Re: [Haskell-cafe] Converting string to System.Time.ClockTime

8 Dec 2011


      On Thu, Dec 8, 2011 at 7:39 PM, Antoine Latter  wrote:
...
On Thu, Dec 8, 2011 at 9:30 AM, dokondr  wrote:
...
Ok, maybe you could advise what packages to use for this simple scenario:
I have two text strings with dates:
s1 = "Wed, 07 Dec 2011 10:09:21 +0000"
s2 = "Wed, 07 Dec 2011 10:11:00 +0000"
I need:
1) Find how many seconds are between  these dates
2) Calculate the date in the middle between these dates
It looks like you already have 1) and 2) finished, using the 'time'
package.
...
3) Print out all three dates in the different format, like these:
 2011,  7 Dec, Wed, 10:11:00
If you need to convert into a specific time-zone you can use the
'utcToLocalTime' function in the 'time' package, which takes a UTCTime
and a TimeZone to create a 'LocalTime'. I'm just guessing that you
might want this, as your output format doesn't include time-zone
information.
Then for formatting, the 'Data.Time.Format' module in the 'time'
package has the function 'formatTime', which uses the same sort of
format string used by 'parseTime'.
I hope that helps! It took me a while to find my way around the 'time'
package properly.
Antoine
Thanks so much for your help! I think I finally :) solved this problem:

import Data.Time.Format
import Data.Time.Clock
import Locale
import Data.Maybe
import Data.Time.Clock.POSIX

timeFormat1 = "%a, %d %b %Y %T %z"
timeFormat2 = "%m/%e/%Y %l:%M:%S %p"

s1 = "Wed, 07 Dec 2011 10:09:21 +0000"
s2 = "Wed, 07 Dec 2011 10:11:00 +0000"
t1 = fromJust $ tryParseTime s1 -- :: UTCTime
t2 = fromJust $ tryParseTime s2
pt1 = utcTimeToPOSIXSeconds t1  -- :: POSIXTime
pt2 = utcTimeToPOSIXSeconds t2
pt3 = pt1 + (pt2 - pt1) / 2
t3 = posixSecondsToUTCTime pt3  -- :: UTCTime

-- formatTime :: FormatTime t => TimeLocale -> String -> t -> String
s3 = formatTime defaultTimeLocale timeFormat2 t3

test = (compare t1 t2) == (compare pt1 pt2)

tryParseTime :: String -> Maybe UTCTime
tryParseTime timeStr = tryFormat (parseTime defaultTimeLocale timeFormat1
timeStr :: Maybe UTCTime)
   where
     tryFormat time
        | time == Nothing = parseTime defaultTimeLocale timeFormat2 timeStr
:: Maybe UTCTime
        | otherwise = time