Re: [Haskell-beginners] Haskeline and forkIO

As a Haskell beginner, I would also like to recommend HLint as a very useful tool to catch these kinds of things. On Sep 3, 2014 12:25 PM, "Peter Jones" wrote:

"Jeff C. Britton" writes:

...

That suggestion works. I will have to continue learning more about Monads.

I should have also mentioned that if you enable GHC warnings it should tell you that having the `return` before `loop` is discarding the value given to it.

...

-----Original Message----- From: Beginners [mailto:beginners-bounces@haskell.org] Sent: Thursday, August 28, 2014 7:11 AM To: beginners@haskell.org Subject: Re: [Haskell-beginners] Haskeline and forkIO

"Jeff C. Britton" writes:

...

loop :: InputT IO () loop = do maybeLine <- getInputLine "Enter a file to compress> " case maybeLine of Nothing -> return () -- user entered EOF Just "" -> return () -- treat no name as "want to quit" Just path -> do return (runWorker path) loop

The other issue you're having is because `runWorker path` is an `IO ()` value but at the point where you use it in the code the type system wants an `InputT IO ()`. To try to satisfy the type system you used `return` to build a `InputT IO (IO ())` value, but that doesn't actually work (as you've noticed). Since `InputT` is a transformer you have an extra layer to work through and so need to *lift* your `IO ()` value into the `InputT IO` layer. Try this:

-- Add this import import Control.Monad.IO.Class

loop :: InputT IO () loop = do maybeLine <- getInputLine "Enter a file to compress> " case maybeLine of Nothing -> return () -- user entered EOF Just "" -> return () -- treat no name as "want to quit" Just path -> do liftIO (runWorker path) loop

You can think of `liftIO` as having this signature (in this context):

liftIO :: IO () -> InputT IO ()

-- Peter Jones, Founder, Devalot.com Defending the honor of good code

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