Feature Request : Qualified module exports

Nicolas Frisby nicolas.frisby at gmail.com
Mon Jul 8 18:51:31 CEST 2013

On Mon, Jul 8, 2013 at 5:14 AM, Erik de Castro Lopo <mle+hs at mega-nerd.com>
> Hi all,
> I've added a feature request ticket to the GHC trac:
>     http://ghc.haskell.org/trac/ghc/ticket/8043
> for qualified module exports. In a nutshell this would allow the
> following 'qualified module' export eg:
>    module MyModule
>        ( qualified module T
>         )
>    import qualified Data.Text as T
> so that any module which imports MyModule has access to the symbols
> of Data.Text under the quaified module name T.

I appreciate the problem — glad you're attacking it.

I worry, though, about the extra layers of indirection for the programmer's
mental name resolution algorithm. If I see a "T.foo" and no import of T,
then I have to investigate each import to see which of them re-exports the
qualified module T.

I have three quick alternative proposals.

1) Use CPP — no GHC patch needed. Presumably, you're importing something
explicitly that would then re-export these qualified modules. One #include
and one symbol occurrence like DEFAULT_IMPORTS sounds like it would do the
trick. Kind of gross, but it clearly indicates where to find that extra
import list.

2) Add a GHC command line option that extends the default imports beyond
just Prelude.  I'm guessing this would be simpler to implement than your
proposal, and it to couches the conceptual changes as merely an expansion
of the familiar implicit import of Prelude. It does, however, mean name
resolution depends on GHC command line options to a new degree, which is
kind of weird. But there's only one extra place to look for imports:
wherever the command line options are coming from.

3) Refine your proposal so that the re-exported module can only itself be
referenced in a qualified way. EG `importing MyModule` from your example
would bring into scope MyModule.T.foo, but not T.foo. Or if MyModule is
imported qualified as X, you'd have have X.T.foo. Would this be complicated
to parse correctly, given the special treatment of '.'?

HTH. Good luck.
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