[Haskell-cafe] Introducing Instances in GHC point releases

[Alexander Dunlap]

Since those types come out of the time library, and that library's
version *has* been bumped (I assume), couldn't you use Cabal to
condition on the version of the time library to determine whether or
not to have CPP set a -DTYPEABLE_IN_TIME flag, and then #ifdef out
your versions of the instances?

I suppose a nice compiler extension would be "recessive instances"
which would take effect only if they did not overlap with another
instance. Probably a lot of corner cases involving multiple recessive
instances to work out, though.

Alex

On Thu, May 21, 2009 at 2:53 PM, John Goerzen <jgoerzen at complete.org> wrote:
> So this is annoying (CCing -cafe)
>
> I need NominalDiffTime and UTCTime to have Typeable instances.  In
> 6.10.1, they didn't ship with them out of the box, so I added them.
> Apparently, in 6.10.3, they DO ship with those instances out of the box.
>
> Annoyingly, that means that my code breaks on 6.10.3.
>
> Even more annoyingly, __GLASGOW_HASKELL__ is still 610, so I can't even
> work around this via cpphs.  There appears to be no way to make code
> that requires those Typeable instances work with both 6.10.1 and 6.10.3.
>
> Yet another reason to avoid API incompatibilities in point releases.
>
> Does anybody have an idea on the best way to handle this?
>
> -- John
>
> Don Stewart wrote:
>> convertible appears broken with 6.10.3. Any thoughts?
>>
>>     Writing new package config file... done.
>>     Downloading convertible-1.0.1...
>>     Configuring convertible-1.0.1...
>>     Preprocessing library convertible-1.0.1...
>>     Preprocessing executables for convertible-1.0.1...
>>     Building convertible-1.0.1...
>>     [1 of 8] Compiling Data.Convertible.Base ( Data/Convertible/Base.hs, dist/build/Data/Convertible/Base.o )
>>     [2 of 8] Compiling Data.Convertible.Utils ( Data/Convertible/Utils.hs, dist/build/Data/Convertible/Utils.o )
>>     [3 of 8] Compiling Data.Convertible.Instances.Map ( Data/Convertible/Instances/Map.hs, dist/build/Data/Convertible/Instances/Map.o )
>>     [4 of 8] Compiling Data.Convertible.Instances.Num ( Data/Convertible/Instances/Num.hs, dist/build/Data/Convertible/Instances/Num.o )
>>     [5 of 8] Compiling Data.Convertible.Instances.C ( Data/Convertible/Instances/C.hs, dist/build/Data/Convertible/Instances/C.o )
>>     [6 of 8] Compiling Data.Convertible.Instances.Time ( Data/Convertible/Instances/Time.hs, dist/build/Data/Convertible/Instances/Time.o )
>>
>>     Data/Convertible/Instances/Time.hs:61:9:
>>         Duplicate instance declarations:
>>           instance Typeable NominalDiffTime
>>             -- Defined at Data/Convertible/Instances/Time.hs:61:9-32
>>           instance Typeable NominalDiffTime
>>             -- Defined in time-1.1.3:Data.Time.Clock.UTC
>>
>>     Data/Convertible/Instances/Time.hs:64:9:
>>         Duplicate instance declarations:
>>           instance Typeable UTCTime
>>             -- Defined at Data/Convertible/Instances/Time.hs:64:9-24
>>           instance Typeable UTCTime
>>             -- Defined in time-1.1.3:Data.Time.Clock.UTC
>>     cabal: Error: some packages failed to install:
>>     HDBC-2.1.0 depends on convertible-1.0.1 which failed to install.
>>     convertible-1.0.1 failed during the building phase. The exception was:
>>
>>
>
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