Haskell get current time as ISO8601 string

Question

I've tried several things I could find online to print the current time as an ISO8601 string, but none of them have worked:

1. This SO answer assumes you already know how to get the current time, but I can't figure that out.

   I keep on getting an error Could not find module ‘System.Locale’ because it seems like System.Locale is deprecated/not installed by default anymore?

   I tried installing old-local but that just gave me new errors, I think caused by incompatibilities between time and old-locale:

   • Couldn't match expected type ‘time-1.9.3:Data.Time.Format.Locale.TimeLocale’
                 with actual type ‘System.Locale.TimeLocale’
     NB: ‘time-1.9.3:Data.Time.Format.Locale.TimeLocale’
           is defined in ‘Data.Time.Format.Locale’ in package ‘time-1.9.3’
         ‘System.Locale.TimeLocale’
           is defined in ‘System.Locale’ in package ‘old-locale-1.0.0.7’
   • In the first argument of ‘formatTime’, namely ‘defaultTimeLocale’
     In the expression: formatTime defaultTimeLocale "%FT%T%QZ"
     In an equation for ‘iso8601’:
         iso8601 = formatTime defaultTimeLocale "%FT%T%QZ"
      |
   49 | iso8601 = formatTime defaultTimeLocale "%FT%T%QZ"
      |                      ^^^^^^^^^^^^^^^^^
   
   

2. This wiki page gets the current time, but doesn't format it as ISO8601.

3. this blog post seems really good, but only has one section on formatting which is actually the same as 1.:

Prelude Data.Time> formatTime defaultTimeLocale "%T, %F (%Z)" myTime

What I want, is to be able to call a function have it print out the current time as an ISO8601 string. I'm guessing this will have to actually be an IO action since it's not a strict function.

Answer 1

I wonder if something from the time package’s Data.Time.Format.ISO8601 module might help? The iso8601Show function turns a UTCTime into a ISO8601 formatted string.

import Data.Time (getCurrentTime)
import Data.Time.Format.ISO8601 (iso8601Show)

main :: IO ()
main = do
  now <- getCurrentTime
  putStrLn (iso8601Show now)

and in the ghci:

>>> import Data.Time (getCurrentTime)
>>> import Data.Time.Format.ISO8601 (iso8601Show)
>>> now <- getCurrentTime
>>> putStrLn (iso8601Show now)
2022-09-09T08:06:21.630747Z