patch applied (ghc): Generalise Package Support

[Simon Marlow]

Malcolm Wallace wrote:

> I would caution against changing the syntax of imports too hastily.  As
> I recall, the outcome of the mailing list discussion (and wiki) on this
> issue was that the proposed syntax changes do not give any new
> expressive power over packages, that we do not already have (via
> OPTIONS_GHC etc).  Given that the user can already say everything about
> packages they need to, in the source code, I question whether
> arbitrarily breaking source-compatibility with other implementations
> (and Haskell'98) is wise.

Well, firstly we don't currently support using -package options inside 
OPTIONS_GHC, but that's certainly one option.

Leaving it there is certainly attractive, because there's not much to do, and as 
you say it keeps the language changes to a minimum.  It's also very close to the 
.NET story.

Package-qualified imports give you two qualitative benefits over per-module 
-package flags:

   - they let you disambiguate imports in a single module

   - they let you use packages without populating the global module namespace

Some people wanted to move in the direction of using package-qualified imports 
for *everything*, that is moving away from a single global module namespace. 
Certainly having a per-package module namespace is useful: for example in the 
GHC package we don't want to have to rename all our modules to be hierarchical, 
but in the absence of package-qualified imports we will probalby be forced to do 
that to avoid probable clashes in the global module namespace.

I see the package-qualified import story as quite attractive.  It's quite 
lightweight, easy to understand and not too hard to implement, and the fact that 
it parallels the story at the module level means that it is intuitive.  I 
wouldn't say I'm completely sold on it (but Simon PJ is, I think).

I'm not sure what the Cabal story would be if package flags are allowed in 
OPTIONS_GHC, it seems a bit hairy.  On the other hand, Cabal works quite 
naturally with package-qualified imports - we just need a way to say in 
build-depends that a package dependency should not populate the global module 
namespace.

Cheers,
	Simon