On Wed, Aug 24, 2011 at 12:33 PM, Max Cantor <mxcantor@gmail.com> wrote:

Hi,

I am trying to use an ErrorT monad inside some Persist code.

I wrote the following instance:

newtype OcrError = OE String
...
instance PersistBackend m => PersistBackend (ErrorT OcrError m) where
 replace      k v = lift $ replace     k v
 update       k l = lift $ update      k l
 updateWhere  f u = lift $ updateWhere f u
 delete       = lift . delete
 insert       = lift . insert
 deleteBy     = lift . deleteBy
 deleteWhere  = lift . deleteWhere
 get          = lift . get
 getBy        = lift . getBy
 count        = lift . count
 selectKeys f = selectKeys f
 selectEnum f o lim off = selectEnum f o lim off

It seems like there is a problem with selectEnum though.

In GHCI, this:

l :: [PersistKV OcrRefMap] <- rs $ selectEnum [] [] 0 0

works fine, while this:

l :: Either OcrError [PersistKV OcrRefMap] <- rs $ runErrorT $ selectEnum [] [] 0 0

hangs.  rs is a small function to run persist calls:
 rs :: (MonadControlIO m) => SqlPersist m a -> m a
 rs f = do
   let conn = DT.pack "user=hachicode password=password host=localhost port=5432 dbname=hachicode"
   withPostgresqlConn conn $ runSqlConn f

and you need -XScopedTypeVariables for the ghci commands.

Any advice?

Thanks,
Max