The document discusses various techniques for extending and improving Perl, including both good and potentially evil techniques. It covers Perl modules that port Perl 6 features to Perl 5 like given/when switches and state variables. It also discusses techniques for runtime introspection and modification like PadWalker and source filters. The document advocates for continuing to extend Perl 5 with modern features to keep it relevant and powerful.

1. Good Evils in Perl
Kang-min Liu <gugod@gugod.org>
YAPC::Asia::2009

2. Perl

3. get things done

4. glue language

5. TIMTOWTDI
There is more then one way to do it

6. the good Perl

7. pragma

8. warnings
gives you good warning messages

9. Can anyone tell me if
there’s any problem in
this small program ?
foo.pl
#!/usr/bin/perl -l
print $foo;
print "Hello";

10. #!/usr/bin/perl -l
print $foo;
print "Hello";

11. #!/usr/bin/perl -l
use warnings;
print $foo;
print "Hello";

12. #!/usr/bin/perl -l
use warnings;
print $foo;
print "Hello";
Use of uninitialized value $foo in print

13. #!/usr/bin/perl -l
use warnings;
print $foo;
print "Hello";

14. it runs!

15. (it should break)

16. $foo is undeclared

17. use strict;

18. it breaks your
program

19. in a nice way :-D

20. feature pragma
Perl 5.10

21. ← Perl6

22. use feature;

23. use feature ‘:5.10’

24. use 5.010;

25. switch
given ($foo) {
when (1) { say "$foo == 1" }
when ([2,3]) {
say "$foo == 2 || $foo == 3"
}
when (/^a[bc]d$/) {
say "$foo eq 'abd' || $foo eq 'acd'"
}
when ($_ > 100) { say "$foo > 100" }
default { say "None of the above" }
}

26. state variables
sub counter {
state $counts = 0;
$counts += 1;
}

27. say
say "hi";

28. say
print "hin";

29. Perl6::*
Perl6 functions implemented in Perl5

30. Perl6::Junctions
any, all, one, none

31. Q:
How to test if an
array contains a
specific value ?

32. Does @a
contains 42 ?

33. my $found = 0;
for my $a (@a) {
if ($a == 42) {
$found = 1;
last;
}
}
if ($found) {
...
}

34. if ( grep { $_ == 42 } @a ) {
...
}

35. if ( grep /^42$/ @a ) {
...
}

36. use Perl6::Junction qw/ all any none one /;

37. if ( any(@ar) == 42 ) {
...
}

38. if ( all(@ar) > 42 ) {
...
}

39. if (none(@ar) > 42 ) {
...
}

40. if ( one(@ar) > 42 ) {
...
}

41. any(values %params) == undef
html form validation

42. any(@birthdays) < str2time("1980/01/01")

43. Can anyone see what it
does now ?
Can anyone write a
nested loop version in 10
seconds ?
if ( any(@a) == any(@b) ) {
...
}

44. • Perl6::Junction (any, all)
• Perl6::Perl
• Perl6::Builtins (system, caller)
• Perl6::Form
• Perl6::Gather

45. autobox

46. my $range = 10−>to(1);
# => [ 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 ]

47. "Hello, world!"−>uc();
# => "HELLO, WORLD!"

48. TryCatch
first class try catch semantics

49. sub foo {
eval {
# some code that might die
return "return value from foo";
};
if ($@) {
...
}
}

50. sub foo {
try {
# some code that might die
return "return value from foo";
}
catch (Some::Error $e where { $_->code > 100 } ) {
...
}
}

51. Try::Tiny
minimal

52. Sub::Alias
easier function alias

53. sub name { "gugod" }
alias get_name => 'name';
alias getName => 'name';

54. self
my $self = shift;

55. package MyClass;
sub myMethod {
my $self = shift;
...
}

56. package MyClass;
use self;
sub myMethod {
...
}

57. Moose
postmodern OO

58. package Point;
use Moose;
has 'x' => (is => 'rw', isa => 'Int');
has 'y' => (is => 'rw', isa => 'Int');
sub clear {
my $self = shift;
$self->x(0);
$self->y(0);
}

59. MooseX::Declare

60. class BankAccunt {
has 'balance' => (
isa => 'Num', is => 'rw', default => 0
);
method deposit (Num $amount) {
$self->balance( $self−>balance + $amount );
}
method withdraw (Num $amount) {
my $current_balance = $self−>balance();
( $current_balance >= $amount )
|| confess "Account overdrawn";
$self->balance( $current_balance − $amount );
}
}

61. Template::Declare

62. h1 {
attr { id => "lipsum" };
outs "Lorem Ipsum";
};
# => <h1 id="lipsum">Lorem Ipsum></h1>

63. Markapl

64. h1(id => "lipsum") {
outs "Lorem Ipsum";
};
# => <h1 id="lipsum">Lorem Ipsum></h1>

65. Rubyish

66. package Cat;
use Rubyish;
attr_accessor "name", "color";
def sound { "meow, meow" }
def speak {
print "A cat goes " . $self−>sound . "n";
}

67. ☺

68. the evil Perl

69. sub prototype

70. grep

71. grep { ... } ...

72. map

73. map { ... } ...

74. sub doMyWork {
my ($arr1, $arr2) = @_;
my @arr1 = @$arr1;
my @arr2 = @$arr2;
...
}
doMyWork(@foo, @bar);

75. sub doMyWork(@@) {
my ($arr1, $arr2) = @_;
my @arr1 = @$arr1;
my @arr2 = @$arr2;
...
}
doMyWork(@foo, @bar);

76. many

77. if (many { $_ > 50 } @arr) {
...
}

78. sub many(&@) {
my ($test_sub, @arr) = @_;
...
}

80. AUTOLOAD

81. sub AUTOLOAD {
my $program = $AUTOLOAD;
$program =~ s/.*:://;
system($program, @_);
}
date();
who('am', 'i');
ls('−l');

82. Android.pm
sub AUTOLOAD {
my ($method) = ($AUTOLOAD =~ /::(w+)$/);
return if $method eq 'DESTROY';
# print STDERR "$0: installing proxy method '$method'n";
my $rpc = rpc_maker($method);
{
# Install the RPC proxy method, we will not came here
# any more for the same method name.
no strict 'refs';
*$method = $rpc;
}
goto &$rpc; # Call the RPC now.
}

83. Source Filter

84. package BANG;
use Filter::Simple;
FILTER {
s/BANGs+BANG!!!/die 'BANG' if $BANG/g;
};
1;

85. use Acme::Morse;
.--.-..--..---.-.--..--.-..--..---.-.--.
.-.-........---..-..---.-..-.--..---.--.
..-.---......-...-...-..--..-.-.-.--.-..
----..-.-.--.-..--..-.-...---.-..---.--.
.-...-..--.---...-.-....

86. Module::Compile

87. perl -MModule::Compile Foo.pm
# => Foo.pmc

88. DB
inheritable built-in debugger

89. # from self.pm
sub _args {
my $level = 2;
my @c = ();
package DB;
@c = caller($level++)
while !defined($c[3]) || $c[3] eq '(eval)';
return @DB::args;
}
sub self { (_args)[0] }

90. # from self.pm
sub _args {
my $level = 2;
my @c = ();
package DB;
@c = caller($level++)
while !defined($c[3]) || $c[3] eq '(eval)';
return @DB::args;
}
sub self { (_args)[0] }

91. Furthermore, when called from within the DB package, caller
returns more detailed information: it sets the list variable
@DB::args to be the arguments with which the subroutine was
invoked.
# from self.pm – perldoc caller
sub _args {
my $level = 2;
my @c = ();
package DB;
@c = caller($level++)
while !defined($c[3]) || $c[3] eq '(eval)';
return @DB::args;
}
sub self { (_args)[0] }

92. PadWalker
runtime stack traveler

93. sub inc_x {
my $h = peek_my(1);
${ $h->{'$x'} }++;
}

94. Binding
easier PadWalker (Rubyish)

95. use Binding;
sub inc_x {
my $b = Binding->of_caller;
$b->eval('$x + 1');
}
sub two {
my $x = 1;
inc_x;
}

96. Devel::Declare
compile-time magician

97. Devel::Declare
compile-time magician
Florian Ragwitz
id:flora

98. Compile time
code injection

99. How it works
• you define “declarator” keywords
• it let compiler stop at the keywords
• your code parse the current line in your way,
maybe re-write it
• you replace current line with the new one
• resumes the compiler on the current line

100. def foo($arg1, $arg2) {
....
}

101. def foo($arg1, $arg2) {
....
}

102. def foo($arg1, $arg2) {
....
}
sub foo {
my ($arg1, $arg2) = @_;
}

103. B::*
more compile time fun

104. Thinking

105. Perl6 is perfect
by Larry Wall
youtube: “Larry Wall Speaks at Google”

106. very3 extensible

107. Perl6 are many
languages

108. Perl5

109. very0.5 extensible

110. • DB
• Devel::Declare, B::*
• prototype

111. Good Evil
Template::Declare prototype
Markapl Devel::Declare
self DB, B::*
TryCatch Devel::Declare
Try::Tiny prototype
some Perl6::* source filter

112. Perl is like the
Force. It has a light
side, a dark side,
and it holds the
universe together.
Larry Wall

114. Perl is old

115. It needs add some
“mondern” traits

116. Extend Perl5 with any
“Modern Sense” of a
modern programming
language.

117. Optimized for
reading

118. the better perl5

119. the extendable
perl5

120. PerlX::*

121. ps: PerlX::Range
is lazy!

122. Thanks for Listening
http://youtube.com/gugod/ for cat videos