Using returned EitherT in haskell program

Question

I'm trying to use the "citation-resolve" package in a Haskell project I'm working on, but I'm having trouble getting my head around using EitherT's in real code. I get that they're monad transformers, and I think I understand what that means, however I can't seem to actually work out how to use them. The toy example that represents what I'm trying to do is as follows:

module Main where
import Text.EditDistance
import Text.CSL.Input.Identifier
import Text.CSL.Reference
import Control.Monad.Trans.Class 
import Control.Monad.Trans.Either 

main = do
    putStrLn "Resolving definition"
    let resRef = runEitherT $ resolveEither "doi:10.1145/2500365.2500595"
    case resRef of 
                Left e -> do 
                    putStrLn ("Got error: "++ e)
                Right ref -> do
                    putStrLn ("Added reference to database: "++ (show ref))

Here, resolveEither has the type:

resolveEither :: (HasDatabase s,
                  Control.Monad.IO.Class.MonadIO m,
                  mtl-2.1.3.1:Control.Monad.State.Class.MonadState s m)
                   => String -> EitherT String m Reference

and runEitherT $ resolveEither "ref" has the type:

runEitherT $ resolveEither "ref"
   :: (HasDatabase s,
       Control.Monad.IO.Class.MonadIO m,
       mtl-2.1.3.1:Control.Monad.State.Class.MonadState s m)
         => m (Either String Reference)

However, this gives the following error:

Main.hs:10:34:
    No instance for (Control.Monad.IO.Class.MonadIO (Either [Char]))
      arising from a use of ‘resolveEither’
    In the first argument of ‘runEitherT’, namely
      ‘(resolveEither "doi:10.1145/2500365.2500595")’
    In the expression:
      runEitherT (resolveEither "doi:10.1145/2500365.2500595")
    In an equation for ‘resRef’:
        resRef = runEitherT (resolveEither "doi:10.1145/2500365.2500595")

Which I have no idea how to resolve, or work around.

Any help would be appreciated, especially pointers to tutorials dealing with monad transformers from a usage perspective, not an implementation one.

Edit:

To reflect the comments on answers by dfeuer and Christian, I still get errors if I change main to the following:

main = do
    putStrLn "Resolving definition"
    resRef <- runEitherT (resolveEither "doi:10.1145/2500365.2500595")
    case resRef of 
                Left e -> do 
                    putStrLn ("Got error: "++ e)
                Right ref -> do
                    putStrLn ("Added reference to database: "++ (show ref))

The error I get now is:

No instance for (MonadState s0 IO)
  arising from a use of ‘resolveEither’
In the first argument of ‘runEitherT’, namely
  ‘(resolveEither "doi:10.1145/2500365.2500595")’
In a stmt of a 'do' block:
  resRef <- runEitherT (resolveEither "doi:10.1145/2500365.2500595")
In the expression:
  do { putStrLn "Resolving definition";
       resRef <- runEitherT (resolveEither "doi:10.1145/2500365.2500595");
       case resRef of {
         Left e -> do { ... }
         Right ref -> do { ... } } }

I'm editing my question as well as commenting, as nice code formatting is substantially easier here than in a comment.

Answer 1

I believe the problem is that you're trying to pattern match on resRef when what you probably want to do is execute it and pattern match on the result.

So you should try this:

main = do
    putStrLn "Resolving definition"
    resRef <- runEitherT $ resolveEither "doi:10.1145/2500365.2500595"
    case resRef of 
                Left e -> do