[Haskell-cafe] Re: Packages and modules

Brian Hulley brianh at metamilk.com
Thu Jul 6 11:37:46 EDT 2006

Simon Peyton-Jones wrote:
> Brian
>> Actually re-reading my post  I realised I may have sounded a bit
>> negative about the hard work you'd done to collate the various
>> responses to form the wiki proposal - my apologies
> Thanks -- email is a fragile medium!
>> I've followed your suggestion and made a separate page at
> http://hackage.haskell.org/trac/ghc/wiki/GhcPackagesAlternativeProposal
> Jolly good, thank you.  I've looked at it.
> Happily, so far as I can see the two proposals are identical!  At
> least I cannot identify any points of difference.  If you think they
> differ, can you say where?

They are more similar than I realised at first. However there are some minor 

1) Qualified names:

  import A.B.C( T1 ) from "foo"
  import A.B.C( T2 ) from "bar"
  type S = A.B.C.T1 -> A.B.C.T2

I'd suggest that the above should give a compiler error that A.B.C is 
ambiguous (as a qualifier), rather than allowing T1 to disambiguate it, 
because otherwise it allows people to write code that could be very hard to 
understand ie within the code, every occurrence of A.B.C as a qualifier 
should refer to the same module. (Otherwise the thing qualified qualifies 
the qualifier that's qualifying it...)

2) Syntax
I think it's important that it's as easy to use per-package namespaces as 
the existing shared namespace, and so my syntax doesn't require the use of 
an extra keyword (ie "from"), and this works syntactically because package 
names are string literals. (I see the original proposal has now changed to 
advocate that package names should indeed be quoted.)

> Your spec is a little unclear about whether the package name is
> compulsory in every import.  Under "The best of both worlds / Shared
> name space" you say that plain "import A.B.C" looks in all exposed
> packages and bleats if its ambiguous.  That's what we propose, and
> it's satisfactorily backward compatible.  And that is what your syntax
> implies too.

In my spec, if you omit the package name you get an "old-style" import using 
the shared namespace, and if you supply a package name you get a "new-style" 
import that only searches in the specified package:

    import A.B.C -- search home + exposed as is done at the moment
    import "" A.B.C -- search home package only
    import "pkg" A.B.C -- search "pkg" only

So the first variant gives backwards compatibility and the other two 
variants give the new per-package namespaces.

So apart from using "" to answer the question about importing from the home 
package, minimalist syntax, and the minor issue about qualification, both 
proposals are indeed now identical, which is great! :-)

Regards, Brian.

Logic empowers us and Love gives us purpose.
Yet still phantoms restless for eras long past,
congealed in the present in unthought forms,
strive mightily unseen to destroy us.


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