Instead of ADTs there are object variants in Nim: http://nim-lang.org/manual.html#object-variants

With ADTs:

  data Shape a =
    Rectangle
      { x :: Float
      , y :: Float
      , width :: Float
      , height :: Float
      }
    | Circle
      { x :: Float
      , y :: Float
      , radius :: Float
      }

With Object Variants, note that we don't have to repeat x and y:

  type
    ShapeKind = enum Rectangle, Circle
  
    Shape = object
      x, y: float
  
      case kind: ShapeKind
      of Rectangle: width, height: float
      of Circle: radius: float

The pattern matching in Nim is quite limited. Something like this works:

  let (x, y) = getCoordinates()

But Nim doesn't have (x,y) = (y,x) for example, instead swap can be used.

You could implement your own pattern matching using metaprogramming: http://www.drdobbs.com/open-source/nimrod-a-new-systems-prog...