Re: [Haskell] Simple question about IO in hugs and ghc

26 Sep 2004


      Vivian McPhail wrote:
...
When I run my main function I get an error:
The prompt does not wait for user input and thus my parser fails with an
empty head.
Is there a switch to not echo to the console?  Is this the problem?  It
happens in both Hugs and GHC
main :: IO ()
main = do 
       putStr "sentence? "
       input <- getLine
       case input of
            "quit" -> putStr "bye.\n"
            _      -> do 
                           let res = get_result $ proof $ do_proof $ getInput input
                                in if res == Nothing
                                   then putStr ("-> Nothing\n")
                                   else putStr ("-> " ++ (show $ unJust res) ++ "\n")
                                   main
The problem isn't in the code above, so it must lie in one of the
functions which you don't show. For comparison, the following code
works (from within Hugs and ghci):
...
main :: IO ()
main = do 
       putStr "sentence? "
       input <- getLine
       case input of
            "quit" -> putStr "bye.\n"
            _      -> do 
                           putStrLn input
                           main
However, for it to work when compiled with ghc, you need to add
"hFlush stdout" after printing the prompt (hugs and ghci make stdout
unbuffered, but it will be line-buffered for a compiled program).

-- 
Glynn Clements <glynn.clements@virgin.net>