At Thu, 16 Sep 2004 18:26:35 +0200, hjgtuyl@chello.nl wrote:
L.S.,
In my enthusiasm to reduce imperative style coding to a minimum, I changed a program to something too lazy to do anything. The following is an extremely simplified version of the program:
import Monad
displayFile1 :: IO (IO ())displayFile1 = liftM putStr contents -- Displays nothing where contents :: IO [Char] contents = readFile "DisplayFile.lhs"
This should display the contents of a file, but nothing appears. The following function should be exactly the same, but this one does display the file:
displayFile2 :: IO () displayFile2 = do contents <- readFile "DisplayFile.lhs" putStr contents
My conclusion is, that "putStr" is evaluated strictly, while "liftM putStr" is not.
Has nothing to do with strict/lazy.
I have the following questions: - Why is this difference?
Notice the type of displayFile1 vs displayFile2 IO(IO()) vs IO () While displayFile2 actually prints something, displayFile1 mearly returns an 'action', which will print something if you 'evaluate' it. Here is an example of how to use displayFile1 the way you wrote it: import Monad displayFile1 :: IO (IO ()) displayFile1 = liftM putStr contents where contents :: IO [Char] contents = readFile "DisplayFile.lhs" doDisplayFile :: IO () doDisplayFile = displayFile1 >>= \action -> action or alternately, you could write doDisplayFile like this (same thing, just a different syntax): doDisplayFile :: IO () doDisplayFile = do action <- displayFile1 action if you wanted displayFile1 to behave more like displayFile2 you could write it like this: displayFile3 :: IO () displayFile3 = contents >>= putStrLn where contents :: IO [Char] contents = readFile "DisplayFile.lhs" or, like this (same thing, different syntax) displayFile3 :: IO () displayFile3 = do c <- contents putStrLn c where contents :: IO [Char] contents = readFile "DisplayFile.lhs" Jeremy Shaw -- This message contains information which may be confidential and privileged. Unless you are the addressee (or authorized to receive for the addressee), you may not use, copy or disclose to anyone the message or any information contained in the message. If you have received the message in error, please advise the sender and delete the message. Thank you.