Question

I’m having a bit of a problem with converting the output of these functions below to an Int

date :: IO (Integer,Int,Int)
date = getCurrentTime >>= return . toGregorian . utctDay

date' :: IO Integer
date' = getCurrentTime >>= return . toModifiedJulianDay . utctDay

Basically I want to take the output from either (not fussed which as long as it’s possible) and turn it into an int. e.g. Today is the 20/4/11 so date would give (2011,4,20) and date' would give 55671.

In an ideal world I want a couple of functions to flatten these IO functions to give an Int – (2011,4,20) to 2011420 and 55671 (currently of type IO Integer) to 55671 as an Int

Please advise me if there are better ways of completing this!

edit: Sorry if it wasnt clear, I meant how can you convert the output of date (2011,4,20) :: IO (Integer,Int,Int) to 20110420 :: Int

Answer 1

If I understood part of your question correctly, you basically want a function, say f, such that f (123,456,789) = 123456789, where all those numbers are numbers. So f :: (Integer, Int, Int) -> Integer.

module Temp where

import Data.Time
import Data.Time.Calendar.WeekDate

date :: UTCTime -> (Integer, Int, Int)
date = toWeekDate . utctDay

f :: (Integer, Int, Int) -> Integer
f (a,b,c) = read $ show a ++ show b ++ show c 

main = do
  utcTime <- getCurrentTime
  putStr "original format: " >>  print (date utcTime)
  putStr "flattened format: " >> print (f . date $ utcTime)

In ghci:

*Temp> main
original format: (2011,16,3)
flattened format: 2011163

edit: Please never forget, that basically, there is no (noncheating) function IO a -> a. So what you want is impossible unless you break purity principles. Please consider just using the function fmap f :: IO (Integer, Int, Int) -> IO Integer.