[Haskell] Re: Trying to install binary-0.4

[Simon Marlow]

Thomas Schilling wrote:
> On Sun, 2007-10-28 at 13:49 -0400, Isaac Dupree wrote:
>> Simon Marlow wrote:
>>> Probably it could be made clearer.  In 4.2 the idea is that instead of 
>>> replacing
>>>
>>>  base-2.0 ==> base-3.0 + directory-1.0 + array-1.0 + ...
>>>
>>> you would replace
>>>
>>>  base-2.0 ==> newbase-1.0 + directory-1.0 + array-1.0 + ...
>>>
>>> and additionally have a package base-3.0 that re-exports the whole of 
>>> (newbase + directory + array + ...).
>> "Macros" in cabal:  Why not just say that depending on base-3.0 actually 
>> means that you have to depend on newbase-1.0, and directory-1.0, etc... 
>> Why is compiler support needed?  is it really possible that I still 
>> don't understand?
> 
> You'd still have the problem that every package has to specify this
> "macro" for itself.  You'd want some global macro-database to avoid
> this.  The better solution would be to just have a package that
> re-exports everything.  I.e., the definition of package base-2.0 would
> look something like this:
> 
>   if has system has base-3.0, directory, array ...
>     re-export
>   else
>     exposed-modules: Data.Maybe, Data.List, ...

And this runs into trouble when base-3.0 has changes to datatypes and/or 
classes relative to base-2.0 (see 4.1 of 
http://hackage.haskell.org/trac/ghc/wiki/PackageCompatibility).  This 
approach doesn't scale; it may be useful for providing backwards-compatible 
versions of smaller packages, but impractical for base.

Cheers,
	Simon