test.nim

type
  RefNode = ref object
    children: ref seq[string]

  Node = ref object
    children: seq[string]


proc main() =
  #var a = RefNode(children: new seq[string])
  var a = RefNode()

  echo("a:", a.repr) # a:ref 00578028 --> [children = nil]

  # Doesn't allocate a the seq[string] object, just a pointer pointing to nil
  new(a.children)

  echo "a:", a.repr # a:ref 00328028 --> [children = ref 00328038 --> nil]

  # a.children.add("Hello World")
  # echo "a:", a.repr

  var b = Node(children: @[])
  echo "b:", b.repr # b:ref 00438048 --> [children = 00438058[]]
  b.children.add("Hello World")

  # Change in object address. Was the previous "children" object copied
  # modified and then reassigned?
  echo "b:", b.repr # b:ref 00578048 --> [children = 0057c368[0057c3c8"Hello World"]]


when isMainModule:
  main()

Here is the output I get:

~/temp_code » nim c -r foo
Hint: system [Processing]
Hint: foo [Processing]
Hint:  [Link]
Hint: operation successful (9891 lines compiled; 0.217 sec total; 14.148MB; Debug Build) [SuccessX]
a:ref 0x10ae23050 --> [children = nil]

a:ref 0x10ae23050 --> [children = ref 0x10ae23068 --> nil]

b:ref 0x10ae23080 --> [children = 0x10ae24050[]]

b:ref 0x10ae23080 --> [children = 0x10ae2c050[0x10ae28530"Hello World"]]