RFC: provide patch-level information at __GLASGOW_HASKELL__

[Austin Seipp]

I agree with Roman. I think just adding a new PATCHLEVEL macro is the
way to go and complementary to the existing setup, without confusing
things.

On Thu, Apr 10, 2014 at 6:18 AM, Roman Cheplyaka <roma at ro-che.info> wrote:
> I've never needed this so far, but if this gets implemented, I'm in favour of
> a separate __GLASGOW_HASKELL_PATCHLEVEL__ macro.
>
> As you say, the existing scheme is somewhat non-intuitive. But having two
> schemes simultaneously carries an even bigger mental overhead.
> And it's too easy to forget that the scheme has changed when testing for newer
> ghc versions.
>
> Roman
>
> * Herbert Valerio Riedel <hvr at gnu.org> [2014-04-10 10:55:47+0200]
>> Hello,
>>
>> I know this has come up various times. So this is mostly an attempt to
>> see what the current position is on this topic:
>>
>> The current scheme is documented as
>>
>> ,----
>> | The value of __GLASGOW_HASKELL__ for a major release x.y.z is the
>> | integer xyy (if y is a single digit, then a leading zero is added, so
>> | for example in version 6.8.2 of GHC we would have
>> | __GLASGOW_HASKELL__==608).
>> `----
>>
>> This has lead to confusion in the past, e.g. the following two values
>>
>>  __GLASGOW_HASKELL__ == 702
>>
>>  __GLASGOW_HASKELL__ == 704
>>
>> were sometimes confused (by me at least) to mean 7.0.2 and 7.0.4
>> respectively. And sometimes when writing conditionals, it also happened
>> that '__GLASGOW_HASKELL__ >= 722' was written to mean >= 7.2.2.
>>
>> Moreover, when GHC 7.2.2 came out, it would have been
>> useful to be able to discriminate 7.2.1 vs 7.2.2 easily, as some
>> SafeHaskell properties changed between 7.2.1 and 7.2.2.
>>
>> Therefore, I'd propose to extend this constant by a patch-level digit
>> for future GHC versions (starting with 7.10.1), i.e.
>>
>> __GLASGOW_HASKELL__ == 7090   -- 7.9 branch
>>
>> __GLASGOW_HASKELL__ == 7100   -- 7.10.1 release candidates
>> __GLASGOW_HASKELL__ == 7101   -- 7.10.1
>> __GLASGOW_HASKELL__ == 7102   -- 7.10.2
>>
>> __GLASGOW_HASKELL__ == 7121   -- 7.12.2
>>
>> NB: this ensures that __GLASGOW_HASKELL__ retains its ordering
>> relation. There's just a steeper jump from 7.8.1 to 7.10.1, but existing
>> code using conditionals such as
>>
>> #if (__GLASGOW_HASKELL__ >= 708) && (__GLASGOW_HASKELL__  < 709)
>>
>> for currently existing GHC versions will continue to work as expected.
>>
>> Alternative ideas:
>>
>>  - Define a __GLASGOW_HASKELL_PATCHLEVEL__ containing only the
>>    patch-level number.
>>
>>    (c.f. GNU GCC's __GNUC_PATCHLEVEL__ constant)
>>
>>    This one might have the least impact, as its existence can be ignored
>>    safely, and we could even use this starting with GHC 7.8.2 with low
>>    risk of affecting users.
>>
>>  - define a __MIN_VERSION_GHC__(x,y,z) macro in the style of
>>    Cabal's MIN_VERSION_<pkgname>() macros
>>
>>    While this has the most structure, this has also the issue of
>>    backward compatibility, as for earlier GHC versions you'd have to
>>    check for the existence of the macro before using it to avoid
>>    compile-time errors.
>>
>>
>> Cheers,
>>   hvr
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-- 
Regards,

Austin Seipp, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/