Using Enum as an integer?

Questions
1

I would like to use values defined inside an Enum as integers. These could be:

  1. used as values when needed
  2. index e.g. an array

Is it possible in Daml?

I seem not to be able to do that. E.g. I’m getting a parse error on the line:
assert $ length myArray == Unused_Last

Error wording:
Couldn't match expected type ‘Int’ with actual type ‘SkipassReductionType’

Here is my Enum type:

data SkipassReductionType
    = NoReduction
    | Youth
    | Child
    | Senior
    | Other
    | Unused_Last
        deriving (Eq, Show, Enum)

Thank you for any hints.

1 Like
2

Hi @mlyczak ,

First, I think it’s worth noting that DAML has only provides linked lists not arrays. Given the linear index time on lists, you are often better off, using maps instead of lists and at that point, you can use the enum type directly as the key type. DAML-LF 1.11 which was released as a preview version in SDK 1.9 includes a DA.Map module which sholud make this fairly easy. To target DAML-LF 1.11, you need to pass --target=1.11 to daml build or add build-options: ["--target=1.11"] to your daml.yaml.

Now, if you need an integer for another reason, you can, as you already noticed, derive the Enum typeclass. This provides you with two methods which you can use to convert back and forth. There are no implicit conversion in DAML, so you have to apply fromEnum everywhere you need an Int.

fromEnum  : Enum a => a -> Int
toEnum : Enum a => Int -> a
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3

Thank you @cocreature. This was very helpful.
Is there a difference between DA.Next.Map and DA.Map? I seem to get pointed in the docs to the former. When should I use the latter?

1 Like
4

DA.Map is only available in the latest LF version (which is still a preview in SDK 1.9) where we added a new Map primitive type with arbitrary key types. Older LF versions only provide a map indexed by Texts. DA.Next.Map wraps the text map provided you define how the type should be converted to and from Text in the MapKey instance. SDK 1.10 will deprecate DA.Next.Map in favor of DA.Map.

2 Likes