Bitwise, List and type alias

Allergies
Allergies in Elm 
type alias AllergyAndScore = {
  allergy: Allergy;
  bitPosition: Int;
}

allergiesAndScores : List AllergyAndScore
allergiesAndScores =
    [ AllergyAndScore Eggs 1
    , AllergyAndScore Peanuts 2
    , AllergyAndScore Shellfish 3
    , AllergyAndScore Strawberries 4
    , AllergyAndScore Tomatoes 5
    , AllergyAndScore Chocolate 6
    , AllergyAndScore Pollen 7
    , AllergyAndScore Cats 8
    ]

toList : Int -> List Allergy
toList score =
  allergiesAndScores
    |> List.map (\{ bitPosition, allergy } -> ( 2 ^ (bitPosition - 1), allergy ))
    |> List.filter (\( s, _ ) -> Bitwise.and s score > 0)
    |> List.map Tuple.second

isAllergicTo : Allergy -> Int -> Bool
isAllergicTo allergy score =
  toList score |> List.member allergy

In this approach

This approach uses a type alias to represent the bit position and the allergy.

The approach is very similar to the Bitwise and List approach, but uses a type alias to make the bit position explicit. It could also be based on the Bitwise and Dict approach.

When to use this approach?

This is the best approach for embracing the domain concept of using bit positions in the allergy score to represent a list of Allergy, but otherwise shares most of the same properties as the Bitwise and List approach it is based on.

The only difference is that the bit positions are hard coded and explicit.

This means that this approach does not guarantee that all bit positions are valid.

It also means that this approach doesn't guarantee that all bit positions are sequential. This is potentially useful, and means that if there was ever a need for non sequential bit positions, we could support it, but it does also make it easier to make a mistake if the bit positions are required to be sequential.

11th Sep 2024 · Found it useful?

Other Approaches to Allergies in Elm

Other ways our community solved this exercise
allergies = [ Eggs, Peanuts, Shellfish, Strawberries, Tomatoes, Chocolate, Pollen, Cats]

toList : Int -> List Allergy
toList score =
  allergies
    |> List.indexedMap (\i allergy -> ( 2 ^ i, allergy ))
    |> List.filter (\( allergyScore, _ ) -> Bitwise.and allergyScore score > 0)
    |> List.map Tuple.second
Bitwise and List

Use the index of list items to represent the bit position of the allergy. 

allergies : Dict Int Allergy
allergies = 
  Dict.fromList [(1, Eggs), (2, Peanuts), (3, Shellfish), (4, Strawberries), (5, Tomatoes), (6, Chocolate), (7, Pollen), (8, Cats)]

toList : Int -> List Allergy
toList score =
  Dict.foldr (\key value list -> if Bitwise.and (2 ^ (key - 1)) score > 0 then (value :: list) else list) [] allergies
Bitwise and Dict

Use the key in a Dict to represent the bit position of the allergy. 

toList : Int -> List Allergy
toList score =
  [ 1, 2, 4, 8, 16, 32, 64, 128 ]
  |> List.filterMap ( \allergenInt -> intToAllergen (Bitwise.and score allergenInt))
Bitwise and case expression

Use a case expression to return the bit position of the allergy.