datatype dlist'' t = Nil | Cons of t * source (dlist'' t) datatype dlist' t = Empty | Nonempty of { Head : dlist'' t, Tail : source (source (dlist'' t)) } con dlist t = source (dlist' t) type position = transaction unit fun headPos [t] (dl : dlist t) = dl' <- get dl; case dl' of Nonempty { Head = Cons (_, tl), Tail = tl' } => cur <- get tl; set dl (case cur of Nil => Empty | _ => Nonempty {Head = cur, Tail = tl'}) | _ => return () fun tailPos [t] (cur : source (dlist'' t)) new tail = new' <- get new; set cur new'; case new' of Nil => set tail cur | _ => return () val create = fn [t] => source Empty fun clear [t] (s : dlist t) = set s Empty fun append [t] dl v = dl' <- get dl; case dl' of Empty => tl <- source Nil; tl' <- source tl; set dl (Nonempty {Head = Cons (v, tl), Tail = tl'}); return (headPos dl) | Nonempty {Tail = tl, ...} => cur <- get tl; new <- source Nil; set cur (Cons (v, new)); set tl new; return (tailPos cur new tl) fun render [ctx] [ctx ~ body] [t] f dl = | Nonempty {Head = hd, Tail = tlTop} => let fun render' prev dl'' = case dl'' of Nil => | Cons (v, tl) => let val pos = case prev of None => headPos dl | Some prev => tailPos prev tl tlTop in {f v pos} end in render' None hd end)}/> fun delete pos = pos fun elements' [t] (dl'' : dlist'' t) = case dl'' of Nil => return [] | Cons (x, dl'') => dl'' <- signal dl''; tl <- elements' dl''; return (x :: tl) fun elements [t] (dl : dlist t) = dl' <- signal dl; case dl' of Empty => return [] | Nonempty {Head = hd, ...} => elements' hd