Well, regarding:

   type DB = SqlPersist Handler

Assuming the definition of Handler is something like:

   type Handler = GHandler MyApp My App

The definition of GHandler is:

   type GHandler sub master = GGHandler sub master (Iteratee ByteString IO) -- or something like that

So you've defined that a database action wraps around an Iteratee. As the text below states, an Iteratee cannot have an instance of MonadPoolIO, and therefore you can't perform Persistent actions against it. Therefore, you'll never be able to do anything. So instead, we need the inner monad for DB to be something with a MonadPoolIO instance, such as:

    type DB = SqlPersist (GGHandler MyApp MyApp IO)

The only problem is that after running a DB action, you'll be left with a GGHandler wrapping around IO, but Yesod expects a GGHandler wrapping around Iteratee. Therefore, you need to call liftIOHandler, which has a type of something like:

    liftIOHandler :: MonadIO m => GGHandler sub master IO a -> GGHandler sub master m a

HTH,
Michael

On Fri, Apr 8, 2011 at 2:53 PM, Aur Saraf <sonoflilit@gmail.com> wrote:
Hello,

----------------->>>>>>>>>><<<<<<<<<--------------------

runDB: need liftIOHandler

⁋For the impatient: stick liftIOHandler at the beginning of runDB like
so: runDB db = liftIOHandler $ fmap connPool getYesod >>=
Settings.runConnectionPool dbNo comments
Add a commentName:

⁋For everyone else: WAI 0.2 contained the request body as an
Enumerator in the Request datatype, and an Application was Request ->
IO Response. In WAI 0.3, an Application is Request -> Iteratee
ByteString IO Response. Long story short: all code in your Handler
function now lives on top of an Iteratee monad so that it can access
the request body.No comments
Add a commentName:

⁋This works great most of the time. The only downside is when you need
to deal with exceptions. It's impossible to define a MonadPeelIO
instance for Iteratee. Therefore, in Yesod 0.7, we have a new datatype
called GGHandler, which is just a generalization of GHandler to allow
an arbitrary inner monad.No comments
Add a commentName:

⁋GHandler defaults to having an Iteratee on the inside. But when you
need a MonadPeelIO (like we do in Persistent), you need to have an IO
on the inside. Eventually though, you'll need to convert your
GGHandler IO to a GHandler. That's what liftIOHandler does.No comments

----------------->>>>>>>>>><<<<<<<<<--------------------


Could anyone please explain this better, and what it means to someone
upgrading an application?

Specifically, what happens to:

   -- | A useful synonym; most of the DB handler functions are of this type
   type DB = SqlPersist Handler

?


Thanks :-),
 Aur

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