anymap.nim

const componentRegistry = CacheSeq"anymap.componentRegistry"

proc makeField(n: NimNode): NimNode =
   let s = n.strVal
   result = ident(toLowerAscii(s[0]) & substr(s, 1))

macro component*(s: untyped): untyped =
   expectKind s, nnkTypeDef
   result = copyNimTree(s)
   componentRegistry.add result[0].basename

macro world*(s: untyped): untyped =
   expectKind s, nnkTypeDef
   result = copyNimTree(s)
   if result[2][2].kind == nnkEmpty:
      result[2][2] = newNimNode(nnkRecList)

   template makeStorage(component): untyped =
      seq[component]

   for c in items(componentRegistry):
      result[2][2].add newIdentDefs(c.makeField, getAst(makeStorage(c)))

when isMainModule:
   type
      Position {.component.} = object
         x, y: float

      Velocity {.component.} = object
         x, y: float

      Acceleration {.component.} = object
         x, y: float

      World {.world.} = object
         registry: seq[uint16]

worth using this pattern instead:

proc `[]`(a: AnyMap, t: typedesc[Position]): seq[Position] =
   result = a.data0

proc `[]`(a: var AnyMap, t: typedesc[Position]): var seq[Position] =
   result = a.data0

proc main =
   var a: AnyMap
   let ent1 = 0'u16

   a[Position].add Position(x: 0, y: 0)
   a[Velocity].add Velocity(x: 1, y: 0)

   a[Position][ent1].x += a[Velocity][ent1].x
   a[Position][ent1].y += a[Velocity][ent1].y

   echo a[Position][ent1].x