Create a map of sets in Elm

DictOfSets.elm

{--
  Some Elm code for a map of sets. This maps prices to the set of items
  with that price.
  
  Run at http://elm-lang.org:1234/try
--}

import Html exposing (text, div, br)
import Dict exposing (Dict)
import Set exposing (Set)

type alias ItemSet = Set String                -- set of named items
type alias ItemsByPrice = Dict Int ItemSet     -- map prices to sets of item names

-- Add item to existing set for that price, or create a new set if there are no existing ones
addItem: String -> Maybe ItemSet -> Maybe ItemSet
addItem newItem existingItems =
  case existingItems of
    Just existingItems -> Set.insert newItem existingItems |> Just
    Nothing -> Set.fromList [newItem] |> Just
    
initial: ItemsByPrice
initial = Dict.empty

-- add $2 apple and $3 pear
-- Note that (addItem "value") is a curried function in the form that update expects
-- in this case of type: Maybe ItemSet -> Maybe ItemSet
couple = Dict.update 2 (addItem "apple") initial
  |> Dict.update 3 (addItem "pear")

-- add fruit with similar prices
similar = Dict.update 2 (addItem "banana") couple
  |> Dict.update 3 (addItem "orange")

main = div [] [
  "initial: " ++ toString initial |> text, br[][],
  "couple: " ++ toString couple |> text, br[][],
  "similar: " ++ toString similar |> text, br[][]
  ]

{--
  Should see the following output:
  
  initial: Dict.fromList []
  couple: Dict.fromList [(2,Set.fromList ["apple"]),(3,Set.fromList ["pear"])]
  similar: Dict.fromList [(2,Set.fromList ["apple","banana"]),(3,Set.fromList ["orange","pear"])]
--}