In the IO monad: f x y z = do let foo = x + y bar = foo * z putStrLn $ "Here's bar" <> (show bar) pure bar If all

In the IO monad: f x y z = do let foo = x + y bar = foo * z putStrLn $ "Here's bar" <> (show bar) pure bar If all you want is to log to the console for debugging from pure code, it would look like f x y z = let foo = x + y bar = foo * z in trace ("Here's bar" <> (show bar)) bar Not recommended, but if you're dead set on commingling your effectful and pure code, you can freely mix them with the function unsafePerformIO. In my experience having to write large applications with significant test coverage, the time savings from not screwing around with stubs and mocks and dealing with the external effects of test runs -- the relative annoyance of using a couple "do"s and "pure"s and separating out data transformation from IO is a small price to pay.