented (OO) or indirect-object (IO) syntax was used, then the +object is probably the correct type and there's no need to become paranoid +about it. Perl isn't a paranoid language anyway. If people subvert the OO +or IO syntax then they probably know what they're doing and you should let +them do it. See rule 1. + +=item 3 + +Use the two-argument form of bless(). Let a subclass use your constructor. +See L<INHERITING A CONSTRUCTOR>. + +=item 4 + +The subclass is allowed to know things about its immediate superclass, the +superclass is allowed to know nothing about a subclass. + +=item 5 + +Don't be trigger happy with inheritance. A "using", "containing", or +"delegation" relationship (some sort of aggregation, at least) is often more +appropriate. See L<OBJECT RELATIONSHIPS>, L<USING RELATIONSHIP WITH SDBM>, +and L<"DELEGATION">. + +=item 6 + +The object is the namespace. Make package globals accessible via the +object. This will remove the guess work about the symbol's home package. +See L<CLASS CONTEXT AND THE OBJECT>. + +=item 7 + +IO syntax is certainly less noisy, but it is also prone to ambiguities that +can cause difficult-to-find bugs. Allow people to use the sure-thing OO +syntax, even if you don't like it. + +=item 8 + +Do not use function-call syntax on a method. You're going to be bitten +someday. Someone might move that method into a superclass and your code +will be broken. On top of that you're feeding the paranoia in rule 2. + +=item 9 + +Don't assume you know the home package of a method. You're making it +difficult for someone to override that method. See L<THINKING OF CODE REUSE>. + +=back + +=head1 INSTANCE VARIABLES + +An anonymous array or anonymous hash can be used to hold instance +variables. Named parameters are also demonstrated. + + package Foo; + + sub new { + my $type = shift; + my %params = @_; + my $self = {}; + $self->{'High'} = $params{'High'}; + $self->{'Low'} = $params{'Low'}; + bless $self, $type; + } + + + package Bar; + + sub new { + my $type = shift; + my %params = @_; + my $self = []; + $self->[0] = $params{'Left'}; + $self->[1] = $params{'Right'}; + bless $self, $type; + } + + package main; + + $a = Foo->new( 'High' => 42, 'Low' => 11 ); + print "High=$a->{'High'}\n"; + print "Low=$a->{'Low'}\n"; + + $b = Bar->new( 'Left' => 78, 'Right' => 40 ); + print "Left=$b->[0]\n"; + print "Right=$b->[1]\n"; + +=head1 SCALAR INSTANCE VARIABLES + +An anonymous scalar can be used when only one instance variable is needed. + + package Foo; + + sub new { + my $type = shift; + my $self; + $self = shift; + bless \$self, $type; + } + + package main; + + $a = Foo->new( 42 ); + print "a=$$a\n"; + + +=head1 INSTANCE VARIABLE INHERITANCE + +This example demonstrates how one might inherit instance variables from a +superclass for inclusion in the new class. This requires calling the +superclass's constructor and adding one's own instance variables to the new +object. + + package Bar; + + sub new { + my $type = shift; + my $self = {}; + $self->{'buz'} = 42; + bless $self, $type; + } + + package Foo; + @ISA = qw( Bar ); + + sub new { + my $type = shift; + my $self = Bar->new; + $self->{'biz'} = 11; + bless $self, $type; + } + + package main; + + $a = Foo->new; + print "buz = ", $a->{'buz'}, "\n"; + print "biz = ", $a->{'biz'}, "\n"; + + + +=head1 OBJECT RELATIONSHIPS + +The following demonstrates how one might implement "containing" and "using" +relationships between objects. + + package Bar; + + sub new { + my $type = shift; + my $self = {}; + $self->{'buz'} = 42; + bless $self, $type; + } + + package Foo; + + sub new { + my $type = shift; + my $self = {}; + $self->{'Bar'} = Bar->new; + $self->{'biz'} = 11; + bless $self, $type; + } + + package main; + + $a = Foo->new; + print "buz = ", $a->{'Bar'}->{'buz'}, "\n"; + print "biz = ", $a->{'biz'}, "\n"; + + + +=head1 OVERRIDING SUPERCLASS METHODS + +The following example demonstrates how to override a superclass method and +then call the overridden method. The B<SUPER> pseudo-class allows the +programmer to call an overridden superclass method without actually knowing +where that method is defined. + + package Buz; + sub goo { print "here's the goo\n" } + + package Bar; @ISA = qw( Buz ); + sub google { print "google here\n" } + + package Baz; + sub mumble { print "mumbling\n" } + + package Foo; + @ISA = qw( Bar Baz ); + + sub new { + my $type = shift; + bless [], $type; + } + sub grr { print "grumble\n" } + sub goo { + my $self = shift; + $self->SUPER::goo(); + } + sub mumble { + my $self = shift; + $self->SUPER::mumble(); + } + sub google { + my $self = shift; + $self->SUPER::google(); + } + + package main; + + $foo = Foo->new; + $foo->mumble; + $foo->grr; + $foo->goo; + $foo->google; + +Note that C<SUPER> refers to the superclasses of the current package +(C<Foo>), not to the superclasses of C<$self>. + + +=head1 USING RELATIONSHIP WITH SDBM + +This example demonstrates an interface for the SDBM class. This creates a +"using" relationship between the SDBM class and the new class Mydbm. + + package Mydbm; + + require SDBM_File; + require Tie::Hash; + @ISA = qw( Tie::Hash ); + + sub TIEHASH { + my $type = shift; + my $ref = SDBM_File->new(@_); + bless {'dbm' => $ref}, $type; + } + sub FETCH { + my $self = shift; + my $ref = $self->{'dbm'}; + $ref->FETCH(@_); + } + sub STORE { + my $self = shift; + if (defined $_[0]){ + my $ref = $self->{'dbm'}; + $ref->STORE(@_); + } else { + die "Cannot STORE an undefined key in Mydbm\n"; + } + } + + package main; + use Fcntl qw( O_RDWR O_CREAT ); + + tie %foo, "Mydbm", "Sdbm", O_RDWR|O_CREAT, 0640; + $foo{'bar'} = 123; + print "foo-bar = $foo{'bar'}\n"; + + tie %bar, "Mydbm", "Sdbm2", O_RDWR|O_CREAT, 0640; + $bar{'Cathy'} = 456; + print "bar-Cathy = $bar{'Cathy'}\n"; + +=head1 THINKING OF CODE REUSE + +One strength of Object-Oriented languages is the ease with which old code +can use new code. The following examples will demonstrate first how one can +hinder code reuse and then how one can promote code reuse. + +This first example illustrates a class which uses a fully-qualified method +call to access the "private" method BAZ(). The second example will show +that it is impossible to override the BAZ() method. + + package FOO; + + sub new { + my $type = shift; + bless {}, $type; + } + sub bar { + my $self = shift; + $self->FOO::private::BAZ; + } + + package FOO::private; + + sub BAZ { + print "in BAZ\n"; + } + + package main; + + $a = FOO->new; + $a->bar; + +Now we try to override the BAZ() method. We would like FOO::bar() to call +GOOP::BAZ(), but this cannot happen because FOO::bar() explicitly calls +FOO::private::BAZ(). + + package FOO; + + sub new { + my $type = shift; + bless {}, $type; + } + sub bar { + my $self = shift; + $self->FOO::private::BAZ; + } + + package FOO::private; + + sub BAZ { + print "in BAZ\n"; + } + + package GOOP; + @ISA = qw( FOO ); + sub new { + my $type = shift; + bless {}, $type; + } + + sub BAZ { + print "in GOOP::BAZ\n"; + } + + package main; + + $a = GOOP->new; + $a->bar; + +To create reusable code we must modify class FOO, flattening class +FOO::private. The next example shows a reusable class FOO which allows the +method GOOP::BAZ() to be used in place of FOO::BAZ(). + + package FOO; + + sub new { + my $type = shift; + bless {}, $type; + } + sub bar { + my $self = shift; + $self->BAZ; + } + + sub BAZ { + print "in BAZ\n"; + } + + package GOOP; + @ISA = qw( FOO ); + + sub new { + my $type = shift; + bless {}, $type; + } + sub BAZ { + print "in GOOP::BAZ\n"; + } + + package main; + + $a = GOOP->new; + $a->bar; + +=head1 CLASS CONTEXT AND THE OBJECT + +Use the object to solve package and class context problems. Everything a +method needs should be available via the object or should be passed as a +parameter to the method. + +A class will sometimes have static or global data to be used by the +methods. A subclass may want to override that data and replace it with new +data. When this happens the superclass may not know how to find the new +copy of the data. + +This problem can be solved by using the object to define the context of the +method. Let the method look in the object for a reference to the data. The +alternative is to force the method to go hunting for the data ("Is it in my +class, or in a subclass? Which subclass?"), and this can be inconvenient +and will lead to hackery. It is better just to let the object tell the +method where that data is located. + + package Bar; + + %fizzle = ( 'Password' => 'XYZZY' ); + + sub new { + my $type = shift; + my $self = {}; + $self->{'fizzle'} = \%fizzle; + bless $self, $type; + } + + sub enter { + my $self = shift; + + # Don't try to guess if we should use %Bar::fizzle + # or %Foo::fizzle. The object already knows which + # we should use, so just ask it. + # + my $fizzle = $self->{'fizzle'}; + + print "The word is ", $fizzle->{'Password'}, "\n"; + } + + package Foo; + @ISA = qw( Bar ); + + %fizzle = ( 'Password' => 'Rumple' ); + + sub new { + my $type = shift; + my $self = Bar->new; + $self->{'fizzle'} = \%fizzle; + bless $self, $type; + } + + package main; + + $a = Bar->new; + $b = Foo->new; + $a->enter; + $b->enter; + +=head1 INHERITING A CONSTRUCTOR + +An inheritable constructor should use the second form of bless() which allows +blessing directly into a specified class. Notice in this example that the +object will be a BAR not a FOO, even though the constructor is in class FOO. + + package FOO; + + sub new { + my $type = shift; + my $self = {}; + bless $self, $type; + } + + sub baz { + print "in FOO::baz()\n"; + } + + package BAR; + @ISA = qw(FOO); + + sub baz { + print "in BAR::baz()\n"; + } + + package main; + + $a = BAR->new; + $a->baz; + +=head1 DELEGATION + +Some classes, such as SDBM_File, cannot be effectively subclassed because +they create foreign objects. Such a class can be extended with some sort of +aggregation technique such as the "using" relationship mentioned earlier or +by delegation. + +The following example demonstrates delegation using an AUTOLOAD() function to +perform message-forwarding. This will allow the Mydbm object to behave +exactly like an SDBM_File object. The Mydbm class could now extend the +behavior by adding custom FETCH() and STORE() methods, if this is desired. + + package Mydbm; + + require SDBM_File; + require Tie::Hash; + @ISA = qw(Tie::Hash); + + sub TIEHASH { + my $type = shift; + my $ref = SDBM_File->new(@_); + bless {'delegate' => $ref}; + } + + sub AUTOLOAD { + my $self = shift; + + # The Perl interpreter places the name of the + # message in a variable called $AUTOLOAD. + + # DESTROY messages should never be propagated. + return if $AUTOLOAD =~ /::DESTROY$/; + + # Remove the package name. + $AUTOLOAD =~ s/^Mydbm:://; + + # Pass the message to the delegate. + $self->{'delegate'}->$AUTOLOAD(@_); + } + + package main; + use Fcntl qw( O_RDWR O_CREAT ); + + tie %foo, "Mydbm", "adbm", O_RDWR|O_CREAT, 0640; + $foo{'bar'} = 123; + print "foo-bar = $foo{'bar'}\n"; + +=head1 SEE ALSO + +L<perlboot>, L<perltoot>, L<perltooc>. |