nim dynamic dispatch test

dispatch.nim

import strformat

type Entity = ref object of RootObj
  tid: int   # tid uniquely identifies which subclass we are (e.g. Foo or Bar)

type Foo = ref object of Entity
  x, y: float
  name: string

type Bar = ref object of Entity
  health: int
  name: string


# constructors
proc newFoo(name:string):Foo = Foo(tid:0, name:name, x:5, y:6)
proc newBar(name:string):Bar = Bar(tid:1, name:name, health:100)

# each type has a different update method
proc updateFoo(self:Foo) = echo "Hello from Foo {self.name} ({self.x},{self.y})".fmt  
proc updateBar(self:Bar) = echo "Hello from Bar {self.name} ({self.health} hp)".fmt

type EntityFn = proc(e:Entity){.nimcall.}

# this allows you to look up the relevant 'update' procedure for a given type (based on tid)
const updateTbl:array[2, EntityFn] = [
  cast[EntityFn](updateFoo),
  cast[EntityFn](updateBar)
]

let entities = @[newFoo("fred"), newBar("bob")]

for e in entities:
  let update = updateTbl[e.tid]  # get the correct procedure for this type of entity
  update(e)