Re: [Haskell-cafe] need help with monad transformers

1 Mar 2012


      It will not work due:

gistfile1.hs:29:36:
    Could not deduce (m ~ ResourceT m0)
    from the context (Base m ~ IO,
                      MonadReader SInit m,
                      MonadIO m,
                      IsSource src,
                      Resource m)
      bound by the type signature for
                 app :: (Base m ~ IO,
                         MonadReader SInit m,
                         MonadIO m,
                         IsSource src,
                         Resource m) =>
                        src m a1 -> Sink a1 m b -> ResourceT m b
      at gistfile1.hs:(29,1)-(59,27)
      `m' is a rigid type variable bound by
          the type signature for
            app :: (Base m ~ IO,
                    MonadReader SInit m,
                    MonadIO m,
                    IsSource src,
                    Resource m) =>
                   src m a1 -> Sink a1 m b -> ResourceT m b
          at gistfile1.hs:29:1
    Expected type: Conduit a1 m a1
      Actual type: Conduit a1 (ResourceT m0) a1
    In the second argument of `(=$=)', namely `process2'
    In the second argument of `($=)', namely `(process1 =$= process2)'


Thu, Mar 01, 2012 at 02:19:29PM +0400, Dmitry Olshansky wrote
...
...
...
If I'm running register outside runWriterT everything will work.
Maybe just
...
lift $ register $ print "freed2"
or I didn't catch something?
2012/3/1 Alexander V Vershilov 
Hello.
I'm trying to add monad stack into network-conduit, and everything
    works except some details [1].
I've run runReaderT $ runTCPServer (wrapper around runResourceT) and
    inside conduit I want to run writer to gather results of inner computation.
    In inner computation I want to use IO, data from outter stack (ReaderT)
    so I'm running {-1-}:
 (k,t) <- lift $ runWriterT $ ask >>= \x -> tell [x]     {- 1 -}
and {-2-}
 (k,t) <- lift $ runWriterT $ do {- 2 -}
                       x <- ask
                       liftIO $ print $x+1
                       tell [x]
and that will work (except I've thought I should not lift runWriterT, but
    calling functions inside.
And finally in computation that will run once I want to register cleaning
    function (for example register $ putStrLn "cleaned") ({-3-})
 (k,t) <- lift $ runWriterT $ do {- 3 -}
                       x <- ask
                       liftIO $ print $x+1
                       register $ print "freed2"
                       tell [x]
but I've got type error. If I'm running register outside runWriterT
    everything
    will work.
I would apperated if there will be any suggestions how to make this code
    better or use register in internal computation (runWriterT)
[1] https://gist.github.com/1941151
    --
    Best regards,
     Alexander V Vershilov
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