[Haskell-beginners] IO String inside a CmdArgs data

[Kamil Stachowski]

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
<daniel.is.fischer at googlemail.com>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
>
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