[Haskell-cafe] Integrate GHC-API with Cabal

[Félix Baylac]

Dear cafe,

I am currently working on a project [1] which aims to index the code published on
Stackage in order to provide a "code example" database. Basically, for each
library,  I want to generate a real-world example corpus for each exported 
symbol.

Before generating this database, I need to retrieve the exported symbol of a 
package. So far, I have been gathering the dependencies and integrating them in
the GHC pkg database using an external stack call, I then parse the cabal file
 and load the exposed modules using the GHC API and gather the exported
 symbols*.

It works pretty reliably on the simple packages, however, I end up with some 
missing dynamic flags for some more advanced packages (missing c includes, c 
libraries, ASM flags, etc.). I then started to parse and load these missing 
attributes to GHC until I stepped back for a moment and realized I was 
re-implementing cabal build.

I then looked for a way to "hook" my GHC API code in cabal build. This would
allow cabal to both handle the dependencies gathering as well as setting up the
correct GHC dyn-flags. The only resource I found was [2]. It's really clever, 
however, looks a bit hacky to me. 

Is there a better way to perform this kind of  "hook"?


[1] https://git.alternativebit.fr/NinjaTrappeur/Exhs
[2] http://blog.ezyang.com/2017/02/how-to-integrate-ghc-api-programs-with-cabal/

* I am aware of the hoogle index generated by haddock. However, this index is
  missing when the haddock documentation of a package cannot be generated.