apecs

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apecs is an Entity Component System inspired by specs and Entitas. It exposes a DSL that translates to fast storage operations, resulting in expressivity without sacrificing performance or safety.

There is an example below, and a tutorial can be found here. For a physics engine written on top of apecs, check out phycs. For a general introduction to ECS, see this talk or here.

Performance

Performance is good. Running ecs-bench shows that apecs is competitive with the fastest Rust ECS frameworks.

	pos_vel build	pos_vel step	parallel build	parallel step
apecs	239	34	777	371
calx	261	21	442	72
constellation	306	10	567	256
froggy	594	13	1565	97
specs	753	38	1016	205

There is a performance guide here.

Example

import Apecs
import Apecs.Stores (Cache)
import Apecs.Concurrent (prmap)
import Linear

newtype Position = Position (V2 Double) deriving Show
-- Turn Position into a component by specifiying the type of its Storage
instance Component Position where
  -- The simplest store is a Map
  type Storage Position = Map Position

newtype Velocity = Velocity (V2 Double)
instance Component Velocity where
  -- We can add a Cache for faster access
  type Storage Velocity = Cache 100 (Map Velocity)

data Player = Player -- A single constructor component for tagging the player
instance Component Player where
  -- Unique contains at most one component. See the Stores module.
  type Storage Player = Unique Player

-- Generate a world type and instances
makeWorld "World" [''Position, ''Velocity, ''Player]

game :: System World ()
game = do
  -- Create new entities
  ety <- newEntity (Position 0)
  -- Components can be composed using tuples
  newEntity (Position 0, Velocity 1)
  newEntity (Position 1, Velocity 1, Player)

  -- set (over)writes components
  set ety (Velocity 2)

  let stepVelocity (Position p, Velocity v) = Position (v+p)

  -- Side effects
  liftIO$ putStrLn "Stepping velocities"
  -- rmap maps a pure function over all entities in its domain
  rmap stepVelocity
  -- prmap n does the same, but in parallel
  prmap 2 stepVelocity

  -- Print all positions
  cmapM_ $ \(Position p) -> liftIO (print p)

main :: IO ()
main = initWorld >>= runSystem game