Without helper functions

th.hs

derivePostgresEnum :: String -> String -> Q [Dec]
derivePostgresEnum s postgresType = do
    ss <- [|SqlOther (pack postgresType)|]
    tpv <- [|PersistText . pack . show|]
    fpv <- [|\dt v ->
                case fromPersistValue v of
                    Left e -> Left e
                    Right s' ->
                        case reads $ unpack s' of
                            (x, _):_ -> Right x
                            [] -> Left $ pack "Invalid " <> pack dt <> pack ": " <> s'|]
    return
        [ InstanceD Nothing [] (ConT ''PersistField `AppT` (ConT $ mkName s)) [ FunD 'toPersistValue
                [ Clause [] (NormalB tpv) [] 
                ]
            , FunD 'fromPersistValue
                [ Clause [] (NormalB (fpv `AppE` LitE (StringL s))) [] 
                ]
            ]
        , InstanceD Nothing [] (ConT ''PersistFieldSql `AppT` (ConT $ mkName s)) [ FunD 'sqlType [ Clause [WildP] (NormalB ss) [] ] ]
        ]