What are comparable types?

Problem

type FruitSort =
    Apple
    | Orange
    | Banana

type alias Fruit =
    { sort: FruitSort
    , name: String
    }

{-| Fruit needs to be "comparable". What do i need to do?
-}
type Basket =
    Dict Fruit Int

Solution

type FruitSort =
    Apple
    | Orange
    | Banana

fruitSortToInt : FruitSort -> Int
fruitSortToInt fruit =
    case fruit of
        Apple -> 1
        Orange -> 2
        Banana -> 3

fruitSortFromInt : Int -> FruitSort
fruitSortFromInt int =
   case int of
       1 -> Apple
       2 -> Orange
       3 -> Banana
       _ -> Apple --be careful, the compiler won't help you there.

type alias Fruit =
    { sort: FruitSort
    , name: String
    }

type alias Item =
    (Int,String)

itemFromFruit : Fruit -> Item
itemFromFruit {sort,name} =
    (sort |> fruitSortToInt,name)

itemToFruit : Item -> Fruit
itemToFruit (int,name) =
    { sort = int |> fruitSortFromInt
    , name = name
    }

type Basket =
    Dict Item Int

Question

Dict needs the first type to be comparable. What does that mean?

Answer

Int, Float, Char, String, Tuple comparable comparable and List comparable are comparable types. So you need to convert Fruit into a comparable type like Tuple Int String.

Further reading

• 📦Package: Use any type with turboMaCk/any-dict.

• 📦Package: use any type with turboMaCk/any-set.