Re: [Haskell-beginners] IO String inside a CmdArgs data

14 Jan 2011

      Wow, that was a quick answer! And it does just what I want. Thanks a lot!

Now, I'll just have to sit down and try to understand it :)

Thanks again,
Kamil Stachowski

On 14 January 2011 19:41, Daniel Fischer
wrote:
...
On Friday 14 January 2011 19:18:58, Kamil Stachowski wrote:
...
Hello,
I am trying to use N. Mitchell's CmdArgs and I would like one of the
fields to store the current time. A minimal example below:
{-# LANGUAGE DeriveDataTypeable #-}
import System.Console.CmdArgs
import System.Time
data FuzzyTimeConf = FuzzyTimeConf { time :: String } deriving (Show,
Data, Typeable)
ftConf = FuzzyTimeConf { time = confDefaultTime }
confDefaultTime = "it is here that I would like to get the current time"
main :: IO ()
main = do
 print =<< cmdArgs ftConf
I want confDefaultTime to store the current time as HH:MM, so I guess I
will also need to extract it in some way.
I've tried many different versions and can't come up with anything
working. confDefaultTime can't be an IO String because than it can't
derive Show and Data which is required for cmdArgs to work in
FuzzyTimeConf. However, I don't seem to be able to find a way to make it
a String and hold the actual current time.
Can you help me, please?
Two things spring to mind:
a) don't use a top-level ftConf, get it inside IO in main:
getFTConf :: IO FuzzyTimeConf
getFTConf = do
 now <- getClockTime -- or whichever time you need
 return $ FuzzyTimeConf { time = convert now }
main = do
 ftConf <- getFTConf
 print =<< cmdArgs ftConf
b) if you absolutely have to have ftConf as a top-level name and it is safe
here:
import System.IO.Unsafe (unsafePerformIO)
{-# NOINLINE ftConf #-}
ftConf :: FuzzyTimeConf
ftConf = unsafePerformIO $ do
 now <- getClockTime
 return $ FuzzyTimeConf { time = convert now }
a) is preferable if it's possible to do thus.
Cheers,
Daniel