gistfile1.hs

class GToHttpApiData a where
  gToQueryParam :: (String -> String) -> a f -> Text

instance GToHttpApiData b => GToHttpApiData (D1 m b) where
  gToQueryParam pref (M1 x) = gToQueryParam pref x
  
instance (GToHttpApiData (a f), GToHttpApiData (b f))
      => GToHttpApiData (a f :+: b f) where
  gToQueryParam pref (L1 a) = gToQueryParam pref a
  gToQueryParam pref (R1 a) = gToQueryParam pref a

instance KnownSymbol consName =>
         (GToHttpApiData (C1 ('MetaCons consName p c) U1)) where
  gToQueryParam pref _ = Text.pack $ pref $ symbolVal (Proxy :: Proxy consName)
  
genericToHttpApiData :: (Generic a, GToHttpApiData (Rep a))
                  => (String -> String) -> a -> Text
genericToHttpApiData pref = gToQueryParam pref . from

data Test = Test1 | Test2

genericToHttpApiData id Test1 
{- -- <interactive>:107:1-28: error:
    • No instance for (GToHttpApiData
                         (C1 ('MetaCons "Test" 'PrefixI 'False) U1
                          :+: C1 ('MetaCons "Test2" 'PrefixI 'False) U1))
        arising from a use of ‘genericToHttpApiData’
    • In the expression: genericToHttpApiData id Test
      In an equation for ‘it’: it = genericToHttpApiData id Test-}