Lens : Smart setter for immutable data

A implementation of lens library in Perl

Transcript

1. 2014೥8݄29೔ Lens : Smart setter for immutable data hiratara @ FreakOut
2. immutable data package Talk; use Moo; has speaker => ( is => ’ro’, requires => 1 ); has name => ( is => ’ro’, requires => 1 );
3. Nested immutable data getter works well $schedule->talk->speaker->name;
4. Nested immutable data setter won’t work well # Can’t write it $new_schedule = $schedule->talk->speaker ->name("hiratara");
5. Nested immutable data Must copy and copy and copy ... $new_speaker = Speaker->new( name => "hiratara", age => $speaker->age, mail => $speaker->age, ); $new_talk = Talk->new( title => $talk->title, speaker => $new_speaker, ); $new_schedule = Schedule->new( talk => $new_talk, kind => $schedule->kind, );
6. Ugly way to solve it Though, I hate mutable data. package Talk; use Moo; has speaker => ( is => ’rw’, requires => 1 ); has name => ( is => ’rw’, requires => 1 );
7. Ugly way to solve it
8. Another way : use Lens Lens acts like an accessor # getter (will get ’hiratara’) (talk . speaker . name)->($schedule); # setter $new_schedule = (talk . speaker . name)->($schedule, ’hiratara’);
9. What is Lens? • A library of Haskell • https://github.com/ekmett/lens/
10. Let’s implement Lens Lens is “costate comonad coalgebra”. We have only to implement it.
11. Let’s implement Lens Lens is “costate comonad coalgebra”. We have only to implement it.
12. Category theory Send e-mail to hira.tara+kenron@gmail.com if you want to buy this
13. What is co- co- means opposite or dual.
14. This is monad M ηM // D D D D D D D D D D D D D D D D MM µ  M Mη oo zzzzzzzz zzzzzzzz M MMM Mµ // µM  MM µ  MM µ // M
15. comonad is a dual of monad W WW ϵW oo Wϵ // W W EEEEEEEE EEEEEEEE δ OO y y y y y y y y y y y y y y y y WWW WW Wδ oo WW δW OO W δ oo δ OO
16. comonad in Perl package Comonad; sub counit; sub coflatten; sub map; sub coflat_map;
17. The type of state monad (A × S)S
18. The type of state monad (A × S)S ≃ (−S ◦ (− × S))(A)
19. costate comonad The dual of state monad AS × S ≃ ((− × S) ◦ −S)(A)
20. costate comonad in Perl package Comonad::Costate; use Moo; extends ’Comonad’; has func => (is => ’ro’); has state => (is => ’ro’); sub counit my $self = shift; $self->func
21. costate comonad in Perl sub map { my ($self, $f) = @_; (ref $self)->new( func => _comp($f, $self->func), state => $self->state, ); }
22. costate comonad in Perl sub coflatten { my $self = shift; my $class = ref $self; my $self_func = $self->func; $class->new( func => sub { my $state = shift; $class->new( func => $self_func, state => $state, ); }, state => $self->state, ); }
23. algebra of monad M MMX Mh // µX  MX h  MX h // X X ηX // C C C C C C C C C C C C C C C C MX h  X
24. coalgebra of comonad W WWX WX Wh oo WX δX OO X h oo h OO X C C C C C C C C C C C C C C C C WX ϵX oo X h OO
25. costate comonad coalgebra in Perl coalgebra => sub { my $data = shift; Comonad::Costate->new( func => sub { my $value = shift; (ref $data)->new( %$data, $field => $value, ) }, state => $data->$field, ); },
26. costate comonad coalgebra costate comonad coalgebra is the combination of setter and getter. This is Lens. A → (AS × S) ≃ (A → AS) × (A → S) • A → AS is setter • A → S is getter
27. composition of Lenses package Lens; use overload ( ’.’ => ’chain’, fallback => 1, ); sub chain { my ($self, $other) = @_; Lens->new( coalgebra => sub { ... }, ); }
28. Field lens sub lens ($) { my $field = shift; Lens->new( coalgebra => sub { my $data = shift; Comonad::Costate->new( func => sub { my $v = shift; (ref $data)->new(%$data, $field => $v) }, state => $data->$field, ); } ); }
29. Lens is more powerful $schedule = (talk . speaker . name)->($schedule, ’hiratara’); # Setter and Getter of a part of string $schedule = (talk . speaker . name . substr_lens(3, 2)) ->($schedule, ’@’); # $schedule->talk->speaker->name will be "hir@ara"
30. CONCLUSION • Lens is a good setter for immutable data • https://github.com/hiratara/p5-Lens To tell the truth, haskell’s lens implementation has the type: Let (−(V))V, −(S): SetSet → Set be functors (−(V))V ⇒ −(S)