haskell - How to use persistent in a monad stack? -

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I'm enjoying learning Haskell and I think I'm making some good progress here with the help of people and #hoschelles. My studies are still mostly on this point where I look at examples and try to apply the techniques that apply to them and apply to my code.

Currently, I have monade piles for various applications and I'm looking to incorporate the functionality of consistent structure in my application. Here's my monade stack: 

  newtype app a = app {unApp :: StateT AppState (SqlPersistT (ResourceT (LoggingT IO))} deriving (functional, Function, Monad, MonadIO, MonadState AppState)

AppState There is only one record data type in this example, there is also a single value.

My main function looks like this: 

  main = runApp "./test.sqlite" (AppState 69) runmigrate

Where runApp should open all monuments: 

  runup :: text - & gt; Upstate - & gt; App A - & gt; IO RunAppleTrSource with runAppTs = runStdoutLoggingT RunResourceT RunSqlConn flip evalStateT S UnApp

and RunMeget to run in app moded In this case I was shooting to get it to run the migration: 

  runmegate :: app () runmigrate = return $ liftPersist $ runMigration migrateAll

The compiler tells me that I do not know what I'm doing with the complaint: 

  Main.lhs: 59: 16 type 'm0 ()' with '() 'Expected type: a () Actual Type: App (M ((M (()) In Expression: Refund $ lift lift $ migrate runmanagement In an equation for 'runmegate': runMigrate = return $ liftPersist $ runMigration migrateAll

Question:

  1. What is the correct way to do this?

  2. What if I had a < Code> ReaderT ? Seeing how SqlPersistT is actually a ReaderT how can I make sure that ask is actually < Match with code> ReaderT Is not SqlPersistT ?

For your first question: return Not the right thing --- The point of this return is that return x does not work for your mood and just gives a value I think You probably want to: 

  runmigrate = app $ lift $ runMigration migrateAll

app your newtype Definitions in your mind; StateT in wrapping lift lifting persistT .

(Incidentally, I recommend naming AppLife RunMigration as it is like runMigrationApp , if you are using it a lot .)


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