ludic/docs/language/ecs/kw-query.md

id	name	category	kind	tokens	sig	tip	order
kw-query	query	ecs	keyword	query	query [PropA, PropB, {Tag}]	Match every model instance carrying all the listed properties.	2

A query selects every live model instance that carries all the listed properties, and is the core way handlers reach data. Use it in a for (…) in query [...] loop, where you write one loop variable per bound property in declaration order; each pass binds those properties for one matching instance and self() gives that instance. A {Model} term in tag position filters by model kind without binding a variable, so query [Position, {Player}] walks only players. Add a where clause to filter on field values (where health.current > 0). Matching is lazy and re-checked per instance as the loop reaches each id, not snapshotted, so spawns and despawns during the loop follow well-defined rules.

program Collisions {
  property Position { column: int = 0, row: int = 0 }
  property Health   { current: int = 100, maximum: int = 100 }
  model Enemy { Position, Health }

  handler Boot phase Start {
    spawn Grunt { Position { column: 5, row: 5 }; Health { current: 10 } }
  }

  handler DrawWorld phase Render {
    Screen.clear(Color.MidnightBlue)
    for (position, health) in query [Position, Health, {Enemy}] where health.current > 0 {
      Screen.fill_rectangle(x: position.column * 16, y: position.row * 16, width: 16, height: 16, color: Color.Crimson)
    }
    Screen.show()
  }
}