allow `validateField` to work on Maybe values

[amitaibu]

ihp/IHP/ValidationSupport/ValidateField.hs

Lines 62 to 69 in d065883

	validateField :: forall field fieldValue model. (
	KnownSymbol field
	, HasField field model fieldValue
	, HasField "meta" model MetaBag
	, SetField "meta" model MetaBag
	) => Proxy field -> Validator fieldValue -> model -> model
	validateField field validator model = attachValidatorResult field (validator (getField @field model)) model
	{-# INLINE validateField #-}

Right now validateField assumes we have required fields. So if we have a Maybe Text for the body it won't work.
So this won't work as is:

buildPost post = post
    |> fill @'["title", "sourceUrl"]
    |> validateField #name nonEmpty
    |> validateField #sourceUrl isUrl

@mpscholten we could add validateMaybeField, but I think validateField "just" working would be nicer. Do you agree?

[amitaibu]

Maybe a simpler option is to add this

-- | Validate a Maybe field.
validateMaybe :: (val -> ValidatorResult) -> Maybe val -> ValidatorResult
validateMaybe _ Nothing = Success
validateMaybe validator (Just value) = validator value

So then we can have this working:

buildPost post = post
    |> fill @'["title", "sourceUrl"]
    |> validateField #name nonEmpty
    |> validateField #sourceUrl (validateMaybe isUrl)

[mpscholten]

The validateMaybe function looks great. Making validateField work automatically with Maybe fields will add a lot of complexity, so I prefer how you solved it 👍