Invalid instance HaskVerdict (a :&& b) r

[fizruk]

@jkarni earlier you've wondered why this test fails.

Here's localized problem (reproducible with stack ghci):

>>> :m + Data.Aeson Verdict.JSON
>>> :set -XOverloadedStrings
>>> decode "\"\"" :: Maybe (Validated (MinLength 1) String)
Nothing
>>> decode "\"\"" :: Maybe (Validated (MaxLength 1 :&& MinLength 10) String)
Just ""

The reason is this instance for :&&:

instance (HaskVerdict a r, HaskVerdict b r) => HaskVerdict (a :&& b) r where
    haskVerdict _ x = And <$> haskVerdict pa x <*> haskVerdict pb x
      where pa = Proxy :: Proxy a
            pb = Proxy :: Proxy b

The Applicative in question is Maybe and for this problem Nothing indicates success.
So this implementation shortcuts with success whenever any of the conditions is met.

To fix this you can use this implementation instead:

instance (HaskVerdict a r, HaskVerdict b r) => HaskVerdict (a :&& b) r where
    haskVerdict _ x =
      case (haskVerdict pa x, haskVerdict pb x) of
        (Just ex, Just ey) -> Just (And ex ey)
        (Just ex, _) -> Just ex
        (_, Just ey) -> Just ey
        _ -> Nothing
      where pa = Proxy :: Proxy a
            pb = Proxy :: Proxy b

[jkarni]

Oh, of course! Thanks!