Haskell: Persist.sqlite - only select - sqlite

Question

I'm trying to use Persist.sqlite to query my DB,..but in all the tutorials I have found so far is that first I have to migrate and insert data. Only after that came the select.
But what if I want to do only select over existing data? No migration and inserts before?
For example in the code below. What if I want to skip buildDb (currently that will not work)?
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE QuasiQuotes, TemplateHaskell, TypeFamilies #-}
{-# LANGUAGE OverloadedStrings, GADTs, FlexibleContexts #-}
{-# LANGUAGE MultiParamTypeClasses #-}
module DBio where
import Data.Text (Text)
import Database.Persist
import Database.Persist.Sqlite (runSqlite, runMigrationSilent)
import Database.Persist.TH (mkPersist, mkMigrate, persistLowerCase,
share, sqlSettings)
import Database.Persist.Sql (insert)
import Data.Conduit (($$))
import Data.Conduit.List as CL
import Control.Monad.IO.Class (liftIO)
share [mkPersist sqlSettings, mkMigrate "migrateTables"] [persistLowerCase|
Tutorial
value Double
url Text
school Bool
deriving Show
|]
run :: IO ()
run = runSqlite "../DB/mind.sqlite" $ do
buildDb
basic <- selectList [TutorialValue >. 0.0] []
liftIO $ print basic
buildDb = do
--runMigrationSilent migrateTables
insert $ Tutorial 2.22 "https://fpcomplete.com/school/basic-haskell-1" True
for example if I comment buildDb, after the table was once created i get following error:
No instance for (Control.Monad.IO.Class.MonadIO m0)
arising from a use of ‘insert’
The type variable ‘m0’ is ambiguous
Relevant bindings include
buildDb :: Control.Monad.Trans.Reader.ReaderT
persistent-2.2:Database.Persist.Sql.Types.SqlBackend
m0
(Key Tutorial)
(bound at DBio.hs:35:1)
Note: there are several potential instances:
instance Control.Monad.IO.Class.MonadIO m =>
Control.Monad.IO.Class.MonadIO
(conduit-1.2.5:Data.Conduit.Internal.Conduit.ConduitM i o m)
-- Defined in ‘conduit-1.2.5:Data.Conduit.Internal.Conduit’
instance Control.Monad.IO.Class.MonadIO m =>
Control.Monad.IO.Class.MonadIO
(conduit-1.2.5:Data.Conduit.Internal.Pipe.Pipe l i o u m)
-- Defined in ‘conduit-1.2.5:Data.Conduit.Internal.Pipe’
instance Control.Monad.IO.Class.MonadIO IO
-- Defined in ‘Control.Monad.IO.Class’
...plus 16 others
In the expression: insert
In a stmt of a 'do' block:
insert
$ Tutorial
2.22 "https://fpcomplete.com/school/basic-haskell-1" True
In the expression:
do { insert
$ Tutorial
2.22 "https://fpcomplete.com/school/basic-haskell-1" True }

Answer 1

I managed to get a working example. I guess that the problem above is with import modules.
{-# LANGUAGE OverloadedStrings, GADTs, TypeFamilies #-}
{-# LANGUAGE QuasiQuotes, TemplateHaskell #-}
{-# LANGUAGE MultiParamTypeClasses, GeneralizedNewtypeDeriving #-}
import Database.Persist ((>.), insertMany, Entity(..))
import Database.Persist.Sqlite (runSqlite,runMigration)
import qualified Database.Persist.TH as TH (share, mkPersist, sqlSettings,
mkMigrate, persistLowerCase)
import Control.Monad (mapM_)
import Database.Persist (selectList, entityVal)
import Database.Persist.Sqlite (runSqlite)
import Control.Monad.IO.Class (liftIO)
TH.share [TH.mkPersist TH.sqlSettings, TH.mkMigrate "migrateAll"] [TH.persistLowerCase|
MyRecord
value Int
deriving Show
|]
create :: IO ()
create = runSqlite "test.sqlite" $ do
let n = 10
runMigration migrateAll
insertMany $ map MyRecord [1..n]
return ()
get :: IO ()
get = runSqlite "test.sqlite" $ do
records <- selectList [MyRecordValue >. 9] []
liftIO $ print records