An attempt to create a different instance of a type depending on the presence of a field in the foreign (JS) object

construct-based-on-fields.purs

data Sh = Sh1 {name :: String} | Sh2 {name :: String, age :: Int}

instance shIsForeign :: IsForeign Sh where
  read x = do
    name <- readProp "name" x
    case (readProp "age" x) of
      Left _ -> pure $ Sh1 {name:name}
      Right age -> pure $ Sh2 {name: name, age: age}

frug :: Foreign -> String
frug f = case ((read f) :: (Either ForeignError Sh)) of
              Left _ -> "neither"
              Right (Sh1 {name:name}) -> "sh1" <> name
              Right (Sh2 {name:name, age: age}) -> "sh2" <> name <> " " <> (show age)