The problem is that the Either SqlError monad is not actually a part of
your "monad stack". It's just something that appears in the result type of
safeConnect and safeInsert. In order to put Either SqlError into your
stack, you'll need a monad transformer which exhibits Either-like
semantics, such as EitherT or ErrorT. Then your safeConnect would have type
"ConnectInfo -> EitherT SqlError IO Connection".
http://hackage.haskell.org/packages/archive/either/3.4.1/doc/html/Control-Mo...
http://hackage.haskell.org/packages/archive/transformers/0.3.0.0/doc/html/Co...
-Karl
On Mon, Aug 26, 2013 at 2:05 PM, Bryan Vicknair
I'm at the beginning of the monad transformers journey, and this is stumping me. I've read "Monad Transformers Step by Step" which made me confident enough to play with simple stacks, but I'm a bit lost in the following example from work. Please excuse any incorrect transformer terminology.
In a WAI web app, I want to chain these two actions inside a monad stack:
safeConnect :: ConnectInfo -> IO (Either SqlError Connection) safeInsert :: Thing -> Connection -> IO (Either SqlError Id)
Here is an example where I would like 'result' to be (Right Id) only if the DB connection *and* the insert were successful, (Left SqlError) otherwise.
add :: Request -> ResourceT IO Response add _ = do result <- liftIO $ safeConnect devConnInfo >>= safeInsert thing case result of (Left e) -> return $ dbErr e (Right _) -> return $ postRedirect where thing = exampleThing
The compiler tells me (edited for brevity):
Expected type: Either SqlError Connection -> GHC.Types.IO a0 Actual type: Connection -> GHC.Types.IO (Either SqlError Id) In the return type of a call of `safeInsert' In the second argument of `(>>=)', namely `safeInsert thing'
How can I chain safeConnect and safeInsert using the (Either SqlError a) monad inside WAI's (ResourceT IO Response) monad stack?
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