datatype lines = End | Line of string * source lines type t = { Head : source lines, Tail : source (source lines) } val create = head <- source End; tail <- source head; return {Head = head, Tail = tail} fun renderL lines = case lines of End => | Line (line, linesS) => {[line]}
and renderS linesS = lines <- signal linesS; return (renderL lines) fun render t = renderS t.Head fun write t s = oldTail <- get t.Tail; newTail <- source End; set oldTail (Line (s, newTail)); set t.Tail newTail