type Stream[T] = concept # interface
  func head(_: Self): T
  func tail(_: Self): Self

proc display[T](stream: Stream[T]) =
  stdout.write stream.head
  stream.tail.display

func consumes(a: Stream[char]) =
  discard

type StreamImpl = seq[char]

func head(stream: StreamImpl): char = stream[0]

func tail(stream: StreamImpl): StreamImpl = stream


let a: StreamImpl = @['h', 'e', 'l', 'l', 'o']


consumes(a)
# works for what it's worth