On Fri, Sep 25, 2015 at 12:18 PM, Eric Seidel <eric@seidel.io> wrote:

I've been meaning to ask about this as well. It also forces tools like
ghc-mod and hdevtools to be cabal-aware, which is an unnecessary source
of complexity IMO.

This would certainly be nice, but...

GHC certainly has enough information to generate these macros, as it
knows which packages (and versions) it's compiling against.

It knows at some point, but it doesn't necessarily know before parsing the module, at which point it is too late. I can have two versions of a package A, and two other packages B and C that depend on different versions of A, and depending on whether a module M uses package B or package C, M will see different versions of package A automatically. This is all slightly magical, and I have to say I don't entirely understand how GHC decides which versions to expose in general, but that's how GHC works today and it's quite convenient.

GHC could provide MIN_VERSION_* macros for packages that have had their versions specified with -package or similar flags (which is how Cabal invokes GHC). That would go only a small way towards the original goals though.

(Also, I wonder how MIN_VERSION_* fits into a Backpack world...)

Regards,

Reid Barton