All new material, this time about one of the fundamental functional datastructures, the Linked List, and the overview of an implementation in Moosey Perl. This covers some of the same material as https://github.com/osfameron/pure-fp-book but perhaps with more explanation (and covering much less material - it was only a 20 minute talk)

1. Functional Pe(a)rls
osfameron @ IPW2011, Turin
the “purely functional data structures”
edition
http://www.fickr.com/photos/jef_saf/3493852795/

2. previously on Functional Pe(a)rls...
(IPW, LPW, YAPC::EU, nwe.pm)
currying
operator references: op(+)
Acme::Monads
Devel::Declare

3. C I A O
vs.
C I A O

4. 0 1 2 3
C I A O

5. 0 1 2 3
C I A O
0th element

6. 0 1 2 3
M I A O
0th element

7. 0 1 2 3
M I A O
0th element
no kittens were harmed
during the making of
this presentation

8. 0 1 2 3
M I A O
2nd element

9. 0 1 2 3
M I A O
4th element

10. 0 1 2 3
M I A O
max: 3

11. 0 1 2 3
M I A O
max: 3

12. 0 1 2 3 4
M I A O W
max: 4
4th element

13. max: 3
0 1 2 3 100,000
M I A O
…

14. 0 1 2 3
M I A O
max: 3

15. 0 1 2 3 4
M I A O W
max: 4
0 1 2 3 4 5
M I A O W !
max: 5

16. 0 1 2 3 4
M I A O W
max: 4
0 1 2 3 4 5
M I A O W !
max: 5

17. Arrays
C I A O
vs.
C I A O

18. Perl @arrays
C I A O
“dynamic array”
vs.
C I A O

19. C I A O

20. C I A O

21. C I A O
Head

22. C I A O
Tail

23. C I A O

24. C I A O

25. I A O

26. A O

27. O

28. C I A O
0th

29. C I A O
nth - 1

30. C I A O
nth[2]? nth[1]?

31. C I A O
nth[2]? nth[0]!

32. C I A O
?

33. ●
C I
tail “ciao” → “iao”
A O
?

34. ●
C I
tail “ciao” → “iao”
A O
?
● tail “iao” → “ao”
● tail “ao” → “o”
● tail “o” → ?

35. ●
C I
tail “ciao” → “iao”
A O
?
● tail “iao” → “ao”
● tail “ao” → “o”
● tail “o” → “” (the empty string)

36. C I A O

37. List =
Head Tail
(another List)
or...

38. Here comes the science^wPerl!

39. Moose(X::Declare)
use MooseX::Declare;
BEGIN { role_type 'List' }
role List {
requires 'head';
requires 'tail';
}

40. List::Link
class List::Link with List {
has head => (
is => 'ro',
isa => 'Any'
);
has tail => (
is => 'ro',
isa => 'List'
),
}

41. List::Link
class List::Link with List {
has head => (
is => 'ro',
isa => 'Any'
);
has tail => (
is => 'ro',
isa => 'List'
),
}

42. List::Link
class List::Link with List {
has head => (
is => 'ro',
isa => 'Any'
);
has tail => (
is => 'ro',
isa => 'List'
),
}

43. List::Empty
class List::Empty with List {
method head {
die "Can't take head of empty list!"
}
method tail {
die "Can't take tail of empty list!"
}
}

44. So we can write:
my $list = List::Link->new(
head => 'c',
tail => List::Link->new(
head => 'i',
tail => List::Link->new(...

45. Sugar!
my $list = List->fromArray(
qw/ c i a o /);

46. Multimethods
use MooseX::MultiMethods;
multi method fromArray ($class:) {
return List::Empty->new;
}

47. Multimethods
use MooseX::MultiMethods;
multi method fromArray ($class:) {
return List::Empty->new;
}

48. Multimethods
multi method fromArray ($class: $head, @tail) {
return List::Link->new(
head => $head,
tail => $class->fromArray(@tail),
);
}

49. Eeek! Recursion!
my $list = List::fromArray(1..1000000);

50. Eeek! Recursion!
my $list = List::fromArray(1..1000000);
Deep recursion on subroutine
"List::fromArray" at foo.pl line 20

51. Eeek! Recursion!
multi method fromArray ($class: $head, @tail) {
return List::Link->new(
head => $head,
tail => $class->fromArray(@tail),
);
}

52. Eeek! Recursion!
fromArray

53. Eeek! Recursion!
fromArray
List::Link->new
(..., fromArray)

54. Eeek! Recursion!
fromArray
List::Link->new
(..., fromArray)
List::Link->new
(..., fromArray)

55. Eeek! Recursion!
fromArray
List::Link->new
(..., fromArray)
List::Link->new
(..., fromArray)
List::Link->new
(..., fromArray)

56. Eeek! Recursion!
fromArray
List::Link->new
(..., fromArray)
List::Link->new
(..., fromArray)
List::Link->new
(..., fromArray)
List::Link->new
(..., fromArray)

57. Eeek! Recursion!
fromArray
List::Link->new
(..., fromArray)
List::Link->new
(..., fromArray)
List::Link->new
(..., fromArray)
List::Link->new
(..., fromArray) = $list

58. Eeek! Recursion!
fromArray
List::Link->new
(..., fromArray)
List::Link->new
(..., fromArray)
List::Link->new
(..., fromArray) = $list

59. Eeek! Recursion!
fromArray
List::Link->new
(..., fromArray)
List::Link->new
(..., fromArray) = $list

60. Eeek! Recursion!
fromArray
List::Link->new
(..., fromArray) = $list

61. Eeek! Recursion!
fromArray = $list

62. Eeek! Recursion!
no warnings 'recursion';
$DB::deep = 100_000_000;

63. Eeek! Recursion!
no warnings 'recursion';
$DB::deep = 100_000_000;
Papering over the cracks

64. Tail Call Elimination
Sub::Call::Tail
Sub::Call::Recur
by nothingmuch

65. Tail call elimination
fromArray

66. Tail call elimination
List::Link->new
(..., fromArray)

67. Tail call elimination
List::Link->new
(..., fromArray)

68. Tail call elimination
List::Link->new
(..., fromArray)

69. Tail call elimination
List::Link->new
(..., fromArray)

70. Tail call elimination
List::Link->new
(..., fromArray) = $list

71. Tail Call Elimination
use Sub::Import 'Sub::Call::Tail'
tail => { -as => 'tail_call' };
multi method fromArray ($self: $head, @tail)
{
tail_call List::Link->new(
head => $head,
tail => $self->fromArray(@tail),
);
}

72. Indexing into List
multi method nth
(List::Empty $self: Int $pos)
{
die "Can't index into an Empty list";
}

73. Indexing into List
multi method nth
(Int $pos where { $_ == 0 })
{
return $self->head;
}

74. Indexing into List
multi method nth
(Int $pos where { $_ > 0 })
{
tail_call $self->tail->nth( $pos - 1 );
}

75. C I A O
M

76. C I A O
M W

77. C I A O
M W
Mutation leads to bugs
(and misspellings!)

78. C I A O
M I A O W

79. C I A O
C I B

80. C I A O
C I B
Copy everything upstream of a change
Downstream of changes can be shared

81. C I A O
C I B O
Doubly linked lists have no downstream!

82. pure-fp-book
● https://github.com/osfameron/pure-fp-book
● Purely Functional Data Structures for the...
● Impure
● Perl Programmer
● Working Programmer
● Mutable, Rank-Scented Many