haskell - Conduit simple file read expression not typechecking -

i'm trying simple conduit example under belt, i'm failing @ type-checking phase. in this example, i'm seeing resourcet being used conduit "executor", if makes sense. know that, time, resourcet has been factored-out it's own package. however, still can't simple example working. here code, inspired this article:

import qualified data.conduit c import qualified data.conduit.binary cb import qualified data.conduit.list cl import control.monad.trans.resource (runresourcet) import data.bytestring.char8 (unpack) import data.bytestring (bytestring (..)) import system.io  printfile file = runresourcet $ cb.sourcefile file c.$$ print'       print' :: c.sink bytestring io ()     print' = cl.mapm_ $ putstrln · unpack 

and here error i'm getting (my package called "conduit-playground":

preprocessing library conduit-playground-0.0.0... [2 of 2] compiling playground       ( src/playground.hs, dist/dist-sandbox-13da96d6/build/playground.o )  src/playground.hs:12:57:     couldn't match type ‘io’                   ‘control.monad.trans.resource.internal.resourcet m’     expected type: c.sink                      bytestring (control.monad.trans.resource.internal.resourcet m) ()       actual type: c.sink bytestring io ()     relevant bindings include       printfile :: filepath -> m () (bound @ src/playground.hs:12:1)     in second argument of ‘(c.$$)’, namely ‘print'’     in second argument of ‘($)’, namely       ‘cb.sourcefile file c.$$ print'’ cabal: error: packages failed install: conduit-playground-0.0.0 failed during building phase. exception was: exitfailure 1 

if leave out runresourcet, i'm still getting typeclass error there not being instance of monadresource io. how should tackle problem?

the problem print' has incompatible type

print' :: c.sink bytestring io () 

you can't have io base monad of conduit if using resourcet (which need in case reading file). simplest monad lets both print , handle file resources resourcet io i.e. io monad transformed resourcet transformer.

print' :: c.sink bytestring (resourcet io) () print' = cl.mapm_ $ liftio . putstrln · unpack 

the liftio needed "lift" expression of type io () resourcet io (). use more generic signature

print' :: monadio m => c.sink bytestring m () 

to make utility function works conduit able perform io.

monadio , liftio can imported module control.monad.io.class.