History.elm

module History where

type History state value =
  { state | undo   : History s a -> Maybe (History s a)
          , redo   : History s a -> Maybe (History s a)
          , add    : a -> History s a -> History s a
          , value  : History s a -> Maybe a
          }

type Infinite a = { past : [a], future : [a] }

infinite : History (Infinite a) a
infinite =
  { past = []
  , future = []
  , undo h =
      case h.past of
        [] -> Nothing
        x::xs -> Just { h | past <- xs, future <- x :: h.future }
  , redo h =
      case h.future of
        [] -> Nothing
        x::xs -> Just { h | past <- x :: h.past, future <- xs }
  , add x h =
      { h | past <- x :: h.past, future <- [] }
  , value h =
      case h.past of
        x::_ -> Just x
        [] -> Nothing
  }