RFC: provide patch-level information at __GLASGOW_HASKELL__

[Roman Cheplyaka]

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
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at haskell.org
> http://www.haskell.org/mailman/listinfo/ghc-devs