Form fields for union types

[nmk]

Assume one needs form fields for the following type Account:

type alias CompanyName = String

type Account
    = Private
    | Business CompanyName

I assume the correct form for the UI would be a radio selection between Private and Business, with a text input for the company name appearing if Business is selected.

What would the correct approach be with elm-simple-form?

The part which is unclear to me is how to set up the validation for have one field depend on another.

[etaque]

Gosh, I'm afraid I don't have any satisfying answer for this, sorry... Not possible to do dependant validations at the moment.
We could look at how Json.Decode does that, implementation should be similar.

[nmk]

I did something like this:

type alias CompanyName =
    String


type AccountType
    = Private
    | Business CompanyName


type alias Registration =
    { email : String
    , accountType : AccountType
    }

validateRegistration : Validation () Registration
validateRegistration =
    let
        company =
            "company" := oneOf [ string, emptyString ]
    in
        Validate.succeed Registration
            |: ("email" := email)
            |: (("accountType" := validateAccountType) |: company)


validateAccountType : Validation e (String -> AccountType)
validateAccountType =
    string
        |> map
            (\s ->
                case s of
                    "business" ->
                        \x -> Business x

                    _ ->
                        \_ -> Private
            )

This works but is not ideal, as it allows Business "" as a valid value.

[etaque]

Okey, you could try using andThen like in Json.Decode, because Validation's andThen follows the same logic:
http://package.elm-lang.org/packages/elm-lang/core/4.0.5/Json-Decode#andThen

[nmk]

This is the first thing I tried. However if the validation is like

validateRegistration : Validation () Registration
validateRegistration =
      Validate.succeed Registration
          |: ("email" := email)
          |: ("accountType" := string `andThen` accountType)

accountType : String -> Validation e AccountType
accountType s =
    case s of
        "business" ->
            succeed Business |: ("companyName" := string)

        _ ->
            succeed Private

then andThen shifts the scope of companyName under accountType (the errors include accountType.companyName: InvalidString). Is this intended? In the case of Json.Decode the "scope" is not shifted.

[etaque]

Okey, that's not intended, as fair as I can remember.

[nmk]

I made an example: https://gist.github.com/nmk/e9fd97dd1a62c04fc52545fec5a9b4ef

[etaque]

Thanks for the example. I'm writing tests first as this change will break things.

[etaque]

Oh... could you try adding parenthesis like this:

(("accountType" := string) `andThen` accountType)

otherwise andThen is applying on string, not on the whole field getter!

[nmk]

You are right! That works and solves the problem nicely. It would be nice if we could get this in the documentation. I will try to get a PR in next week.

[nmk]

Just for completeness, here is a link to a working gist: https://gist.github.com/nmk/e9fd97dd1a62c04fc52545fec5a9b4ef