Example of phantom type in Elm

phantom.elm

module Main exposing (main)

import Html exposing (Html, text)


type Distance a
    = Distance Float


type Kilometer
    = Kilometer


type Mile
    = Mile


marathonDistance : Distance Kilometer
marathonDistance =
    Distance 42.195


distanceKmToMiles : Distance Kilometer -> Distance Mile
distanceKmToMiles (Distance km) =
    Distance (0.621371 * km)


marathonDistanceInMiles : Distance Mile
marathonDistanceInMiles =
    distanceKmToMiles marathonDistance


main : Html msg
main =
    text (distanceKmToMiles marathonDistanceInMiles)