Stream.elm

module Stream where


import Dict exposing (Dict)


type alias Stream comparable v = Signal (comparable, v)


leftJoin : Stream comparable v1 -> Stream comparable v2 -> Stream comparable (v1, Maybe v2)
leftJoin stream1 stream2 =
  let
      buffer : Signal (Dict comparable v2)
      buffer = Signal.foldp step Dict.empty stream2

      step : (comparable, v2) -> Dict comparable v2 -> Dict comparable v2
      step (k, v2) = Dict.insert k v2

      lookup : (comparable, v1) -> Dict comparable v2 -> (comparable, (v1, Maybe v2))
      lookup (k, v1) state = (k, (v1, Dict.get k state))
  in
      Signal.map2 lookup stream1 buffer