possible regression in time 1.8.0.1 / ghc 8.2 rc2 when converting UTCTime to a string

[DougBurke]

In testing out one of my packages with ghc 8.2 rc2, I found a failing test which I've boiled down to the following: is this a bug in time, or in ghc, or allowed behavior?

The problem is that I am trying to round-trip the string '1970-01-01T00:00:00Z' to UTCTime and then back to a string, and with time 1.8.0.1 this returns '1970-01-01T00:00:00.Z' - note the decimal point.

Details below:

% cat prob.hs
module Main where

import Data.Time
import Data.Maybe

fmt :: String
fmt = "%FT%T%QZ"

torig :: String
torig = "1970-01-01T00:00:00Z"

tconv :: UTCTime
tconv = fromJust (parseTimeM True defaultTimeLocale fmt torig)

tnew :: String
tnew = formatTime defaultTimeLocale fmt tconv

main :: IO ()
main = do
  putStrLn ("Original:  " ++ torig)
  putStrLn ("Converted: " ++ tnew)

I see the following on a ubuntu laptop

a) ghc 8.0.2, time 1.6.0.1

% ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.0.2
% ghc-pkg list time
/usr/lib/ghc/package.conf.d
    time-1.6.0.1
/home/djburke/.ghc/x86_64-linux-8.0.2/package.conf.d
    (no packages)
% runghc prob.hs
Original:  1970-01-01T00:00:00Z
Converted: 1970-01-01T00:00:00Z

b) ghc 8.2 rc2, time 1.8.0.1

% ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.2.0.20170507
% ghc-pkg list time
/home/djburke/local/ghc-8.2.0.20170507/lib/ghc-8.2.0.20170507/package.conf.d
    time-1.8.0.1
% runghc prob.hs
Original:  1970-01-01T00:00:00Z
Converted: 1970-01-01T00:00:00.Z

[bgamari]

It looks like this regressed with 85904c5.

[DougBurke]

Ta for digging that up - my reading of the commit - particularly the test formatUnitTest "%Q" 0 "." - is that this is an intentional change in the behavior of %Q, but there's enough moving parts here (and I have little enough spare brain power) that I'll leave it to @AshleyYakeley to determine what the best thing to do here.

[AshleyYakeley]

Yes, this is a bug. The documentation for %Q says "For a whole number of seconds, %Q produces the empty string." To always get a decimal point (the current wrong behaviour of %Q), one should use ".%-q".

[AshleyYakeley]

Fixed in c29513e.

[AshleyYakeley]

I will do a point release for this.

[DougBurke]

Cheers.