Re: [Haskell-beginners] Date Time in Haskell

Hi Kostiantyn, Thanks for introducing me to package "Safe" !! main = do currentTime <- getCurrentTime -- in all remaining code, currentTime has type "UTCTime" ... getDifference currentTime userTime This doesn't work for me. Correct me if I'm doing something wrong here. Here's my code: main = do now <- getCurrentTime diffUTCTime now bree bree = readMay "1981-06-16 04:35:25" :: UTCTime This gives an error like this; io.hs:28:5: Couldn't match expected type ‘IO b’ with actual type ‘NominalDiffTime’ Relevant bindings include main :: IO b (bound at io.hs:26:1) In a stmt of a 'do' block: diffUTCTime now bree In the expression: do { now <- getCurrentTime; diffUTCTime now bree } On Thu, May 14, 2015 at 3:55 PM, Kostiantyn Rybnikov wrote:

Hi Dananji!

First of all, for more explicit failure-handling I suggest using "readMay" from package "safe" [0] instead of "read" whenever possible.

Alternative to using "read", if you know date format of input-data, you can take a look at "UNIX-style parsing" section at Data.Time.Format module [1]. If you have older "time" package, you'd probably use parseTime, for newer versions it is recommended to use parseTimeM.

Here's an example:

``` ➜ ~ ghci GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help Prelude> import Data.Time.Format Prelude Data.Time.Format> import System.Locale Prelude Data.Time.Format System.Locale> import Data.Time.Clock Prelude Data.Time.Format System.Locale Data.Time.Clock> parseTime defaultTimeLocale "%F %X" "2014-01-02 11:12:30" :: Maybe UTCTime Just 2014-01-02 11:12:30 UTC ```

If you have a value of type "IO <something>", you can "extract" it when being in IO monad like this:

main = do currentTime <- getCurrentTime -- in all remaining code, currentTime has type "UTCTime" ... getDifference currentTime userTime

In terms of "sugar-less" way to use a value from "IO <something>", you can also use it like this:

getCurrentTime >>= \currentTime -> doSomething currentTime

Hope this helps.

[0]: https://hackage.haskell.org/package/safe [1]: http://hackage.haskell.org/package/time-1.5.0.1/docs/Data-Time-Format.html

On Thu, May 14, 2015 at 10:20 AM, Dananji Liyanage wrote:

...

Hi All,

I'm trying out the date time conversions in Haskell.

How do I convert a keyboard input (IO String) to UTCTime?

This is how I'm doing (probably there's a better way), convDay x = read (x ++ " 00:00:00") :: UTCTime ​​

Another thing, how do I convert getCurrentTime (IO UTCTime) to UTCTime in order to get the time difference between the keyboard input date and today?

-- Regards, Dananji Liyanage

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-- Regards, Dananji Liyanage