type Stream[T] = concept s, type S # interface
  s.head() is T
  s.tail is S

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

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

type StreamImpl[T] = seq[T] # list

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

func tail[T](stream: StreamImpl[T]): StreamImpl[T] =
  stream[1..^1] & stream[0] # dumb

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


consumes(a)
# works just fine