type
  Loopable*[T] = concept x
    x.len() is int
    x[int] is T
    x.items is T

iterator loop*[T](a: Loopable[T]): T = # TODO cannot access fields in iterator
  for each in a:
    yield each

type
  Elem* = object
    baa*: string

proc foo(elems: seq[Elem]): string =
  for elem in elems.loop():
    result &= "\n  "
    result &= $(elem.baa) # TODO Error: undeclared field: 'baa'
    result &= "\n"

let elems = @[Elem(baa: "one"), Elem(baa: "two")]
echo foo(elems)