type alias Props = { start: Date, stop: Date, assignee: String}
type Item
    = Task Props
    | SubTask Props
    | Bug Props
    | Epic Props
    | ...
assignTo: String -> Item -> Item
assignTo person item =
    let
        changeAssignee props = {props | assignee = person}
    in
        case item of
            Task p -> Task (changeAssignee p)
            SubTask p -> SubTask (changeAssigne p)
            ...