Nesting a multiple field validation loses the error

[nmk]

I am trying to write a validation which checks that two fields together satisfy some condition.

Here is a working example: https://ellie-app.com/mjLjf9kDma1/3

Select One and Two from the dropdowns and the validation succeeds. Select One and Three and the validation fails and the error is displayed.

However, when I embed this validation in another one, the validation still fails, yet the error is lost, resulting in getOutput f == Nothing without errors. Here is an example: https://ellie-app.com/mjLjf9kDma1/4

I suspect this is a grouping issue, but I cannot figure out what the correct expression grouping in the second example should be. I would greatly appreciate any help with this one!

[etaque]

Hello,
Finally found the time to look at your issue, it's effectively a grouping problem.

In there:

thingCombinationValidation : Validation MyError ( Thing, Thing )
thingCombinationValidation =
    field "firstThing" thingValidation
        |> andThen
            (\fst ->
                field "secondThing" thingValidation
                    |> andThen
                        (\snd ->
                            if List.member ( fst, snd ) forbiddenCombinations then
                                fail (customError InvalidCombination)
                            else
                                succeed ( fst, snd )
                        )
            )

The fail (customError InvalidCombination) isn't attached to a field, like that, so error is lost in errors tree, see how it's done in lib:
https://github.com/etaque/elm-form/blob/master/src/Form/Validate.elm#L131

So what you can do is to use field to attach your custom error, like so:

field "firstThing" (fail (customError InvalidCombination))

Maybe that kind of error could be caught by better types. Worth investigating!

[nmk]

Thank you for looking into this! I was pretty sure I had answered during the holidays, but just noticed I hadn't. Sorry about that!

Using field to add an error works, but is not really intuitive. Maybe adding a function like failAtField implemented in terms of field and fail as shown above would make it clearer.