Source code for http://forum.nim-lang.org/t/673

ex.nim

#########################
# CODE INCLUDED BY BOTH
# COMPILATION UNITS.

# Trait definition
type DrawableVTable* = tuple[
  drawType: int,
  preDraw: proc(self: ref TObject): bool {.nimcall.},
  draw: proc(self: ref TObject) {.nimcall.}
  ]

# Fat pointer: (object with type erased, pointer to v-table).
type
  Drawable* = tuple[self: ref TObject, vtable: ptr DrawableVTable]

# Convenience functions for the trait.
proc drawType*(d: Drawable): int {.inline.} =
  result = d.vtable.drawType
proc preDraw*(d: Drawable): bool {.inline.} =
  result = d.vtable.preDraw(d.self)
proc draw*(d: Drawable) {.inline.} =
  d.vtable.draw(d.self)


#########################
# COMPILATION UNIT 1
# (dynamic library)

# Main library with general-purpose drawObject
# function which uses all drawable types.

type
  DrawingEngine* = object
    objs : seq[Drawable]

# Function using trait.
proc drawObjects(engine: DrawingEngine) =
  for obj in engine.objs:
    echo("----")
    if obj.preDraw():
      echo("drawType = " & $obj.drawType)
      obj.draw()

proc addDrawable*(engine: var DrawingEngine, obj: Drawable) =
  engine.objs.add(obj)

# Global engine. Should also be exposed through a trait.
var engine = DrawingEngine(objs: @[])


#########################
# COMPILATION UNIT 2
# 3-party plugin
# (dynamic library)

import strutils

# Object implementing that trait
type Circle = object
  x, y, r : float

proc preDrawC(self: ref TObject): bool {.procvar.} =
  # Some data-ready check.
  let circle = cast[ref Circle](self)
  echo("Pre-draw for circle (x: $1, y: $2, r: $3)")
  result = circle.r != 0

proc drawC(self: ref TObject) {.procvar.} =
  let circle = cast[ref Circle](self)
  echo(format("Drawing circle (x: $1, y: $2, r: $3)",
    circle.x, circle.y, circle.r))

proc toDrawable(circle: ref Circle): Drawable =
  # Global vtable for Circle's Drawable implementation.
  var CircleVTable {.global.} = DrawableVTable((
    drawType: 5,
    preDraw: preDrawC,
    draw: drawC))
  
  result = (cast[ref TObject](circle), addr CircleVTable)

proc newCircle(x, y, r: float): ref Circle =
  result.new
  result.x = x
  result.y = y
  result.r = r

# The drawing engine would be exposed.
var circle1 = newCircle(0, 0, 1)
var circle2 = newCircle(0, 0, 0)
engine.addDrawable(circle1.toDrawable())
engine.addDrawable(circle2.toDrawable())

#########################
# This would be called eventually within the engine itself.
engine.drawObjects()