Remove cabal flag - undecidable in or out?

[DanBurton]

Eep, your API changes based on compile-time settings. I think this is
a bad idea, because other packages cannot depend on a flag, so
realistically other packages cannot depend on the instances existing,
so they're nearly useless.

UndecidableInstances is excessively maligned and usually fine anyway.
If it compiles, it won't go wrong.

~ Ben Millwood
http://www.haskell.org/pipermail/haskell-cafe/2012-August/102731.html

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I find it unlikely that other packages will depend on Tardis, but in order to make it so they can, this should be addressed in some form, if only in documentation that states "INSTALL WITH FLAG AT YOUR OWN RISK".

[DanBurton]

I've added a warning to the .description. I'm leaving this issue open until I've decided for sure whether or not to keep the flag. I'm sort of thinking no.

[bmillwood]

Sorry for reporting this in the wrong place initially :) but I think many people would agree with me when I say UndecidableInstances isn't very scary.