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13936 :: A game entity system built with concepts, generic procs, and mixins: Great idea or abomination?

• Charles (orginal) [2026-05-21T17:25:51+02:00] view original

  I've been building my game engine in Nim for quite a while and it's coming along well. Last night it occurred to me that I could replace the component system using Nim concepts instead.

  I don't have a specific question, I'm just curious what you guys think of this as I've almost never used concepts before. Is this a terrible idea?

  The behaviour is defined in distinct modules:

  movement.nim

  
  type
    Positionable = concept p
      p.position is Vec3
      p.direction is Vec3
    
    Movable = concept m, var mv
      m is Positionable
      m.velocity is Vec3
    
    IsGrounded = concept g
      g.heightOffTheFloor is float32
  
  template implementsPositionable* {.dirty.} =
    position: Vec3
    direction: Vec3
  
  template implementsMovable* {.dirty.} =
    position: Vec3
    direction: Vec3
    velocity: Vec3
  
  template implementsGrounded* {.dirty.} =
    heightOffTheFloor: float32
  
  proc applyInputs[T: Movable](m: var T, vel: Vec3) =
    m.velocity = vel
  
  proc applyMovement[M: Movable](m: var M, dt: float) =
    m.position += m.velocity * dt
  
  proc applyGrounding*[T: Positionable and IsGrounded](entity: var T, bvh: Node) =
    entity.position = bvh.getHeight(entity.position) + [0'f32, entity.heightOffTheFloor, 0].Vec3
  Run

  And then I have an entity macro that allows me to create mixins of sorts:

  
  import std/macros
  import ../navigation/movement
  import ../animation/state_machine
  {.experimental: "dynamicBindSym".}
  
  macro entity*(name: untyped, body: untyped): untyped =
    let fields = newNimNode(nnkRecList)
    
    for item in body:
      case item.kind
      of nnkIdent, nnkSym: # Behaviour name (implementsMovable)
        let sym = bindSym(item.strVal)
        let impl = sym.getImpl()
        let templateBody = impl[^1]
        for field in templateBody:
          if field.kind == nnkCall:
            fields.add newIdentDefs(postfix(field[0], "*"), field[1][0])
      of nnkCall: # field: Type
        fields.add newIdentDefs(postfix(item[0], "*"), item[1][0])
      else:
        error "Unexpected node in entity body: " & $item.kind, item
    
    let objTy = newNimNode(nnkObjectTy)
    objTy.add newEmptyNode()
    objTy.add newEmptyNode()
    objTy.add fields
    
    let typeDef = newNimNode(nnkTypeDef)
    typeDef.add postfix(name, "*")
    typeDef.add newEmptyNode()
    typeDef.add objTy
    
    let typeSection = newNimNode(nnkTypeSection)
    typeSection.add typeDef
    
    result = typeSection
  #   echo result.repr
  Run

  And the implementation of the concepts can be defined just like a type with the templates from the relevant behaviour modules:

  
  entity Character:
    implementsMovable
    implementsGrounded
    someOtherField: int
  Run

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