I still have problems and your code won't typecheck on my machine printing the following error: Test.hs:9:34: No instance for (Control.Monad.CatchIO.MonadCatchIO (InterpreterT IO)) arising from a use of `catch' at Test.hs:9:34-53 Possible fix: add an instance declaration for (Control.Monad.CatchIO.MonadCatchIO (InterpreterT IO)) In the first argument of `runInterpreter', namely `(code `catch` handler)' In the second argument of `(=<<)', namely `(runInterpreter (code `catch` handler))' In the expression: print =<< (runInterpreter (code `catch` handler)) I assume we are using different versions of some packages. Could you please send me the output of your 'ghc-pkg list'. Thanks, Martin