I'm not familiar with persistent, but I might have some ideas. You are using template haskell. So while you are not directly using json, you are using it indirectly, via the code generated by mkPersist [1]. Presumably mkPersist must create an PersistEntity Account instance. Looking at the superclass constraints of PersistEntity [2] we see ToJSON (Key record) and FromJSON (Key record) where the record type variable is you Account type. You can easily create instances of FromJSON and ToJSON by using Generics [3]. But this only works if you type is an instance of the Generic type class. I think that the code generated by mkPersist depends on the Key Account type having an instance for Generic. Notice that Key is a data family (type level function) and thus Key Account equals some other type. I can really recommend the -ddump-splices GHC option [4] when debugging code involving template haskell. It is a lot easier to understand a problem when you can just read the generated Haskell code. 1 - https://hackage.haskell.org/package/persistent-template-2.8.0.1/docs/Databas... 2 - https://hackage.haskell.org/package/persistent-2.10.4/docs/Database-Persist-... 3 - https://hackage.haskell.org/package/aeson-1.4.6.0/docs/Data-Aeson.html#v:par... 4 - https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/debugging.ht... Op vr 17 jan. 2020 om 18:29 schreef Debasish Ghosh

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Hi -

I have an entity declaration in persistent as follows:

share [mkPersist sqlSettings { mpsGenerateLenses = True, mpsPrefixFields = False }, mkMigrate "migrateAll"] [persistLowerCase| Account accountNo Text accountType AccountType accountHolderName Text accountOpenDate UTCTime default=CURRENT_TIME accountCloseDate UTCTime Maybe default=NULL currentBalance MoneyUSD rateOfInterest Double Primary accountNo deriving Show |]

I am getting this compilation error ..

• No instance for (Generic (Key Account)) arising from the 'deriving' clause of a data type declaration Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself • When deriving the instance for (aeson-1.4.6.0:Data.Aeson.Types.ToJSON.ToJSON (Key Account)) | 40 | share [mkPersist sqlSettings { mpsGenerateLenses = True, mpsPrefixFields = False }, mkMigrate "migrateAll"] [persistLowerCase| | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

and another exactly similar for FromJSON

What exactly is going wrong here ? I am not using json in the above definition.

regards. -- Debasish Ghosh