Collecting all external names in a module

[Claus Reinke]

Johan:
> I started writing a manual traversal of the RenamedSource AST 
> (as I want qualified names) but I thought I check if I'm going 
> about this right before I spend all the time required to write 
> the traversal for the whole AST.

Manual traversal code on ASTs tends to consist mainly
of boilerplate, so I'd like to emphasize Simon's suggestion:

Simon:
> You could even use generic programming to do it (all the Hs 
> things are in class Data I think).

There is a page on the GHC wiki on generic AST traversals

    http://hackage.haskell.org/trac/ghc/wiki/GhcApiAstTraversals

by now probably out of date (eg, the instances may now be
available without needing ghc-syb, I think?), but it should 
get you started (using generic programming is a lot easier 
than trying to understand its types, you just need to make 
sure that your traversals avoid some holes in the GHC 
ASTs; there's a Data-based show variant that illustrates
the idea), and you might be able to correct out-of-date info.

Claus