iso8601Show $ CalendarDiffTime (-1) (-1) fails with "*** Exception: formatShow: bad value"

[tysonzero]

Parsing of course has the equivalent issue:

iso8601ParseM @IO @CalendarDiffTime "P-1MT-1S"
*** Exception: user error (no parse of "P-1MT-1S")

Whilst it seems like the ISO8601 standard may not explicitly support negative interval durations, the Haskell type does, and a variety of tools including PostgreSQL understand negative values within intervals, so I think crashing here is a greater evil than extending the formatting standard to support values the type already supports.

As an example reference: PostgreSQL specifically supports minus signs in the components but not a preceding sign over the entire time interval:

select 'P-1MT1H-1S' :: interval;
P-1MT59M59S

select 'P-1MT1S' :: interval;
P-1MT1S

select 'P-1MT-1S' :: interval;
P-1MT-1S

select '-P1MT1S' :: interval;
ERROR:  invalid input syntax for type interval: "-P1MT1S"

[tysonzero]

Ah ok looks like this has been fixed in 1.14, and luckily it seems like you can use time-1.14 with ghc-9.8.2/ghc-9.10.1, which I was not aware was possible given the whole boot package thing (https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/libraries/version-history).

Didn't see anything in the changelog so I initially assumed the upgrade wouldn't fix it, I suppose the "fix show of CalendarDiffTime" may have also included iso8601 show/parseM instances as well?