[Haskell-beginners] Haskeline and forkIO

[Peter Jones]

"Jeff C. Britton" <jcb at iteris.com> 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 at haskell.org]
> Sent: Thursday, August 28, 2014 7:11 AM
> To: beginners at haskell.org
> Subject: Re: [Haskell-beginners] Haskeline and forkIO
>
> "Jeff C. Britton" <jcb at iteris.com> 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