4 ms·

Even languages with heavy-weight MOPs do not offer a programming interface to unapply/uncompose, the reverse of: https://metacpan.org/dist/Moose/source/lib/Moo

by bmn__ 5y ago

Even languages with heavy-weight MOPs do not offer a programming interface to unapply/uncompose, the reverse of: https://metacpan.org/dist/Moose/source/lib/Moose/Meta/Role/Application/ToInstance.pm#L22 https://metacpan.org/dist/Moose/source/lib/Moose/Meta/Role/A... https://github.com/rakudo/rakudo/blob/master/src/Perl6/Metamodel/ClassHOW.nqp#L96 https://github.com/rakudo/rakudo/blob/master/src/Perl6/Metam... Reading the source codes, I think this is due to no one yet having requested the ability to do so. To me it feels like implementing this is possible, but tedious. The simple work-around for the problem would be to rebless the instance. Example: class Structure {} role Floatable {} role Inhabitable {} class Houseboat extends Structure with Floatable with Inhabitable {} $hb = Houseboat new $s = Structure new $s apply Floatable, Inhabitable # houseboat at run-time # stop swimming ### NYI ### $s unapply Floatable # make a new house that cannot swim $s1 = Structure new $s1 apply Inhabitable $s1 = $s rebless_into $s1

lizmat 5y ago
I guess in Raku you would create a Boat role, and you would create a clone of a House instance with the Boat role mixed in, when needed. In Raku speak: my $house = House.new; my $houseboat = $house but Boat; See: https://docs.raku.org/routine/but https://docs.raku.org/routine/but There would be no need to de-compose.
- fenomas 5y ago
  But note that the point of an ECS is being able to dynamically change roles without recreating new entities. If changing a house into a houseboat means creating a new entity, then everyone who had a reference to the house would need to be informed of the new reference.
  - lizmat 5y ago
    Doesn't de-composing have the same issue, though?
    - fenomas 5y ago
      I'm not sure I follow. In ECSs that I'm familiar with, entities are integer IDs, not class objects. So adding or removing components (roles) to an entity doesn't make use of any language-level features, it's just calls to the ECS library. E.g. in JS: var id = ecs.createEntity() // returns an int ecs.addComponent(id, 'house') ecs.addComponent(id, 'boat') So, very different from the class-based approach being floated a few comments ago. Does that answer your question?
      - raiph 5y ago
        Liz suggested `but`, but that creates a new entity with the role mixed in. Instead one can use `does` to mix in to an existing entity: role House {} role Boat {} my \id = 42; id does House; id does Boat; say id ~~ House; # True (What's missing is the ability to drop a mixin once it's been added via `does`.)

by bmn__ 5y ago

lizmat 5y ago

I guess in Raku you would create a Boat role, and you would create a clone of a House instance with the Boat role mixed in, when needed. In Raku speak: my $house = House.new; my $houseboat = $house but Boat; See: https://docs.raku.org/routine/but https://docs.raku.org/routine/but There would be no need to de-compose.

fenomas 5y ago
But note that the point of an ECS is being able to dynamically change roles without recreating new entities. If changing a house into a houseboat means creating a new entity, then everyone who had a reference to the house would need to be informed of the new reference.
- lizmat 5y ago
  Doesn't de-composing have the same issue, though?
  - fenomas 5y ago
    I'm not sure I follow. In ECSs that I'm familiar with, entities are integer IDs, not class objects. So adding or removing components (roles) to an entity doesn't make use of any language-level features, it's just calls to the ECS library. E.g. in JS: var id = ecs.createEntity() // returns an int ecs.addComponent(id, 'house') ecs.addComponent(id, 'boat') So, very different from the class-based approach being floated a few comments ago. Does that answer your question?
    - raiph 5y ago
      Liz suggested `but`, but that creates a new entity with the role mixed in. Instead one can use `does` to mix in to an existing entity: role House {} role Boat {} my \id = 42; id does House; id does Boat; say id ~~ House; # True (What's missing is the ability to drop a mixin once it's been added via `does`.)