One attempt at simple adding or updating a record in a list in Elm

Update.elm

import UpdateList exposing (addOrUpdateById)

update msg model =
  case msg of
    NewOrUpdatedItem item ->
      ({model | items = model.items |> addOrUpdateById item}, Cmd.none)

UpdateList.elm

module UpdateList exposing (addOrUpdateById)

type alias Identifyable a = { a | id : Int }

addOrUpdateById : Identifyable a -> List (Identifyable a) -> List (Identifyable a)
addOrUpdateById record list =
  if isNewRecord record list then
    List.concat [ [ record ], list ]
  else
    List.map (updateById record) list

updateById : Identifyable a -> Identifyable a -> Identifyable a
updateById record r =
  if r.id == record.id then
    record
  else
    r

isNewRecord : Identifyable a -> List (Identifyable a) -> Bool
isNewRecord record list =
  not
    (
      list
      |> List.map (\r -> r.id)
      |> List.member record.id
    )