I'm just learning Haskell and monad transformers and I've found myself with an IO (IO ()) that I'd like to flatten into just IO (). I'm sure that I'm doing something wrong, but can't pinpoint exactly where I'm getting lost.
Here is a simplified example of what I'm trying to do. This is a convoluted way of implementing
echo, but it illustrates the problem.
userInput :: Monad m => ReaderT (IO String) m (IO String) userInput = ask echo :: Monad m => ReaderT (IO String) m (IO ()) echo = userInput >>= /input -> -- unwrap ReaderT to get an IO String input >>= (/s -> -- unwrap IO String to get a String putStrLn s) -- print out the String & return -- rewrap into a ReaderT main :: IO (IO ()) -- How to turn IO (IO ()) to IO ()? main = runReaderT echo getLine
In my real application, I have a Spock app that makes HTTP requests to an upstream server. Spock apps use a monad transformer stack called
SpockCtxT and I'd like to insert a
ReaderT into the stack to abstract the HTTP request so that I can swap it out for a mock implementation in my tests.
Fundamentally, the idea is a monad transformer stack where one of the transformers gives you an
IO whether it be an HTTP request or
getLine. Am I thinking about this incorrectly or is there some way to do this?
join. It has the type signature
join :: Monad m => m (m a) -> m a
which specializes to
join :: IO (IO ()) -> IO ()
you can use hoogle to find this out. It is a command-line tool. We can search by type signature:
hoogle "IO (IO ()) -> IO ()"
Control.Monad join :: Monad m => m (m a) -> m a Control.Composition (.$) :: Monad m => m (m a) -> m a RIO join :: Monad m => m (m a) -> m a Universum.Monad.Reexport join :: Monad m => m (m a) -> m a Stack.Prelude join :: Monad m => m (m a) -> m a Relude.Monad.Reexport join :: Monad m => m (m a) -> m a Intro join :: Monad m => m (m a) -> m a Hledger.Web.Import join :: Monad m => m (m a) -> m a Data.Edison.Seq concat :: Sequence s => s (s a) -> s a Data.Edison.Seq.Defaults concatUsingFoldr :: Sequence s => s (s a) -> s a -- plus more results not shown, pass --count=20 to see more
which has several functions that are exactly what you want.