ludic/examples/lang/qdecl.ludic

# ============================================================================
# qdecl.ludic — a handler whose query lives in a @Queries annotation.
#
#   @Queries(these: [Battle{hp <= 0 and side == 1}, Pos], on: Foe)
#   handler CleanBattle phase LateUpdate { … }
#
# is the same thing as writing `for (Battle, Pos) in query [Battle, Pos, {Foe}]
# where Battle.hp <= 0 and Battle.side == 1 { … }` around the whole body — the
# loop header moves into the annotation. Each property binds by name, the body
# runs once per matching entity, and self() is that entity.
# ============================================================================
program QueryDecl {
  property Battle { hp: int = 0, side: int = 0 }
  property Pos { x: int = 0, y: int = 0 }
  model Foe { Battle, Pos }

  handler Seed phase Start {
    spawn Foe { Battle { hp: 3, side: 1 } }
    spawn Foe { Battle { hp: 0, side: 1 } }
    spawn Foe { Battle { hp: -2, side: 0 } }
  }

  # a per-property constraint `Battle{...}` qualifies its bare fields to that
  # property; `on: Foe` adds the {Foe} kind filter.
  @Queries(these: [Battle{hp <= 0 and side == 1}, Pos], on: Foe)
  handler CleanBattle phase LateUpdate {
    print(Pos.x)
    despawn self()
  }

  @Queries(these: [Battle])
  handler Census phase Render {
    print(Battle.hp)
  }

  handler Bye phase Render { quit() }
}