RFC: provide patch-level information at __GLASGOW_HASKELL__

[Simon Marlow]

On 10/04/2014 09:55, Herbert Valerio Riedel wrote:
> 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.

We omitted the patchlevel deliberately, because patchlevel releases of 
GHC are not supposed to change any APIs, so there should be no reason 
why you would want to distinguish them in the source code.  If we 
changed something in SafeHaskell in a patchlevel release, then that was 
probably a mistake.

We could add a __GLASGOW_HASKELL_PATCHLEVEL__ macro, but I'd like to 
understand more about why people need this, and whether we should be 
more careful about what we do in patchlevel releases.

Cheers,
Simon



>
> 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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