(Apologies for the long post)
I need some help integrating a typeclass based functionality within an existing API. I am using Control.Monad.Validate for my validation of objects. Here's a simple function based on this ..
debit :: forall m. (MonadReader Env m,
MonadValidate [Error] m) => Y.Dense "USD" -> Account -> m Account
debit amount = updateBalance ((-1) * amount)
Now I want to make a change in the API where I would like to fetch the account from some database based on account number. For this I have a typeclass as below ..
class (Monad m) => AccountDB m where
query :: Text -> m (Maybe Account)
.. elided details
and I am using persistent - so I have an instance of this class as follows ..
-- | Instance of AccountDB for `SqlPersistT` (which is `ReaderT SqlBackend`)
instance (MonadIO m) => AccountDB (SqlPersistT m) where
query ano = get (AccountKey ano)
.. other details elided
My question is how can I integrate this typeclass within the above API ? I tried as yet another constraint in the function ..
debit :: forall m. (MonadReader Env m,
MonadValidate [Error] m,
AccountDB m) => Y.Dense "USD" -> Text -> m Account
debit amount accNo = do
maybeAccount <- query accNo
maybe (refuteErr $ InvalidAccountNumber accNo) (updateBalance ((-1) * amount)) maybeAccount
This compiles but I don't get the proper instance of the typeclass when I run it - possibly due to the reason that the "m" cannot be the same for AccountDB as the other constraints. And I don't want to hardcode SqlPersistT in the function debit as I would like to be able to run it with other instances of the typeclass as well (example for testing I may like to test against a HashMap).
What would be an idiomatic approach to this kind of composition ?
Any help will be appreciated.
regards.