Example of a graph in Elm

routes.elm

import Html       exposing (text)
import List       exposing (concatMap, filter)
import List.Extra exposing (uniqueBy)
    
type alias CityId = Int
    
type alias City =
  { id: CityId
  , name: String
  }

type alias Route =
  { from: City
  , to: City
  , trafficLevel: Float
  }

type alias World =
  { cities: List City -- not really necessary, except perhaps as a memoization optimization
  , routes: List Route
  }
  
portland  = { id = 1, name = "Portland"  }
seattle   = { id = 2, name = "Seattle"   }
vancouver = { id = 3, name = "Vancouver" }

newWorld routes =
  { cities = citiesFromRoutes routes
  , routes = routes
  }

citiesFromRoutes : List Route -> List City
citiesFromRoutes routes =
  let 
    allCities = concatMap (\r -> [r.from, r.to]) routes
  in
    uniqueBy (\c -> c.id) allCities

world = newWorld
  [ { from = portland,  to = seattle,   trafficLevel = 4 }
  , { from = portland,  to = vancouver, trafficLevel = 3 }
  , { from = seattle,   to = portland,  trafficLevel = 2 }
  , { from = vancouver, to = portland,  trafficLevel = 0 }
  ]

routesFrom city world =
  filter (\r -> r.from.id == city.id) world.routes

fromPortland = routesFrom portland

routesBetween origin destination world =
  filter (\r -> r.from.id == origin.id && r.to.id == destination.id) world.routes

main = text (toString (routesBetween portland vancouver world))