instance Enum DiffTime dangerous

[David Fox]

As you probably know, there are 86,400,000,000,000,000 picoseconds in a day. This is a 57 bit integer, which is larger than the Int type on many machines. Therefore, when you say fromEnum diff you will get unpleasant results at most times of day.)

{-# LANGUAGE TypeApplications #-}
import Data.Time
main :: IO ()
main = do
  mapM_ test
    ["2022-06-25 00:00:00.00001 UTC",
     "2022-06-25 00:00:00.0001 UTC",
     "2022-06-25 00:00:00.001 UTC",
     "2022-06-25 00:00:00.002 UTC",
     "2022-06-25 00:00:00.003 UTC",
     "2022-06-25 00:00:27.581804038 UTC",
     "2022-06-25 12:59:27.581804038 UTC",
     "2022-06-25 23:59:27.581804038 UTC"]
    where
      test :: String -> IO ()
      test s = do
        putStrLn ("expected: " <> show (utctDayTime (read s :: UTCTime)))
        putStrLn ("  actual: " <> show (toEnum @DiffTime $ fromEnum $ utctDayTime (read s :: UTCTime)))

Output:

$ ghcjs test.hs -o test
[1 of 1] Compiling Main             ( test.hs, test.js_o )
Linking test.jsexe (Main)
$ nodejs ./test.jsexe/all.js
expected: 0.00001s
  actual: 0.00001s
expected: 0.0001s
  actual: 0.0001s
expected: 0.001s
  actual: 0.001s
expected: 0.002s
  actual: 0.002s
expected: 0.003s
  actual: -0.001294967296s
expected: 27.581804038s
  actual: -0.000475936912s
expected: 46767.581804038s
  actual: 0.000745473904s
expected: 86367.581804038s
  actual: -0.00145057192s

[Ashley Yakeley]

Surely this is no more dangerous than instance Enum Integer?

Prelude> fromEnum (10000000000000000000 :: Integer)
-8446744073709551616

[David Fox]

I'm not sure how much of a comfort this is. In my personal experience the DiffTime instance has actually caused a problem that I had to track down.

[Ashley Yakeley]

Don't use fromEnum I guess?

[Ashley Yakeley]

Not seeing a change to be made here.