[Haskell] Re: Trying to install binary-0.4

[Thomas Schilling]

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, ...