How do I model inheritance in Haskell?

Question

I am attempting to create a game engine that is composed of a few different types:

data Camera = Camera ...
data Light = SpotLight ... | DirectionalLight ...         


        

Answer 1

You'll want to begin to depend on things like typeclasses and object encodings. The first method is to encode the common interface as a typeclass each type inherits from.

class PhysicalObject o where
  pos      :: o -> Vector3
  velocity :: o -> Vector3

The second is to build a common object

data PhysicalObject = PhysicalObject { poPos :: Vector3, poVelocity :: Vector3 }

data Monster = Monster { monsterPO :: PhysicalObject
                       , ... monsterStuff ...
                       }

which could even be used to instantiate the first typeclass

instance PhysicalObject PhysicalObject where
  pos      = poPos
  velocity = poVelocity

instance PhysicalObject Monster where
  pos      = pos      . monsterPO
  velocity = velocity . monsterPO

Be careful with typeclass encodings like this, though, as too great a use of them often causes ambiguity when reading code. It can be difficult to understand the types and know which instance is being used.