Container.nim

####################################### Definition
type Container* = ref object
  list: seq[int]

proc newContainer(): Container =        # Explicit new
  new(result)
  result.list = @[]

proc newContainer(a: var Container) =   # Safe new
  if a == nil: a = newContainer()

proc length*(a: Container): int =       # Safe length
  return if a == nil: 0 else: a.list.len

proc isEmpty*(a: Container): bool =     # Safe isEmpty
  return a.length == 0

proc add*(a: var Container, x: int) =   # Safe add
  a.newContainer
  a.list.add x

iterator items*(a: Container): int =    # Safe iterator
  if not a.isEmpty:
    for i in a.list: yield i

####################################### Example
var a: Container                        # Nil, which represents a safe empty container, no need to check for nil

echo "is empty? ", a.isEmpty            #> true
for i in a: echo i                      # Nothing is echoed, no error

echo "Length is:", a.length             #> 0
a.add 5                                 # Nil until 5 was added
a.add 6                                 # Container at this point
echo "Length is:", a.length             #> 2

for i in a: echo i                      #> 5
                                        #> 6